Home » 2013

Yearly Archives: 2013

DIVING REGULATOR

6 February 2013 14.39 / Leave a comment

A diving regulator is a pressure regulator used in scuba or surface supplied diving equipment that reduces pressurized breathing gas to ambient pressure and delivers it to the diver. The gas may be air or one of a variety of specially blended breathing gases. The gas may be supplied from a cylinder worn by the diver (as in a scuba set), or via a hose from a compressor or a bank of cylinders on the surface (as in surface-supplied diving). A gas pressure regulator has one or more valves in series, which reduces pressure from the source in a controlled manner, lowering pressure at each stage.

The terms “regulator” and “demand valve” are often used interchangeably, but a demand valve is the part of a regulator that delivers gas only while the diver is breathing in and reduces the gas pressure to ambient. In single hose regulators, it is part of the second stage held in the diver’s mouth by a mouthpiece. In double hose regulators it is part of the regulator attached to the cylinder.

Types of diving regulator

Demand valve

A demand valve detects when the diver starts inhaling and supplies the diver with a breath of gas at ambient pressure.

The demand valve was invented in 1838 in France,and forgotten in the next few years; another workable demand valve was not invented until 1860.

On November 14, 1838, Dr. Manuel Théodore Guillaumet of Argentan, Normandy, France, filed a patent for a twinhose demand regulator; the diver was provided air through pipes from the surface. The apparatus was demonstrated to, and investigated by, a committee of the French Academy of Sciences: “Mèchanique appliquée — Rapport sur une cloche à plongeur inventée par M. Guillaumet” (Applied mechanics — Report on a diving bell invented by Mr. Guillaumet), Comptes rendus, vol. 9, pages 363-366 (September 16, 1839).
Illustration of diving apparatus invented by Dr. Manuel Théodore Guillaumet from: Alain Perrier, 250 Réponses aux questions du plongeur curieux [250 Answers to the questions of the curious diver] (Aix-en-Provence, France: Éditions du Gerfaut, 2008), page 45.
On June 19, 1838, in London, England, a Mr. William Edward Newton first filed a patent (no. 7695: “Diving apparatus”) for a diaphram-actuated, twin-hose demand valve for divers. (See: John Bevan (1990) “The First Demand Valve?,” SPUMS Journal [SPUMS = South Pacific Underwater Medicine Society], vol. 20, no. 4, pages 239-240.) However, it is believed that Mr. Newton was merely filing a patent on behalf of Dr. Guillaumet. (See: le scaphandre autonome (scuba diving): Un brevet semblable est déposé en 1838 par William Newton en Angleterre. Il y a tout lieu de penser que Guillaumet, devant les longs délais de dépôt des brevets en France, a demandé à Newton de faire enregistrer son brevet en Angleterre où la procédure est plus rapide, tout en s’assurant les droits exclusifs d’exploitation sur le brevet déposé par Newton. (A similar patent was filed in 1838 by William Newton in England. There is every reason to think that owing to the long delays in filing patents in France, Guillaumet asked Newton to register his patent in England where the procedure was faster, while ensuring the exclusive rights to exploit the patent filed by Newton. [Note: The illustration of the apparatus in Newton’s patent application is identical to that in Guillaumet’s patent application; furthermore, Mr. Newton was apparently an employee of the British Office for Patents, who applied for patents on behalf of foreign applicants.]
Also from “le scaphandre autonome” Web site: Reconstruit au XXe siècle par les Américains, ce détendeur fonctionne parfaitement, mais, si sa réalisation fut sans doute effective au XIXe, les essais programmés par la Marine Nationale ne furent jamais réalisés et l’appareil jamais commercialisé. (Reconstructed in twentieth century by the Americans, this regulator worked perfectly; however, although it was undoubtedly effective in the nineteenth century, the test programs by the French Navy were never conducted and the apparatus was never sold.))

In 1860 a mining engineer from Espalion (France), Benoît Rouquayrol, had invented a demand valve with an iron air reservoir to let miners breathe in flooded mines. He called his invention régulateur (‘regulator’).

In 1864 Rouquayol met the French Imperial Navy officer Auguste Denayrouze and both worked together to adapt Rouquayrol’s regulator to diving. The Rouquayrol-Denayrouze apparatus was mass produced, though with some interruptions, from 1864 to 1965.

As of 1865 it was acquired as a standard by the French Imperial Navy but never was entirely accepted by the French divers because of a lack of safety and autonomy.

In 1926 Maurice Fernez and Yves Le Prieur patented a hand-controlled regulator (not a demand valve then) which used a full-face mask (the air escaping from the mask at constant flow).

It was not until December 1942 that the demand valve was definitely improved in the way we know nowadays, when Frenchmen Jacques-Yves Cousteau (navy officer) and Émile Gagnan (engineer) met for the first time in Paris. Gagnan, employee at Air Liquide, had miniaturized and adapted a Rouquayrol-Denayrouze regulator to gas generators (following severe fuel restrictions due to the German occupation of France) and Cousteau suggested to adapt it again to diving, which was in 1864 its original purpose. Smaller than the large Rouquayrol-Denayrouze regulator and equipped with a safer reservoir (three gas cylinders at the time) the modern demand valve was born. Another French inventor, Georges Commeinhes from Alsace, had patented in 1937 and 1942 a diving demand valve, air-supplied by two gas cylinders through a full-face mask. Commeinhes died in 1944 during the liberation of Strasbourg and his invention was soon forgotten. In any case the Commeinhes demand valve was also an adaptation of the Rouquayoul-Denayrouze mechanism, but not as precise and miniaturized as was the Cousteau-Gagnan apparatus.

The demand valve has a chamber, which in normal use contains breathing gas at ambient pressure. A valve which supplies medium pressure gas can vent into the chamber. Either a mouthpiece or a full-face mask is connected to the chamber, for the diver to breathe from. On one side of the chamber is a flexible diaphragm to control the operation of the valve.

Modern demand valves use both breathing systems, mouthpiece or full-face mask, depending on the purpose of the dive. Modern full-face masks, for example, allow the use of underwater communication systems (usually called intercoms). Historically old demand valves also used one or the other system: the 1838 Guillaumet, 1864 Rouquayrol-Denayrouze, 1926 Fernez-Le Prieur and 1943 Cousteau-Gagnan apparati used all of them mouthpieces to provide the air to the diver (although the 1838 Guillaumet’s demand valve wasn’t independent from the surface and the Fernez-Le Prieur patent wasn’t a demand valve). The 1933 Le Prieur and 1942 Commeinhes apparati used full-face masks.

When the diver starts to breathe in, the inhalation lowers the pressure inside the chamber, which moves the diaphragm inwards operating a system of levers. This operates against the closing spring and lifts the valve off its seat, opening the valve and releasing gas into the chamber. The medium pressure gas, at about 10 bar/140 psi over ambient pressure, expands, reducing its pressure to ambient pressure, blowing out any water in the chamber and supplying the diver with gas to breathe. When the chamber is full and the lowering of pressure has been reversed, the diaphragm expands outwards to its normal position to close the medium pressure valve when the diver stops breathing in.

When the diver exhales, one-way valves, made from a flexible and air-tight material, flex outwards under the pressure of the exhalation allowing gas to escape from the chamber. They close making a seal when the exhalation stops and the pressure inside the chamber reduces to ambient pressure.

The diaphragm is protected by a cover, which the outside water can enter freely through holes or slits.

Demand valves can be of both the open circuit or reclaim types. The vast majority of demand valves are open circuit, which means that the exhaled gas is discharged into the surrounding environment and lost. Reclaim systems allow the used gas to be returned to the surface or (more often) diving bell for re-use after removing the carbon dioxide and making up the oxygen. This process, referred to as “push-pull” is technologically complex and expensive, and is only used for deep commercial diving on heliox mixtures, as the saving on helium compensates for the expense and complications of the system.

Free-flow regulator

These are generally used in surface supply diving with free-flow masks and helmets, and are usually simply a large high flow rated industrial gas regulator, which is manually controlled at the gas panel on the surface to the pressure required to provide to desired flow rate to the diver. Free flow is not normally used on scuba equipment as the high gas flow rates are inefficient and wasteful.

Rebreather regulators

The scuba rebreather systems also recycle the breathing gas, but are not based on a demand valve system for their primary function, as the breathing loop is carried by the diver and remains at ambient pressure while in use. Regulators used in scuba rebreathers are described below.

Automatic diluent valve (ADV)

These are used in rebreathers to add gas to the loop to compensate automatically for volume reduction due to pressure increase with greater depth, or to make up gas lost from the system by the diver exhaling through the nose while clearing the mask or as a method of flushing the loop. They are often provided with a purge button to allow manual flushing of the loop. The ADV is virtually identical in function to the open circuit demand valve.

Bailout valve (BOV)

This is an open circuit demand valve built into a rebreather mouthpiece, or other part of the breathing loop, which can be isolated while the diver is using the rebreather to recycle breathing gas, and opened at the same time as isolating the breathing loop when a problem causes the diver to bail out onto open circuit. The main distinguishing feature of the BOV is that the same mouthpiece is used for open and closed circuit, and the diver does not have to shut the Dive/Surface valve, remove it from his/her mouth, and find and insert the bailout demand valve in order to bail out onto open circuit. This reduction in critical steps makes the integrated BOV a significant safety advantage, though they are costly.

Constant mass flow addition valve

These are used to supply a constant mass flow of fresh gas to an active type semi-closed rebreather, to replenish the gas used by the diver and to maintain an approximately constant composition of the loop mix. Two main types are used: the fixed orifice and the adjustable orifice, usually a needle valve. The constant mass flow valve is usually based on a gas regulator which is isolated from the ambient pressure so that it provides an absolute pressure regulated output (not compensated for ambient pressure). This limits the depth range in which constant mass flow is possible through the orifice, but provides a relatively predictable gas mixture in the breathing loop. An overpressure valve is used to protect the output hose.

Manual and electronically controlled addition valves

These are used on manual and electronically controlled closed circuit rebreathers (mCCR, eCCR), to add oxygen to the loop to maintain set-point. A manually or electronically controlled valve is used to release oxygen from the outlet of a standard scuba regulator first stage into the breathing loop. An overpressure valve is necessary to protect the hose (see below)

Structure and function of diving regulators

A diving regulator A-clamp type first stage

A DIN fitting stage with 2 medium pressure and 1 high pressure hose

A depth gauge and standard contents gauge

A button contents gauge on an ‘A’ clamp type first stage

A 1960s era Sportsways “Waterlung” Regulator with “J” Valve incorporated

The parts of a regulator are described as the major functional groups in downstream order as following the gas flow from the cylinder to its final use, and accessories which are not part of the primary functional components, but are commonly found on contemporary regulators. Some historically interesting models and components are described in a later section.

Single hose two-stage open circuit demand regulators

A “single-hose” aqualung with the first stage on top of the cylinder and the second stage demand valve on the left hand hose

Most contemporary diving regulators are single hose two-stage regulators. They consist of a first stage regulator, and a second stage demand valve. An intermediate pressure hose connects these components to transfer air, and allows relative movement within the constraints of hose length and flexibility. Other medium pressure hoses run to various equipment listed below.

The first make of this sort of scuba was the Porpoise which was made in Australia and was invented by Ted Eldred. At the same time in France, the Cristal Explorer (single hose) was designed by Bronnec & Gauthier.

The first stage

The first stage of the regulator is mounted to the cylinder valve via one of the standard connectors. It reduces cylinder pressure to a middle or intermediate pressure, usually about 10 bars (150 psi) higher than the ambient pressure. The breathing gas then passes through a hose to the second stage.

A balanced regulator first stage automatically keeps a constant pressure difference between the interstage pressure and the ambient pressure even as the tank pressure drops with consumption. The balanced regulator design allows the first stage orifice to be as large as needed without incurring performance degradation as a result of changing tank pressure.

The first stage generally has several low-pressure outlets (ports) for second-stage regulators, BCD inflators and other equipment; and one or more high-pressure outlets, which allow a submersible pressure gauge (SPG) or gas-integrated diving computer to read the cylinder pressure. The valve may be designed so that one low-pressure port is designated “Reg” for the primary second stage regulator, because that port allows a higher flow rate to give less breathing effort at maximum demand. A small number of manufacturers have produced regulators with a larger than standard hose and port diameter for this primary outlet.

Types of first stage

Diagram of the internal components of a balanced piston-type first stage

Diagram of the internal components of a diaphragm-type first stage

Diagram of the internal components of an unbalanced diaphragm first stage

Diagram of the internal components of a balanced diaphragm first stage

Animation of the internal components of a diaphragm-type first stage during the breathing cycle

The mechanism inside the first stage can be of the diaphragm type or the piston type. Both types can be balanced or unbalanced. Unbalanced regulators have the cylinder pressure pushing the first stage upstream valve closed, which is opposed by the intermediate stage pressure and a spring. As cylinder pressure falls the closing force is less, so the regulated pressure increases at lower tank pressure. To keep this pressure rise within acceptable limits the high-pressure orifice size was limited, but this decreased the total flow capacity of the regulator. A balanced regulator keeps about the same ease of breathing at all depths and pressures, by using the cylinder pressure to also indirectly oppose the opening of the first stage valve.

Piston type first stage

Some components of piston-type first stages are easier to manufacturer and have a simpler design than the diaphragm type. They need more careful maintenance because some internal moving parts are exposed to water and any contaminants in the water.

The piston in the first stage is rigid and acts directly on the seat of the valve. The pressure in the medium (aka intermediate) pressure chamber drops when the diver inhales from the second stage valve, this causes the piston to lift off the stationary valve seat as the piston slides into the intermediate pressure chamber. The now open valve permits high pressure gas to flow into the medium pressure chamber until the pressure in the chamber has risen enough to push the piston back into its original position against the seat and thus close the valve.

Diaphragm type first stage

Diaphragm-type first stages are more complex and have more components than the piston type. This design means that they are particularly suited to cold water diving and to working in saltwater and water containing a high degree of suspended particles, silt, or other contaminating materials, since the only parts exposed to the water are the valve opening spring and the diaphragm, all other parts are sealed off from the environment. In some cases the diaphragm and spring are also sealed from the environment.

The diaphragm is a flexible cover to the medium (intermediate) pressure chamber. When the diver consumes gas from the second stage, the pressure falls in the medium pressure chamber and the diaphragm deforms inwards pushing against the valve lifter. This opens the high pressure valve permitting gas to flow past the valve seat into the medium-pressure chamber. When the diver stops inhaling, pressure in the medium pressure chambers rises and the diaphragm returns to its neutral flat position and no longer presses on the valve lifter shutting off the flow until the next breath is taken.

Connection of first stage regulator to the cylinder valve or cylinder manifold

In an open-circuit scuba set, the first-stage of the regulator has an A-clamp, also known as a “yoke” or “international” connection, or a DIN fitting to connect it to the pillar valve of the diving cylinder. There are also European standards for scuba regulator connectors for gases other than air.

Yoke valves are the most popular in North America and many countries with large numbers of recreational diving tourists; it clamps an open hole on the regulator against an open hole on the cylinder. The user screws the clamp in place finger-tight, and once the cylinder valve is opened, gas pressure completes the seal along with an O-ring. The diver must take care not to screw the yoke down too tightly, or it may prove impossible to remove without tools. Conversely, failing to tighten sufficiently can lead to O-ring extrusion and a loss of cylinder gas, which can be a serious problem if it happens when the diver is at depth. Yoke fittings are rated up to a maximum of 240 bar working pressure.

The DIN fitting is a type of direct screw-in connection to the cylinder. While less common worldwide, the DIN system has the advantage of withstanding greater pressure, up to 300 bar, permitting the use of high-pressure steel cylinders. They are also less susceptible to blowing the O-ring seal if banged against something. DIN fittings are the standard in much of central Europe and are available in most countries. The DIN fitting is considered more secure and therefore safer by many Technical divers.

Adapters are available enabling a DIN first-stage to be attached to a cylinder with a yoke fitting valve, and for a Yoke first stage to be attached to a DIN cylinder valve.

Most cylinder valves are currently of the K-valve type, which is a simple manually operated screw-down on-off valve. In the mid-1960s, J-valves were widespread. J-valves contain a spring-operated valve that is restricts or shuts off flow when tank pressure falls to 300-500 psi, causing breathing resistance and warning the diver that he or she is dangerously low on air. The reserve air is released by pulling a reserve lever on the valve. J-valves fell out of favor with the introduction of pressure gauges, which allow divers to keep track of their air underwater, especially as the valve-type is vulnerable to accidental release of reserve air and increases the cost and servicing of the valve. J-valves are occasionally still used when work is done in visibility so poor that the pressure gauge cannot be seen, even with a light.

Risk of the regulator becoming blocked with ice

As gas leaves the cylinder it decreases in pressure in the first stage, becoming very cold due to adiabatic expansion. Where the ambient water temperature is less than 5°C any water in contact with the regulator may freeze. If this ice jams the diaphragm or piston spring, preventing the valve closing, a free-flow may ensue that can empty a full cylinder within a minute or two, and the free-flow causes further cooling in a positive feedback loop. Generally the water that freezes is in the ambient pressure chamber around a spring that keeps the valve open and not moisture in the breathing gas from the cylinder, but that is also possible if the air is not adequately filtered.

The modern trend of using more plastics, instead of metals, within the regulators encourages freezing because it insulates the inside of a cold regulator from the warmer surrounding water.

Cold water kits can be used to reduce the risk of freezing inside the regulator. Some regulators come with this as standard, and some others can be retrofitted. Environmental sealing of the diaphragm main spring chamber using a soft secondary diaphragm and hydrostatic transmitter or a silicone, alcohol or glycol/water mixture antifreeze liquid in the sealed spring compartment can be used for a diaphragm regulator. Silicone grease in the spring chamber can be used on a piston first stage.

The Poseidon Xstream first stage insulates the external spring and spring housing from the rest of the regulator, so that it is less chilled by the expanding air, and provides large slots in the housing so that the spring can be warmed by the water, thus avoiding the problem of freezing up the external spring.

Interstage hose

A medium (intermediate) pressure hose is used to allow breathing gas (typically at between 9 and 13 atmospheres above ambient) to flow from the first stage regulator to the second stage, or demand valve, which is held in the mouth by the diver, or attached to the full face mask or diving helmet.

Second stage or Demand valve

Types of second stage

A pair of demand valves

Animation of demand valve function during the breathing cycle

Air flow through the exhaust valve

Twin-hose open circuit demand scuba regulators

The “twin”, “double” or “two” hose type of scuba demand valve was the first in general use.

This type of regulator has two large bore corrugated breathing tubes. One tube is to supply air from the regulator to the mouthpiece, and the second tube is for exhalation; it is not for rebreathing but to keep the air inside the breathing tube at the same pressure as the water at the regulator diaphragm. This second breathing tube returns the exhaled air to the regulator on the wet side of the diaphragm, where it is released through a rubber duck-bill one-way valve, and comes out of the holes in the cover.

In Cousteau’s original aqualung prototype, there was no exhaust hose, and the exhaled air exited through a one-way valve at the mouthpiece. It worked out of water, but when he tested the aqualung in the river Marne air escaped from the regulator before it could be breathed when the mouthpiece was above the regulator. After that, he had the second breathing tube fitted.

Even with both tubes fitted, raising the mouthpiece above the regulator increases the flow of gas and lowering the mouthpiece increases breathing resistance. As a result, many aqualung divers, when they were snorkeling on the surface to save air while reaching the dive site, put the loop of hoses under an arm to avoid the mouthpiece floating up causing free flow.

Diver orientation changes breathing characteristic of regulators. With twin hose regulator on back at shoulder level, if the diver rolls on his or her back the released air pressure is higher than in the lungs. Divers learned to restrict flow by using their tongue to close the mouthpiece. When the cylinder pressure was running low and air demand effort rising, a roll to the right side made breathing easier.

Divers had to carry more weight underwater to compensate for the buoyancy of the air in the hoses. An advantage with this type of regulator is that the bubbles leave the regulator behind the diver’s head, increasing visibility, and not interfering with underwater photography. Twin hose regulators have been superseded by single hose regulators and became obsolete for most diving in the 1980s.

Some modern twin-hose regulators have one or more low-pressure ports that branch off between the two valve stages, which can be used to supply direct feeds for suit or BC inflation and/or a secondary single hose demand valve, and a high pressure port for a submersible pressure gauge.

Someone made a twin-hose type regulator where the energy released as the air expands from cylinder pressure to the surrounding pressure as the diver breathes in, is not thrown away but used to power a propeller.

The twin-hose arrangement with a mouthpiece or full-face mask has reappeared in modern rebreathers, but as part of the breathing loop, not as part of a regulator. The associated demand valve comprising the bail-out valve is always a single hose regulator.

Old-style “twin-hose” twin cylinder aqualung

Nemrod twin-hose regulator made in the 1980s. It has one low-pressure port, which feeds the left (inhalation) hose. Its mouthpiece can be strapped in.

The Draeger two stage twin hose regulator

Twin 7l cylinders with Draeger harness, valves, manifold and regulator from c1965

Two stage twin hose open circuit demand regulators

Early open circuit scuba demand regulators were mostly twin hose designs. The mechanism of the regulator is packaged in a usually circular metal housing mounted on the cylinder valve behind the diver’s neck, and the air flows through a pair of corrugated rubber hoses to and from the mouthpiece. The supply hose is connected to one side of the regulator body and supplies air to the mouthpiece through a non-return valve, and the exhaled air is returned to the regulator housing on the outside of the diaphragm, also through a non-return valve on the other side of the mouthpiece, and usually through another non-return exhaust valve in the regulator housing, often a “duckbill” type. The demand valve component of a two stage twin hose regulator is thus mounted in the same housing as the first stage regulator, and in order to prevent free-flow, the exhaust valve is located at the same depth as the diaphragm, and the only reliable place to do this is in the same housing.

Single stage twin hose open circuit demand regulators

Beuchat “Souplair” single stage twin hose regulator

Some early twin hose regulators were of single stage design. The first stage functions in a way similar to the second stage of two-stage demand valves, but would be connected directly to the cylinder valve and reduced high pressure air from the cylinder directly to ambient pressure on demand. This could be done by using a longer lever and larger diameter diaphragm to control the valve movement, but there was a tendency for cracking pressure, and thus work of breathing, to vary as the cylinder pressure dropped.

Rebreather Automatic Diluent Valves

Some passive semi-closed circuit rebreathers use a form of demand valve, which senses the volume of the loop and injects more gas when the volume falls below a certain level.

Upstream vs downstream

Most modern demand valves use a downstream rather than an upstream valve mechanism. In a downstream valve, the moving part of the valve opens in the same direction as the flow of gas and is kept closed by a spring. In an upstream valve, the moving part works against the pressure and opens in the opposite direction as the flow of gas. If the first stage jams open and the medium pressure system over-pressurizes, the second stage downstream valve opens automatically resulting in a “freeflow”. With an upstream valve, the result of over-pressurization may be a blocked valve. This will stop the supply of breathing gas, and possibly result in a ruptured hose or the failure of another second stage valve, such as one that inflates a buoyancy device. When a second stage upstream tilt valve is used a relief valve should be included by the manufacture on the first stage regulator to protect the intermediate hose.

If a shut-off valve is fitted between the first and second stages, as is found on scuba bailout systems used for commercial diving, and in some technical diving configurations, the demand valve will normally be isolated and unable to function as a relief valve. In this case an overpressure valve must be fitted to the first stage if it does not already have one. As very few contemporary (2011) scuba regulator first stages are factory fitted with overpressure relief valves, they are available as aftermarket accessories which can be screwed into any low pressure port available on the first stage.

Regulator accessories

Pressure relief valve

A downstream demand valve serves as a fail safe for over-pressurization: if a first stage with a demand valve malfunctions and jams in the open position, the demand valve will be over-pressurized and will “free flow”. Although it presents the diver with an imminent “out of air” crisis, this failure mode lets gas escape directly into the water without inflating buoyancy devices. The effect of unintentional inflation might be to carry the diver quickly to the surface causing the various injuries that can result from an over-fast ascent. There are circumstances where regulators are connected to inflatable equipment such as a rebreather’s breathing bag, a buoyancy compensator or a drysuit but without the need for demand valves. Examples of this are argon suit inflation sets, and “off board” or secondary diluent cylinders for closed-circuit rebreathers. When no demand valve is connected to a regulator, it should be equipped with a pressure relief valve’, unless it has a built in over pressure valve, so that over-pressurization does not inflate any buoyancy devices connected to the regulator.

Submersible pressure gauge (SPG)

To monitor breathing gas pressure in the diving cylinder, a diving regulator usually has a high pressure hose leading to a contents gauge(also called pressure gauge). The port for this hose leaves the first-stage upstream of all pressure-reducing valves. The contents gaugeis a pressure gauge measuring the gas pressure in the diving cylinder so the diver knows how much gas remains in the cylinder. It is also known as submersible pressure gauge or SPG. There are several types of contents gauge:-

Standard type

This is an analogue gauge that can be held in the palm of a hand and is connected to the first stage by a high pressure hose. It displays with a pointer moving over a dial. Sometimes they are fixed in a console, which is a plastic or rubber case that holds the air pressure gauge and also a depth gauge and/or a dive computer and/or a compass.

Button gauges

These are coin-sized analogue pressure gauges located on the first stage. They are compact, have no dangling hoses and few points of failure. They are generally not used on back mounted cylinders, because the diver cannot easily see them there when underwater. They are sometimes used on side slung stage cylinders. Due to their small size, it can be difficult to read the gauge to a resolution of less than 20 bar / 300 psi.

Air integrated computers

Some dive computers are designed to measure, display, and monitor pressure in the diving cylinder. This can be very beneficial to the diver, but if the dive computer fails, the diver can no longer monitor his or her gas reserves. Most divers using a gas-integrated computer will also have a standard air pressure gauge. The computer is either connected to the first stage by a high pressure hose, or has two parts, the pressure transducer on the first stage and the display at the wrist or console, which communicate by radio link; the signals are encoded to eliminate the risk of one diver’s computer picking up a signal from another diver’s transducer, or radio interference from other sources.

Mechanical reserve valves

In the past, some types of diving cylinder had a mechanical reserve valve that restricted air flow when the pressure was below 500 psi. Alerted to having a low gas supply the diver would pull a lever to open the reserve valve and surface using the reserve gas. Occasionally, a diver would inadvertently trigger the mechanism while donning gear or performing a movement underwater and, unaware that the reserve had already been accessed, could find himself out of breathing gas with no warning. These valves are known as “J valves” due to the letter J being next to that valve in the US Divers product catalog. Valves without the reserve lever are called “K valves” for the same reason; being the next item in the catalog they were denoted by the letter K. Modern divers using “J valves” dive with the reserve valve in the open position and depend on a contents gauge or computer to monitor gas supply.

Secondary demand valve (Octopus)

A combined diving regulator demand valve and BC inflation valve

As a nearly universal standard practice in modern recreational diving, the typical single-hose regulator has a spare demand valve fitted for emergency use by the diver’s buddy, typically referred to as the octopus because of the extra hose, or secondary DV. The medium pressure hose on the octopus is usually longer than the medium pressure hose on the primary DV that the diver uses, and the demand valve and/or hose may be colored yellow to aid in locating during an emergency. The secondary regulator should be clipped to the diver’s harness in a position where it can be easily seen and reached by both the diver and the potential sharer of air. The longer hose is used for convenience when sharing air, so that the divers are not forced to stay in an awkward position relative to each other. Technical divers frequently extend this feature and use a 5′ or 7′ hose, which allows divers to swim in single file while sharing air, which may be necessary in restricted spaces inside wrecks or caves.

The secondary demand valve can be a hybrid DV and buoyancy compensator inflation valve. Both types are sometimes called alternate air sources. A DV on a regulator connected to a separate independent diving cylinder would also be called an “alternate air source”, and also a redundant air source, as it is totally independent of the primary air source.

Full face mask or helmet

This is stretching the concept of accessory a bit, as it would be equally valid to call the regulator an accessory of the full face mask or helmet, but the two items are closely connected, and generally found in use together.

Most full face masks and probably most diving helmets currently in use are open circuit demand systems, using a demand valve (in some cases more than one) and supplied from a scuba regulator and frequently also a surface supply umbilical from a surface supply panel using a surface supply regulator to control the pressure of primary and reserve air or other breathing gas.

Lightweight diving helmets are almost always surface supplied, but full face masks are used equally appropriately with scuba open circuit, scuba closed circuit (rebreathers) and surface supplied open circuit.

The demand valve is usually firmly attached to the helmet or mask, but there are a few models of full face mask which have removable demand valves with quick connections, allowing them to be exchanged under water. These include the Draeger Panorama and Kirby-Morgan 48 Supermask.

Buoyancy compensator and dry suit inflation hoses

A drysuit direct feed a.k.a. a power inflator. CEJN 221 type.

Hoses may be fitted to low pressure ports of the regulator first stage to provide gas for inflating buoyancy compensators and/or dry suits. These hoses usually have quick-connector end with an automatically sealing valve which blocks flow if the hose is disconnected from the BC or suit. There are two basic styles of connector, which are not compatible with each other. The high flow rate fitting has a larger bore and allows gas flow at a fast enough rate for use as a connector to a demand valve. This is sometimes seen in a combination BC inflator/deflator mechanism with integrated secondary DV (octopus), such as in the AIR II unit from Scubapro. The low flow rate connector is more common and is the industry standard for BC inflator connectors, and is also popular on dry suits, as the limited flow rate reduces the risk of a blow-up if the valve sticks open. The high flow rate connector is used by some manufacturers on dry suits.

Various minor accessories are available to fit these hose connectors. These include interstage pressure gauges, which are used to troubleshoot and tune the regulator (not for use underwater), noisemakers, used to attract attention underwater and on the surface, and valves for inflating tires and inflatable boat floats, making the air in a scuba cylinder available for other purposes.

Instrument consoles (Combo consoles)

These are usually rubber or tough plastic moldings which enclose the SPG and have mounting sockets for other diver instrumentation, such as decompression computers, underwater compass, timer and/or depth gauge and occasionally a small plastic “slate” on which notes can be written either before or during the dive. There instruments would otherwise be carried somewhere else, such as strapped to the wrist or forearm, or in a pocket, and are only regulator accessories for convenience of transport and access.

Exotic examples

Twin-hose without visible regulator valve (fictional)

This type is mentioned here because it is very familiar in comics and other drawings, as a wrongly-drawn twin-hose two-cylinder aqualung, with one wide hose coming out of each cylinder top with no apparent regulator valve and going to the mouthpiece, much more often than a correctly-drawn twin-hose regulator. It would not work in the real world.

Demone regulator

This type was designed by Robert J. Dempster and made at his factory in Illinois, USA, from 1961 to 1965. It operates like a single-hose regulator. The second-stage looks like the mouthpiece of a twin-hose regulator, but with a small diaphragm on the front. The second-stage valve is inside one end of the mouthpiece tube. The exhaled air goes into a twin-hose-type exhalant tube which surrounds the intermediate-pressure hose and blows out at its end about 60% of the way back to the first-stage, to keep the bubbles away from the diver’s face. Near the mouthpiece is a one-way valve to let outside water into the exhalant hose to avoid free flow if the diaphragm (at the mouth) is below the open end of the exhalant hose. Many Demone regulators have two intermediate-pressure tubes and two exhalant hoses and two second-stages, one assembly on each side of the diver’s head, causing a superficial resemblance to the fictional “Twin-hose without visible regulator valve”.

Practical Mechanics design

This design was described in Practical Mechanics magazine in January 1955, as a home-made aqualung with a first-stage on the cylinder top leading through an intermediate-pressure hose to a large round second-stage (a converted Calor Gas regulator) on the diver’s chest connected to the diver’s mouthpiece by a twin-hose loop.

Twin-hose, home-made

In 1956 and for some years afterwards in Britain, factory-made aqualungs were very expensive, and many aqualungs of this type were made by sport divers in diving clubs’ workshops, using miscellaneous industrial and war-surplus parts. One necessary raw material was a Calor Gas bottled butane gas regulator, whose 1950s version was like an aqualung regulator’s second stage but operated constant-flow because its diaphragm was spring-loaded; conversion included changing the spring and making several big holes in the wet-side casing. The cylinder was often an ex-RAF pilot’s oxygen cylinder; some of these cylinders were called tadpoles from their shape.

In least one version of Russian twin-hose aqualung, the regulator did not have an A-clamp but screwed into a large socket on the cylinder manifold; that manifold was thin, and meandered somewhat. It had two cylinders and a pressure gauge. There is suspicion that those Russian aqualungs started as a factory-made improved descendant of an aqualung home-made by British sport divers and obtained unofficially by a Russian and taken to Russia.

Constant flow

In constant-flow regulators the first stage is a pressure regulator providing a constant reduced pressure, and the second stage is a plain on/off valve. These are the earliest type of breathing set flow control. The diver must open and close the supply valve to regulate flow. Constant flow valves in an open-circuit breathing set consume gas less economically than demand valve regulators because gas flows even when it is not needed.

Before 1939, diving and industrial open-circuit breathing sets with constant-flow regulators were designed and made, but did not get into general use due to excessively short dive duration for its weight. Design complications resulted from the need to put the second-stage on/off valve where it could be easily operated by the diver. Examples were:

“Ohgushi’s Peerless Respirator”. The valve was operated by the diver’s teeth.
Commandant le Prieur’s breathing sets: see Timeline of underwater technology. They were used for some sport diving on the French Riviera.

Full-face mask regulator

diagram of the 1946 version of the Le Prieur breathing set

There have been some cases of a single-hose-type regulator last stage built into a full-face mask so that the mask’s big front window plus the flexible rubber seal joining it to its frame, was a very big and thus very sensitive regulator diaphragm:

Several versions of the Le Prieur breathing set. Yves Le Prieur first patented with Maurice Fernez, in 1926, a breathing apparatus using a mouthpiece, but as of 1933 he removed the mouthpiece and included a circular full-face mask in all following patents (like 1937, 1946 or 1947).
In 1934 René Commeinhes, from Alsace (France), adapted a Rouquayrol-Denayrouze apparatus for the use of firefighters. With new 1937 and 1942 patents (GC37 and GC42) his son Georges adapted this invention to underwater breathing by means of a single hose connected to a full-face mask.
Captain Trevor Hampton invented independently from Le Prieur a similar regulator-mask in the 1950s and submitted it for patent. The Royal Navy requisitioned the patent, but found no use for it and eventually released it. By then, the market had moved on and it was too late to make this regulator-mask in bulk for sale.

Performance of regulators

In Europe, EN250:2000 defines the minimum requirements for breathing performance of regulators.

The original Cousteau twin-hose diving regulators could deliver about 140 litres of air per minute, and that was officially thought to be adequate; but divers sometimes needed a faster rate, and had to learn not to “beat the lung”, i.e. to try to breathe faster than the regulator could supply. Between 1948 and 1952 Ted Eldred designed his Porpoise air scuba to supply 300 liters/minute if the diver need to breathe that fast, and that soon became British and Australian naval standard.

In the United States Military, scuba regulators must adhere to performance specifications as outlined by the Mil-R-24169B which was based on equipment performance until recently.

Various breathing machines have been developed and used for assessment of breathing apparatus performance. ANSTI has developed a testing machine that measures the inhalation and exhalation effort in using a regulator; publishing results of the performance of regulators in the ANSTI test machine has resulted in big performance improvements.

Manufacturers

Air Liquide: La Spirotechnique, Apeks and Aqua Lung
Apollo Sports
Atomic Aquatics
Beuchat
Cressi-Sub
Dive Rite
Draeger
HTM Sports: Dacor and Mares
Poseidon
ROMI Enterprises: Aeris and Oceanic
Ocean Divers Supply
Scubapro
Tusa
Zeagle
Edge-HOG (Highly Optimized Gear)
Swagelok Speciality Regulators {Breathing Air}

Value Added Reseller

Dive Rite
Coltri
Edge-HOG
Genesis
Halcyon
OMS
Seacsub
Sherwood
Tigullio
XS Scuba

Source :

http://en.wikipedia.org/wiki/Diving_regulator

PEARL

3 February 2013 01.51 / Leave a comment

A pearl is a hard object produced within the soft tissue (specifically the mantle) of a living shelled mollusc. Just like the shell of a clam, a pearl is made up of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes of pearls (baroque pearls) occur. The finest quality natural pearls have been highly valued as gemstones and objects of beauty for many centuries, and because of this, the word pearl has become a metaphor for something very rare, fine, admirable, and valuable.

The most valuable pearls occur spontaneously in the wild, but they are extremely rare. These wild pearls are referred to as natural pearls. Cultured or farmed pearls from pearl oysters and freshwater mussels make up the majority of those that are currently sold. Imitation pearls are also widely sold in inexpensive jewelry, but the quality of their iridescence is usually very poor, and often, artificial pearls are easily distinguished from genuine pearls. Pearls have been harvested and cultivated primarily for use in jewelry, but in the past they were also stitched onto lavish clothing. Pearls have also been crushed and used in cosmetics, medicines, and in paint formulations.

Whether wild or cultured, gem quality pearls are almost always nacreous and iridescent, as is the interior of the shell that produces them. However, almost all species of shelled molluscs are capable of producing pearls (formally referred to as “calcareous concretions” by some sources) of lesser shine or less spherical shape. Although these may also be legitimately referred to as “pearls” by gemological labs and also under U.S. Federal Trade Commission rules, and are formed in the same way, most of them have no value, except as curiosities.

Evolutionary significance

A pearl being extracted from an akoya pearl oyster.

A black pearl and a shell of the black-lipped pearl oyster. The iridescent colors originate from nacre layers.

Pearls are commonly viewed by scientists as a by-product of an adaptive immune system-like function.

Etymology

The English word pearl comes from the French perle, originally from the Latin perna meaning leg, after the ham- or mutton leg-shaped bivalve.

Definition

Almost any shelled mollusk can, by natural processes, produce some kind of “pearl” when an irritating microscopic object becomes trapped within the mollusk’s mantle folds, but the great majority of these “pearls” are not valued as gemstones. Nacreous pearls, the best-known and most commercially-significant pearls, are primarily produced by two groups of molluscan bivalves or clams. A nacreous pearl is made from layers of nacre, by the same living process as is used in the secretion of the mother of pearl which lines the shell.

A “natural pearl” or “wild pearl” is one that forms without any human intervention at all, in the wild, and is very rare. Many hundreds of pearl oysters or pearl mussels have to be gathered and opened, and thus killed, to find even one wild pearl, and for many centuries that was the only way pearls were obtained. This was the main reason why pearls fetched such extraordinary prices in the past. A cultured pearl is formed in a pearl farm, using human intervention as well as natural processes.

One family of nacreous pearl bivalves – the pearl oyster – lives in the sea, while the other – a very different group of bivalves – lives in freshwater; these are the river mussels such as the freshwater pearl mussel. Saltwater pearls can grow in several species of marine pearl oysters in the family Pteriidae. Freshwater pearls grow within certain (but by no means all) species of freshwater mussels in the order Unionida, the families Unionidae and Margaritiferidae.

Structure of nacre layers, wherein aragonite plates are separated by biopolymers, such as chitin, lustrin and silk-like proteins

Physical properties

Electron microscopy image of a fractured surface of nacre

The unique luster of pearls depends upon the reflection, refraction, and diffraction of light from the translucent layers. The thinner and more numerous the layers in the pearl, the finer the luster. The iridescence that pearls display is caused by the overlapping of successive layers, which breaks up light falling on the surface. In addition, pearls (especially cultured freshwater pearls) can be dyed yellow, green, blue, brown, pink, purple, or black. The very best pearls have a metallic mirror-like luster.

Because pearls are made primarily of calcium carbonate, they can be dissolved in vinegar. Calcium carbonate is susceptible to even a weak acid solution because the crystals of calcium carbonate react with the acetic acid in the vinegar to form calcium acetate and carbon dioxide.

Freshwater and saltwater pearls

Freshwater and saltwater pearls may sometimes look quite similar, but they come from different sources.

Freshwater pearls form in various species of freshwater mussels, family Unionidae, which live in lakes, rivers, ponds and other bodies of fresh water. These freshwater pearl mussels occur not only in hotter climates, but also in colder more temperate areas such as Scotland (where they are totally protected under law). However, most freshwater cultured pearls sold today come from China.

Saltwater pearls grow within pearl oysters, family Pteriidae, which live in oceans. Saltwater pearl oysters are usually cultivated in protected lagoons or volcanic atolls.

Creation of a pearl

Diagram comparing a cross-section of a cultured pearl, upper, with a natural pearl, lower

The difference between wild and cultured pearls focuses on whether the pearl was created spontaneously by nature – without human intervention – or with human aid. Pearls are formed inside the shell of certain mollusks as a defense mechanism against a potentially threatening irritant such as a parasite inside its shell, or an attack from outside, injuring the mantle tissue. The mollusk creates a pearl sac to seal off the irritation.

The mantle of the mollusk deposits layers of calcium carbonate (CaCO₃) in the form of the mineral aragonite or a mixture of aragonite and calcite (polymorphs with the same chemical formula, but different crystal structures) held together by an organic horn-like compound called conchiolin. The combination of aragonite and conchiolin is called nacre, which makes up mother-of-pearl. The commonly held belief that a grain of sand acts as the irritant is in fact rarely the case. Typical stimuli include organic material, parasites, or even damage that displaces mantle tissue to another part of the mollusk’s body. These small particles or organisms gain entry when the shell valves are open for feeding or respiration. In cultured pearls, the irritant is typically an introduced piece of the mantle epithelium, with or without a spherical bead (beaded or beadless cultured pearls).

Natural pearls

Natural pearls are nearly 100% calcium carbonate and conchiolin. It is thought that natural pearls form under a set of accidental conditions when a microscopic intruder or parasite enters a bivalve mollusk, and settles inside the shell. The mollusk, being irritated by the intruder, forms a pearl sac of external mantle tissue cells and secretes the calcium carbonate and conchiolin to cover the irritant. This secretion process is repeated many times, thus producing a pearl. Natural pearls come in many shapes, with perfectly round ones being comparatively rare.

Typically, the build-up of a natural pearl consists of a brown central zone formed by columnar calcium carbonate (usually calcite, sometimes columnar aragonite) and a yellowish to white outer zone consisting of nacre (tabular aragonite). In a pearl cross-section such as the diagram, these two different materials can be seen. The presence of columnar calcium carbonate rich in organic material indicates juvenile mantle tissue that formed during the early stage of pearl development. Displaced living cells with a well-defined task may continue to perform their function in their new location, often resulting in a cyst. Such displacement may occur via an injury. The fragile rim of the shell is exposed and is prone to damage and injury. Crabs, other predators and parasites such as worm larvae may produce traumatic attacks and cause injuries in which some external mantle tissue cells are disconnected from their layer. Embedded in the conjunctive tissue of the mantle, these cells may survive and form a small pocket in which they continue to secrete their natural product: calcium carbonate. The pocket is called a pearl sack, and grows with time by cell division; in this way the pearl grows also. The juvenile mantle tissue cells, according to their stage of growth, produce columnar calcium carbonate, which is secreted from the inner surface of the pearl sack. With ongoing time the external mantle cells of the pearl sack proceed to the formation of tabular aragonite. When the transition to nacre secretion occurs, the brown pebble becomes covered with a nacreous coating. As this process progresses, the shell itself grows, and the pearl sack seems to travel into the shell. However, it actually stays in its original relative position within the mantle tissue. After a couple of years, a pearl will have formed and the shell might be found by a lucky pearl fisher.

Cultured pearls

Nuclei from Toba Pearl Island, Japan

Cultured pearls are the response of the shell to a tissue implant. A tiny piece of mantle tissue from a donor shell is transplanted into a recipient shell. This graft will form a pearl sac and the tissue will precipitate calcium carbonate into this pocket. There are a number of options for producing cultured pearls: use freshwater or seawater shells, transplant the graft into the mantle or into the gonad, add a spherical bead or do it non-beaded. The majority of saltwater cultured pearls are grown with beads. The trade name of the cultured pearls are Akoya, white or golden South sea, and black Tahitian. The majority of beadless cultured pearls are mantle-grown in freshwater shells in China, known as freshwater cultured pearls.

Cultured pearls can be distinguished from natural pearls by X-ray examination. Nucleated cultured pearls are often ‘pre-formed’ as they tend to follow the shape of the implanted shell bead nucleus. Once the pre-formed beads are inserted into the oyster, it secretes a few layers of nacre around the outside surface of the implant before it is removed after six months or more.

When a cultured pearl with bead is X-rayed, it reveals a different structure to that of a natural pearl. A beaded cultured pearl shows a solid center with no concentric growth rings, whereas a natural pearl shows a series of concentric growth rings. A beadless cultured pearl (whether of freshwater or saltwater origin) may show growth rings, but also a complex central cavity, witness of the first precipitation of the young pearl sac.

Imitation pearls

Some imitation pearls are simply made of mother-of-pearl, coral or conch shell, while others are made from glass and are coated with a solution containing fish scales called essence d’Orient. Although imitation pearls look the part, they do not have the same weight or smoothness as real pearls, and their luster will also dim greatly.

Gemological identification

A well-equipped gem testing laboratory can distinguish natural pearls from cultured pearls by using gemological X-ray equipment to examine the center of a pearl. With X-rays it is possible to see the growth rings of the pearl, where the layers of calcium carbonate are separated by thin layers of conchiolin. The differentiation of natural pearls from non-beaded cultured pearls can be very difficult without the use of this X-ray technique.

Natural and cultured pearls can be distinguished from imitation pearls using a microscope. Another method of testing for imitations is to rub two pearls against each other. Imitation pearls are completely smooth, but natural and cultured pearls are composed of nacre platelets, making both feel slightly gritty.

Value of a natural pearl

A brooch and a set of earrings from the 19th century made from gold and natural pearls

Quality natural pearls are very rare jewels. The actual value of a natural pearl is determined in the same way as it would be for other “precious” gems. The valuation factors include size, shape, color, quality of surface, orient and luster.

Single, natural pearls are often sold as a collector’s item, or set as centerpieces in unique jewelry. Very few matched strands of natural pearls exist, and those that do often sell for hundreds of thousands of dollars. (In 1917, jeweler Pierre Cartier purchased the Fifth Avenue mansion that is now the New York Cartier store in exchange for a matched, double strand of natural pearls that he had been collecting for years; valued at the time at $1 million USD.)

The Great Depression effectively slashed the value of the natural pearl, but there is no doubt that it had been some time coming. The introduction and advance of the cultured pearl hit the pearl industry hard; it had pearl dealers publicly disputing over the authenticity of these new cultured pearls, and left many consumers uneasy and confused about the much lower prices. Essentially, it damaged the image of both natural and cultured pearls alike. By the 1950s, an era of every woman being able to own her own pearl necklace had begun, and natural pearls were reduced to a small, exclusive niche in the pearl industry.

Origin of a natural pearl

Previously, natural pearls were found in many parts of the world. Present day natural pearling is confined mostly to seas off Bahrain. Australia also has one of the world’s last remaining fleets of pearl diving ships. Australian pearl divers dive for south sea pearl oysters to be used in the cultured south sea pearl industry. The catch of pearl oysters is similar to the numbers of oysters taken during the natural pearl days. Hence significant numbers of natural pearls are still found in the Australian Indian Ocean waters from wild oysters. X-ray examination is required to positively verify natural pearls found today.

Types of cultured pearls

A blister pearl, a half-sphere, formed flush against the shell of the pearl oyster.

Keshi pearls, although they often occur by chance, are not considered natural pearls. They are a byproduct of the culturing process, and hence do not happen without human intervention. These pearls are quite small: typically a few millimeters in size. Keshi pearls are produced by many different types of marine mollusks and freshwater mussels in China. Keshi pearls are actually a mistake in the cultured pearl seeding process. In seeding the cultured pearl, a piece of mantle muscle from a sacrificed oyster is placed with a bead of mother of pearl within the oyster. If the piece of mantle should slip off the bead, a pearl forms of baroque shape about the mantle piece which is entirely nacre. Therefore, a Keshi pearl could be considered superior to cultured pearls with a mother of pearl bead center. In the cultured pearl industry, the resources used to create a mistaken all nacre baroque pearl is a drain on the production of round cultured pearls. Therefore, they are trying to improve culturing technique so that keshi pearls do not occur. All nacre pearls may one day be limited to natural found pearls. Today many “keshi” pearls are actually intentional, with post-harvest shells returned to the water to regenerate a pearl in the existing pearl sac.

Tahitian pearls, frequently referred to as black pearls, are highly valued because of their rarity; the culturing process for them dictates a smaller volume output and they can never be mass produced because, in common with most sea pearls, the oyster can only be nucleated with one pearl at a time, while freshwater mussels are capable of multiple pearl implants. Before the days of cultured pearls, black pearls were rare and highly valued for the simple reason that white pearl oysters rarely produced naturally black pearls, and black pearl oysters rarely produced any natural pearls at all.

Mary, Queen of Scots by an unknown artist after François Clouet (c. 1559)
London, Victoria and Albert Museum

The Queen is shown wearing her rope of famous black pearls.

Since the development of pearl culture technology, the black pearl oyster found in Tahiti and many other Pacific Island areas has been extensively used for producing cultured pearls. The rarity of the black cultured pearl is now a “comparative” issue. The black cultured pearl is rare when compared to Chinese freshwater cultured pearls, and Japanese and Chinese akoya cultured pearls, and is more valuable than these pearls. However, it is more abundant than the South Sea pearl, which is more valuable than the black cultured pearl. This is simply because the black pearl oyster Pinctada margaritifera is far more abundant than the elusive, rare, and larger south sea pearl oyster Pinctada maxima, which cannot be found in lagoons, but which must be dove for in a rare number of deep ocean habitats or grown in hatcheries.

Black pearls are very rarely black: they are usually shades of green, purple, aubergine, blue, grey, silver or peacock (a mix of several shades, like a peacock’s feather).

Black cultured pearls from the black pearl oyster – Pinctada margaritifera – are not South Sea pearls, although they are often mistakenly described as black South Sea pearls. In the absence of an official definition for the pearl from the black oyster, these pearls are usually referred to as “black pearls”.

The correct definition of a South Sea pearl – as described by CIBJO and GIA – is a pearl produced by the Pinctada maxima pearl oyster. South Sea pearls are the color of their host Pinctada maxima oyster – and can be white, silver, pink, gold, cream, and any combination of these basic colors, including overtones of the various colors of the rainbow displayed in the pearl nacre of the oyster shell itself.

South Sea pearls are produced in various parts of the world. White ones tend to come from the Broome area of Australia while golden ones are from the Philippines. Pearls are also produced in the Cook Islands and one farm in the Sea of Cortez, Mexico, from Concha Nácar the rainbow lipped oyster; these pearls fluoresce red under ultraviolet light.

Pearls from other species

A shell of the Indian volute, Melo melo, surrounded by a number of pearls from this species

Biologically speaking, under the right set of circumstances, almost any shelled mollusk can produce some kind of pearl, however, most of these molluscan pearls have no luster or iridescence. The great majority of mollusk species produce pearls which are not attractive, and are sometimes not even very durable, such that they usually have no value at all, except perhaps to a scientist, a collector, or as a curiosity. These objects used to be referred to as “calcareous concretions” by some gemologists, even though a malacologist would still consider them to be pearls. Valueless pearls of this type are sometimes found in edible mussels, edible oysters, escargot snails, and so on. The GIA and CIBJO now simply use the term ‘pearl’ (or, where appropriate, the more descriptive term ‘non-nacreous pearl’) when referring to such items and, under Federal Trade Commission rules, various mollusc pearls may be referred to as ‘pearls’, without qualification.

Pearl of Lao Tzu, the largest known pearl came from a giant clam

A few species produce pearls that can be of interest as gemstones. These species include the bailer shell Melo, the giant clam Tridacna, various scallop species, Pen shells Pinna, and the Haliotis iris species of abalone. Abalone, or Pāua are Mabe pearls unique to New Zealand waters and are commonly referred to as ‘Blue Pearls’. They are admired for their incredible luster and naturally bright vibrant colors that are often compared to Opal. Another example is the conch pearl (sometimes referred to simply as the ‘pink pearl’), which is found very rarely growing between the mantle and the shell of the queen conch or pink conch, Strombus gigas, a large sea snail or marine gastropod from the Caribbean Sea. These pearls, which are often pink in color, are a by-product of the conch fishing industry, and the best of them display a shimmering optical effect related to chatoyance known as ‘flame structure’. In 1999, the world auction record for a Melo pearl was US$488,311 for a single pearl.

Somewhat similar gastropod pearls, this time more orange in hue, are (again very rarely) found in the horse conch Pleuroploca gigantea.

The largest pearl known was found in the Philippines in 1934 and is known as the Pearl of Lao Tzu. It is a naturally-occurring, non-nacreous, calcareous concretion (pearl) from a giant clam. Because it did not grow in a pearl oyster it is not pearly; instead the surface is glossy like porcelain. Other pearls from giant clams are known to exist, but this is a particularly large one, weighing 14 lb (6.4 kg).

History

Pearl hunting

A 14th-century piece of clothing used by Kuwaiti divers searching for pearls in the Arabian Sea

For thousands of years, most seawater pearls were retrieved by divers working in the Indian Ocean, in areas like the Persian Gulf, the Red Sea, and in the Gulf of Mannar. Starting in the Han Dynasty (206 BC–220 AD), the Chinese hunted extensively for seawater pearls in the South China Sea. In the 14th-century Arabian Sea, the traveller Ibn Battuta provided the earliest known description of pearl diving by means of attaching a cord to the diver’s waist.

Catching of pearls, Bern Physiologus (9th century)

When Spanish conquistadors arrived in the Western Hemisphere, they discovered that around the islands of Cubagua and Margarita, some 200 km north of the Venezuelan coast, was an extensive pearl bed (a bed of pearl oysters). One discovered and named pearl, La Peregrina pearl, was offered to the Spanish queen. According to Garcilasso de la Vega, who says that he saw La Peregrina at Seville in 1607, (Garcilasso, “Historie des Incas, Rois du Perou,” Amsterdam, 1704, Vol. II, P. 352.) this was found at Panama in 1560 by a slave worker who was rewarded with his liberty, and his owner with the office of alcalde of Panama.

Margarita pearls are extremely difficult to find today and are known for their unique yellowish color. The most famous Margarita necklace that any one can see today is the one that then Venezuelan President Romulo Betancourt gave to Jacqueline Kennedy when she and her husband, President John F. Kennedy paid an official visit to Venezuela.

Before the beginning of the 20th century, pearl hunting was the most common way of harvesting pearls. Divers manually pulled oysters from ocean floors and river bottoms and checked them individually for pearls. Not all mussels and oysters produce pearls. In a haul of three tons, only three or four oysters will produce perfect pearls.

Pearl farming

Today, the cultured pearls on the market can be divided into two categories. The first category covers the beaded cultured pearls, including Akoya, South Sea and Tahiti. These pearls are gonad grown, and usually one pearl is grown at a time. This limits the number of pearls at a harvest period. The pearls are usually harvested after one year for akoya, 2–4 years for Tahitian and South Sea, and 2–7 years for freshwater. This perliculture process was first developed by the British biologist William Saville-Kent who passed the information along to Tatsuhei Mise and Tokichi Nishikawa from Japan. The second category includes the non-beaded freshwater cultured pearls, like the Biwa or Chinese pearls. As they grow in the mantle, where on each wing up to 25 grafts can be implanted, these pearls are much more frequent and saturate the market completely. An impressive improvement in quality has taken place in the last ten years when the former rice grain-shaped pebbles are compared with the near round pearls of today. In the last two years large near perfect round bead nucleated pearls up to 15mm in diameter have been produced with metallic luster.

The nucleus bead in a beaded cultured pearl is generally a polished sphere made from freshwater mussel shell. Along with a small piece of mantle tissue from another mollusk (donor shell) to serve as a catalyst for the pearl sac, it is surgically implanted into the gonad (reproductive organ) of a saltwater mollusk. In freshwater perliculture, only the piece of tissue is used in most cases, and is inserted into the fleshy mantle of the host mussel. South Sea and Tahitian pearl oysters, also known as Pinctada maxima and Pinctada margaritifera, which survive the subsequent surgery to remove the finished pearl, are often implanted with a new, larger beads as part of the same procedure and then returned to the water for another 2–3 years of growth.

Despite the common misperception, Mikimoto did not discover the process of pearl culture. The accepted process of pearl culture was developed by the British Biologist William Saville-Kent in Australia and brought to Japan by Tokichi Nishikawa and Tatsuhei Mise. Nishikawa was granted the patent in 1916, and married the daughter of Mikimoto. Mikimoto was able to use Nishikawa’s technology. After the patent was granted in 1916, the technology was immediately commercially applied to akoya pearl oysters in Japan in 1916. Mise’s brother was the first to produce a commercial crop of pearls in the akoya oyster. Mitsubishi’s Baron Iwasaki immediately applied the technology to the south sea pearl oyster in 1917 in the Philippines, and later in Buton, and Palau. Mitsubishi was the first to produce a cultured south sea pearl – although it was not until 1928 that the first small commercial crop of pearls was successfully produced.

The original Japanese cultured pearls, known as akoya pearls, are produced by a species of small pearl oyster, Pinctada fucata martensii, which is no bigger than 6 to 8 cm in size, hence akoya pearls larger than 10 mm in diameter are extremely rare and highly priced. Today, a hybrid mollusk is used in both Japan and China in the production of akoya pearls.

Recent pearl production

In 2010, China overtook Japan in akoya pearl production. Japan has all but ceased its production of akoya pearls smaller than 8 mm. Japan maintains its status as a pearl processing center, however, and imports the majority of Chinese akoya pearl production. These pearls are then processed (often simply matched and sorted), relabeled as product of Japan, and exported.

In the past two decades, cultured pearls have been produced using larger oysters in the south Pacific and Indian Ocean. The largest pearl oyster is the Pinctada maxima, which is roughly the size of a dinner plate. South Sea pearls are characterized by their large size and warm luster. Sizes up to 14 mm in diameter are not uncommon. South Sea pearls are primarily produced in Australia, Indonesia, and the Philippines.

Mitsubishi commenced pearl culture with the south sea pearl oyster in 1916, as soon as the technology patent was commercialized. By 1931 this project was showing signs of success, but was upset by the death of Tatsuhei Mise. Although the project was recommenced after Tatsuhei’s death, the project was discontinued at the beginning of WWII before significant productions of pearls were achieved.

After WWII, new south sea pearl projects were commenced in the early 1950s in Burma and Kuri Bay and Port Essington in Australia. Japanese companies were involved in all projects using technicians from the original Mitsubishi south sea pre-war projects.

Freshwater pearl farming

In 1914, pearl farmers began growing cultured freshwater pearls using the pearl mussels native to Lake Biwa. This lake, the largest and most ancient in Japan, lies near the city of Kyoto. The extensive and successful use of the Biwa Pearl Mussel is reflected in the name Biwa pearls, a phrase which was at one time nearly synonymous with freshwater pearls in general. Since the time of peak production in 1971, when Biwa pearl farmers produced six tons of cultured pearls, pollution has caused the virtual extinction of the industry. Japanese pearl farmers recently cultured a hybrid pearl mussel – a cross between Biwa Pearl Mussels and a closely related species from China, Hyriopsis cumingi, in Lake Kasumigaura. This industry has also nearly ceased production, due to pollution.

Japanese pearl producers also invested in producing cultured pearls with freshwater mussels in the region of Shanghai, China. China has since become the world’s largest producer of freshwater pearls, producing more than 1,500 metric tons per year (in addition to metric measurements, Japanese units of measurement such as the kan and momme are sometimes encountered in the pearl industry).

Led by pearl pioneer John Latendresse and his wife Chessy, the United States began farming cultured freshwater pearls in the mid 1960s. National Geographic magazine introduced the American cultured pearl as a commercial product in their August 1985 issue. The Tennessee pearl farm has emerged as a tourist destination in recent years, but commercial production of freshwater pearls has ceased.

Momme Weight

For many cultured pearl dealers and wholesalers, the preferred weight measure used for loose pearls and pearl strands is momme. Momme is a weight measure used by the Japanese for centuries. Today, momme weight is still the standard unit of measure used by most pearl dealers to communicate with pearl producers and wholesalers. One momme corresponds to 1/1000 kan. Reluctant to give up tradition, in 1891, the Japanese government formalized the kan measure as being exactly 1 kan = 3.75 kilograms or 8.28 pounds. Hence, 1 momme = 3.75 grams or 3750 milligrams.

In the United States, during the 19th and 20th centuries, through trade with Japan in silk cloth the momme became a unit indicating the quality of silk cloth.

Though millimeter size range is typically the first factor in determining a cultured pearl necklace’s value, the momme weight of pearl necklace will allow the buyer to quickly determine if the necklace is properly proportioned. This is especially true when comparing the larger south sea and Tahitian pearl necklaces.

Pearls in jewelry

Ring of Tahitian pearl

The value of the pearls in jewelry is determined by a combination of the luster, color, size, lack of surface flaw and symmetry that are appropriate for the type of pearl under consideration. Among those attributes, luster is the most important differentiator of pearl quality according to jewelers.

All factors being equal, however, the larger the pearl the more valuable it is. Large, perfectly round pearls are rare and highly valued. Teardrop-shaped pearls are often used in pendants.

Shapes

Pearls come in eight basic shapes: round, semi-round, button, drop, pear, oval, baroque, and circled. Perfectly round pearls are the rarest and most valuable shape. Semi-rounds are also used in necklaces or in pieces where the shape of the pearl can be disguised to look like it is a perfectly round pearl. Button pearls are like a slightly flattened round pearl and can also make a necklace, but are more often used in single pendants or earrings where the back half of the pearl is covered, making it look like a larger, rounder pearl.

Portrait of Empress Maria Fiodorovna in a Head-Dress Decorated with Pearls by Ivan Kramskoi (1880s)
Saint Petersburg, Hermitage Museum

Drop and pear shaped pearls are sometimes referred to as teardrop pearls and are most often seen in earrings, pendants, or as a center pearl in a necklace. Baroque pearls have a different appeal; they are often highly irregular with unique and interesting shapes. They are also commonly seen in necklaces. Circled pearls are characterized by concentric ridges, or rings, around the body of the pearl.

In general, cultured pearls are less valuable than natural pearls, whereas imitation pearls almost have no value. One way that jewelers can determine whether a pearl is cultured or natural is to have a gemlab perform an X-ray examination of the pearl. If X-rays reveals a nucleus, the pearl is likely a bead-nucleated saltwater pearl. If no nucleus is present, but irregular and small dark inner spots indicating a cavity are visible, combined with concentric rings of organic substance, the pearl is likely a cultured freshwater. Cultured freshwater pearls can often be confused for natural pearls which present as homogeneous pictures which continuously darken toward the surface of the pearl. Natural pearls will often show larger cavities where organic matter has dried out and decomposed.

Lengths of pearl necklaces

Portrait of Caterina Sagredo Barbarigo by Rosalba Carriera, cir. 1740. The subject is wearing a single-strand pearl collar and pendant pearl earrings

Queen of Italy, Margherita of Savoy, owned one of the most famous collections of natural pearls. She is wearing a multi-strand choker and a rope of pearls, possibly with matching bracelet and earrings

There is a special vocabulary used to describe the length of pearl necklaces. While most other necklaces are simply referred to by their physical measurement, pearl necklaces are named by how low they hang when worn around the neck. A collar, measuring 10 to 13 inches or 25 to 33 cm in length, sits directly against the throat and does not hang down the neck at all; collars are often made up of multiple strands of pearls. Pearl chokers, measuring 14 to 16 inches or 35 to 41 cm in length, nestle just at the base of the neck. A strand called a princess length, measuring 17 to 19 inches or 43 to 48 cm in length, comes down to or just below the collarbone. A matinee length, measuring 20 to 24 inches or 50 to 60 cm in length, falls just above the breasts. An opera length, measuring 28 to 35 inches or 70 to 90 cm in length, will be long enough to reach the breastbone or sternum of the wearer; and longer still, a pearl rope, measuring more than 45 inches or 115 cm in length, is any length that falls down farther than an opera.

Necklaces can also be classified as uniform, or graduated. In a uniform strand of pearls, all pearls are classified as the same size, but actually fall in a range. A uniform strand of akoya pearls, for example, will measure within 0.5 mm. So a strand will never be 7 mm, but will be 6.5–7 mm. Freshwater pearls, Tahitian pearls, and South Sea pearls all measure to a full millimeter when considered uniform.

A graduated strand of pearls most often has at least 3 mm of differentiation from the ends to the center of the necklace. Popularized in the United States during the 1950s by the GIs bringing strands of cultured akoya pearls home from Japan, a 3.5 momme, 3 mm to 7 mm graduated strand was much more affordable than a uniform strand because most of the pearls were small.

Colors of pearl jewelry

Earrings and necklaces can also be classified on the grade of the color of the pearl. While white, and more recently black, saltwater pearls are by far the most popular, other color tints can be found on pearls from the oceans. Pink, blue, champagne, green, black and even purple saltwater pearls can be encountered, but to collect enough of these rare colors to form a complete string of the same size and same shade can take years.

Religious references

Hindu scriptures

The Hindu tradition describes the sacred Nine Pearls which were first documented in the Garuda Purana, one of the books of the Hindu mythology. Ayurveda contains references to pearl powder as a stimulant of digestion and to treat mental ailments. According to Marco Polo, the kings of Malabar wore a necklace of 104 rubies and pearls which was given from one generation of kings to the next. The reason was that every king had to say 104 prayers every morning and every evening. At least until the beginning of the 20th century it was a Hindu custom to present a completely new, undrilled pearl and pierce it during the wedding ceremony.

The Pearl or Mukta in Sanskrit is also associated with many Hindu deities. The most famous being the Koustubha which Lord Vishnu wears on his chest. Apart from religious connotations, stories and folklore abound of pearls occurring in snakes, the Naaga Mani, and elephants, the Gaja Mukta.

Hebrew scriptures

According to Rebbenu Bachya, the word Yahalom in the verse Exodus 28:18 means “pearl” and was the stone on the Hoshen representing the tribe of Zebulun. This is generally disputed among scholars, particularly since the word in question in most manuscripts is actually Yasepheh – the word from which jasper derives; scholars think that refers to green jasper (the rarest and most prized form in early times) rather than red jasper (the most common form). Yahalom is usually translated by the Septuagint as an “onyx”, but sometimes as “beryl” or as “jasper”; onyx only started being mined after the Septuagint was written, so the Septuagint’s term “onyx” probably does not mean onyx – onyx is originally an Assyrian word meaning ring, and so could refer to anything used for making rings. Yahalom is similar to a Hebrew word meaning hit hard, so some people think that it means diamond. The variation in possibilities of meaning for this sixth stone in the Hoshen is reflected in different translations of the Bible – the King James Version translates the sixth stone as diamond, the New International Version translates it as emerald, and the Vulgate translates it as jaspis – meaning jasper. There is a wide range of views among traditional sources about which tribe the stone refers to.

New Testament scriptures

Religious pendant showing Christ blessing, framed with rubies and pearls, from the Byzantine empire, 12th or 13th century

In a Christian New Testament parable, Jesus compared the Kingdom of Heaven to a “pearl of great price” in Matthew 13: 45–46. “Again, the kingdom of heaven is like unto a merchant man, seeking goodly (fine) pearls: Who, when he had found one pearl of great price, went and sold all that he had, and bought it.”

The twelve gates of the New Jerusalem are reportedly each made of a single pearl in Revelation 21:21, that is, the Pearly Gates. “And the twelve gates were twelve pearls; every gate was of one pearl: and the streets of the city were pure gold, as if transparent glass.”

Holy things are compared to pearls in Matthew 7:6. “Do not give dogs what is holy, and do not throw your pearls before pigs, lest they trample them underfoot and turn to attack you.”

Pearls are also found in numerous references showing the wickedness and pride of a people, as in Revelation 18:16. “And saying, Alas, alas, that great city, that was clothed in fine linen, in purple and scarlet, and decked with gold, and precious stones, and pearls!”

Islamic scriptures

The Qur’an often mentions that dwellers of paradise will be adorned with pearls:

22:23 God will admit those who believe and work righteous deeds, to Gardens beneath which rivers flow: they shall be adorned therein with bracelets of gold and pearls; and their garments there will be of silk.

35:33 Gardens of Eternity will they enter: therein will they be adorned with bracelets of gold and pearls; and their garments there will be of silk.

52:24 Round about them will serve, [devoted] to them, youths [handsome] as pearls well-guarded.

Other scriptures

The metaphor of a pearl appears in the longer Hymn of the Pearl, a poem respected for its high literary quality, and use of layered theological metaphor, found within one of the texts of Gnosticism.

The Pearl of Great Price is a book of scripture in The Church of Jesus Christ of Latter-day Saints.

Source :

http://en.wikipedia.org/wiki/Pearl

Pearl

DIVING CYLINDER

2 February 2013 04.34 / Leave a comment

A diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set. It provides gas to the scuba diver through the demand valve of a diving regulator.

Diving cylinders typically have an internal volume of between 3 and 18 litres (0.11 and 0.64 cu ft) and a maximum pressure rating from 200 to 300 bars (2,900 to 4,400 psi). The internal cylinder volume is also expressed as “water capacity” – the volume of water which could be contained by the cylinder. When pressurised, a cylinder carries a volume of gas greater than its water capacity because gas is compressible. 600 litres (21 cu ft) of gas at atmospheric pressure is compressed into a 3-litre cylinder when it is filled to 200 bar. Cylinders also come in smaller sizes, such as 0.2, 1.5 and 2 litres, however these are not generally used for breathing, instead being used for purposes such as Surface Marker Buoy, drysuit and buoyancy compensator inflation.

Divers use gas cylinders above water for many purposes including storage of gases for oxygen first aid treatment of diving disorders and as part of storage “banks” for diving air compressor stations. They are also used for many purposes not connected to diving. For these applications they are not diving cylinders.

The term “diving cylinder” tends to be used by gas equipment engineers, manufacturers, support professionals, and divers speaking British English. “Scuba tank” or “diving tank” is more often used colloquially by non-professionals and native speakers of American English. The term “oxygen tank” is commonly used by non-divers when referring to diving cylinders; however, this is a misnomer. These cylinders typically contain (atmospheric) breathing air, or an oxygen-enriched air mix. They rarely contain pure oxygen, except when used for rebreather diving, shallow decompression stops in technical diving or for in-water oxygen recompression therapy. Breathing oxygen at depths greater than 20 feet (6.1 m)(equivalent to a partial pressure of oxygen of 1.6 ATA) can result in oxygen toxicity, a highly dangerous condition that can trigger seizures and thus lead to drowning.

Parts of a cylinder

A steel 15l cylinder with net and boot and a bare 12l aluminium cylinder

Two 12l steel cylinders connected by an isolation manifold and tank bands

The diving cylinder consists of several parts:

The pressure vessel

The pressure vessel is normally made of cold-extruded aluminium or forged steel. An especially common cylinder available at tropical dive resorts is an “aluminium-80” which is an aluminium cylinder with an internal volume of 0.39 cubic feet (11 L) rated to hold about 80 cubic feet (2,300 L) of atmospheric pressure gas at its rated pressure of 3,000 psi (210 bar). Aluminium cylinders are also often used where divers carry many cylinders, such as in technical diving in warm water where the dive suit does not provide much buoyancy, because the greater buoyancy of aluminium cylinders reduces the extra buoyancy the diver would need to achieve neutral buoyancy. They are also preferred when carried as “sidemount” or “sling” cylinders as the near neutral buoyancy allows them to hang comfortably along the sides of the diver’s body, without disturbing trim, and can be handed off to another diver with a minimal effect on buoyancy. In cold water diving, where a diver wearing a highly buoyant thermally insulating dive suit has a large excess of buoyancy, steel cylinders are often used because they are denser than aluminium cylinders. Kevlar wrapped composite cylinders are used in fire fighting breathing apparatus and oxygen first aid equipment, but are rarely used for diving, due to their high positive buoyancy.

The aluminium alloys used for diving cylinders are 6061 and 6351. 6351 alloy is subject to sustained stress cracking and cylinders manufactured of this alloy should be periodically eddy current tested according to national legislation and manufacturer’s recommendations.

The neck of the cylinder is internally threaded to fit a cylinder valve. There are several standards for neck threads, these include:

Taper thread (17E), with a 12% taper right hand thread, standard Whitworth 55° form with a pitch of 14 threads per inch and pitch diameter at the top thread of the cylinder of 18.036mm. These connections are sealed using thread tape and torqued to between 120 and 150 N.m on steel cylinders, and 75 to 140 N.m on aluminium cylinders

Parallel threads are made to several standards:

M25x2 parallel thread, which is sealed by an O-ring and torqued to 100 to 130 N.m on steel, and 95 to 130 N.m on aluminium cylinders
M18x1.5 parallel thread, which is sealed by an O-ring, and torqued to 100 to 130 N.m on steel cylinders, and 85 to 100 N.m on aluminium cylinders
3/4″x14 BSP parallel thread. This has a 55° Whitworth thread form, a pitch diameter of 25.279mm and a pitch of 14 threads per inch (1,814mm)
3/4″x14 NGS (NPSM)parallel thread, sealed by an O-ring, torqued to 40 to 50 ft.lbf on aluminium cylinders This has a 60° thread form, a pitch diameter of 0.9820″ to 0.9873″, and a pitch of 14 threads per inch.
3/4″x16 UNF, sealed by an O-ring, torqued to 40 to 50 ft.lbf on aluminium cylinders

The 3/4″NGS and 3/4″BSP are very similar, whaving the same pitch and a pitch diameter that only differs by about 0.2mm, but they are not compatible, as the thread forms are different.

All taper thread valves are sealed using an O-ring at top of the neck thread which seals in a chamfer or step in the cylinder neck and against the flange of the valve.

The shoulder of the cylinder carries stamp markings providing required information about the cylinder

Stamp markings on an American manufacture aluminum 40 cu.ft. 3000 psi cylinder

Stamp markings on an American manufacture aluminum 80 cu.ft. 3000 psi cylinder

Stamp markings on an Italian manufacture steel 6 l 300 bar cylinder

The cylinder valve

the pillar valve or cylinder valve is the point at which the pressure vessel connects to the diving regulator. The purpose of the pillar valve is to control gas flow to and from the pressure vessel and to form a seal with the regulator. Some countries require that the pillar valve includes a burst disk, a type of pressure ‘fuse’, that will fail before the pressure vessel fails in the event of overpressurization.

a rubber o-ring forms a seal between the metal of the pillar valve and the metal of the diving regulator. Fluoroelastomer (i.e. “viton”) o-rings are used with cylinders storing oxygen-rich gas mixtures to reduce the risk of fire.

Types of cylinder valve

Cylinder valves are classified by three basic aspects: The connection with the cylinder, the connection to the regulator, and other distinguishing features.

Cylinder thread variations

Cylinder threads are in two basic configurations: Taper thread and parallel thread. The thread specification must match the neck thread of the cylinder.

Draeger 300 bar taper thread DIN cylinder valve

A 232 bar DIN connection cylinder valve with parallel thread cylinder connection

Connection to the regulator

There are three types of cylinder valve in general use for Scuba cylinders containing air:

A-clamp or yoke – the connection on the regulator surrounds the valve pillar and presses the output O-ring of the pillar valve against the input seat of the regulator. The yoke is screwed down snug by hand (overtightening can make the yoke impossible to remove later without tools) and the seal is created by pressure when the valve is opened. This type is simple, cheap and very widely used worldwide. It has a maximum pressure rating of 232 bar and the weakest part of the seal, the O-ring, is not well protected from overpressurisation.
232 bar DIN (5-thread, G5/8) – the regulator screws into the cylinder valve trapping the O-ring securely. These are more reliable than A-clamps because the O-ring is well protected, but many countries do not use DIN fittings widely on compressors, or cylinders which have DIN fittings, so a European diver with a DIN system abroad in many places will need to take an adaptor.
300 bar DIN (7-thread, G5/8) – these are similar to 5-thread DIN fitting but are rated to 300 bar working pressures. The 300 bar pressures are common in European diving and in US cave diving, but their acceptance in U.S. sport diving has been hampered by the fact that United States Department of Transportation rules presently prohibit the transport of metal scuba cylinders on public roads with pressures above about 230 bar, even if the cylinders and air delivery systems have been rated for these pressures by the American agencies which oversee cylinder testing and equipment compatibility for SCUBA (Occupational Safety and Health Administration and Compressed Gas Association).

Pressure rating

DIN valves are produced in 200 bar and 300 bar pressure ratings. The number of threads and the detail configuration of the connections is designed to prevent incompatible combinations of filler attachment or regulator attachment with the cylinder valve.

Yoke valves are rated between 200 and 240 bar, and there does not appear to be any mechanical design detail preventing connection between any yoke fittings, though some older yoke clamps will not fit over the popular 232/240 bar combination DIN/yoke cylinder valve as the yoke is too narrow.

Adaptors are available to allow connection of DIN regulators to yoke cylinder valves (A-clamp or yoke adaptor), and to connect yoke regulators to DIN cylinder valves. (plug adaptors and block adaptors) Plug adaptors are rated for 232/240 bar. Block adaptors are generally rated for 200 bar.

There are also cylinder valves for Scuba cylinders containing gases other than air:

The new European Norm EN 144-3:2003 introduced a new type of valve, similar to existing 232 bar or 300 bar DIN valves, however, with a metric M 26×2 fitting on both the cylinder and the regulator. These are to be used for breathing gas with oxygen content above that normally found in natural air in the Earth’s atmosphere (i.e. 22–100%). From August 2008, these shall be required for all diving equipment used with nitrox or pure oxygen. The idea behind this new standard is to prevent a rich mixture being filled to a cylinder that is not oxygen clean. However even with use of the new system there still remains nothing except human procedural care to ensure that a cylinder with a new valve remains oxygen-clean – which is exactly how the current system works.
A male thread cylinder valve was supplied with some Dräger semi-closed circuit recreational rebreathers (Dräger Ray) for use with nitrox mixtures.

A yoke (A-clamp) to DIN adaptor allows connection of a DIN regulator to a Yoke cylinder valve

Din plug adaptor

DIN valve with plug adaptor for yoke attachment fitted

Other distinguishing features

Plain valves

The most commonly used cylinder valve type is the single outlet plain valve, sometimes known as a “K” valve, which allows connection of a single regulator, and has no reserve function. It simply opens to allow gas flow, or closes to shut it off. Several configurations are used, with options of DIN or A-clamp connection, and vertical or transverse spindle arrangements. The valve is operated by turning a knob, usually rubber or plastic, which affords a comfortable grip. Several turns are required to fully open the valves. Some DIN valves are convertible to A-clamp by use of an insert which is screwed into the opening.
Y and H cylinder valves have two outlets, each with its own valve, allowing two regulators to be connected to the cylinder. If one regulator “freeflows”, which is a common failure mode, or ices up, which can happen in water below about 5°C, its valve can be closed and the cylinder breathed from the regulator connected to the other valve. The difference between an H valve and a Y valve is that the Y valve body splits into two posts roughly 90° to each other and 45° from the vertical axis, looking like a Y, while an H valve is usually assembled from a valve designed as part of a manifold system with an additional valve post connected to the manifold socket, with the valve posts parallel and vertical, which looks a bit like an H. Y-valves are aso known as “slingshot valves” due to their appearance.

Reserve valves

Reserve lever or “J-valve” (obsolescent). Until the 1970s, when submersible pressure gauges on regulators came into common use, diving cylinders often used a mechanical reserve mechanism to indicate to the diver that the cylinder was nearly empty. The gas supply was automatically cut-off when the gas pressure reached the reserve pressure. To release the reserve, the diver pulled down on a rod that ran along the side of the cylinder and which activated a lever on the valve. The diver would then finish the dive before the reserve (typically 300 pounds per square inch (21 bar)) was consumed. On occasion, divers would inadvertently trigger the mechanism while donning gear or performing a movement underwater and, not realizing that the reserve had already been accessed, could find themselves out of air at depth with no warning whatsoever. The J-valve got its name from being item number J in one of the first scuba equipment manufacturer catalogs. The standard non-reserve yoke valve at the time was item K, and is often still referred to as a K-valve. J-valves are still occasionally used by professional divers in zero visibility, where the SPG can not be read.
Less common in the 1950s thru 1970s was an R-valve which was equipped with a restriction that caused breathing to become difficult as the cylinder neared exhaustion, but that would allow less restricted breathing if the diver began to ascend and the ambient water pressure lessened, providing a larger pressure differential over the orifice. It was never particularly popular because, were it necessary for the diver to descend (as is often necessary in cave and wreck diving, breathing would become progressively more difficult as the diver went deeper, eventually becoming impossible until the diver could begin his or her ascent.

A “J” valve from c.1960

Draeger taper thread cylinder valve with reserve lever

“H”-valve with DIN connections

Draeger 200 bar cylinder valves with manifold and reserve lever

Draeger 200 bar cylinder manifold

Left side cylinder valve for barrel seal manifold with blanking plug and DIN connection

Accessories

Additional components for convenience, protection or other functions, not directly required for the function as a pressure vessel.

Manifolds

A cylinder manifold is a tube which connects two cylinders together so that the contents of both can be supplied to one or more regulators. There are three commonly used configurations of manifold:

The earliest type is a tube with a connector on each end which is attached to the cylinder valve outlet, and an outlet connection in the middle, to which the regulator is attached. A variation on this pattern includes a reserve valve at the outlet connector. The cylinders are isolated from the manifold when closed, and the manifold can be attached or disconnected while the cylinders are pressurised.
More recently, manifolds have become available which connect the cylinders on the cylinder side of the valve, leaving the outlet connection of the cylinder valve available for connection of a regulator. This means that the connection cannot be made or broken while the cylinders are pressurised, as there is no valve to isolate the manifold from the interior of the cylinder. This apparent inconvenience allows a regulator to be connected to each cylinder, and isolated from the internal pressure independently, which allows a malfunctioning regulator on one cylinder to be isolated while still allowing the regulator on the other cylinder access to all the gas in both cylinders.
These manifolds may be plain or may include an isolation valve in the manifold, which allows the contents of the cylinders to be isolated from each other. This allows the contents of one cylinder to be isolated and secured for the diver if a leak at the cylinder neck thread, manifold connection, or burst disk on the other cylinder causes its contents to be lost.

A relatively uncommon manifold system is a connection which screws directly into the neck threads of both cylinders, and has a single valve to release gas to a connector for a regulator. These manifolds can include a reserve valve, either in the main valve or at one cylinder. This system is mainly of historical interest.

Cylinder bands

Cylinder bands are straps, usually of stainless steel, which are used to clamp two cylinders together as a twin set. The cylinders may be manifolded or independent. It is usual to use a cylinder band near the top of the cylinder, just below the shoulders, and one lower down. The standard distance between centrelines for bolting to a backplate is 11 inches (280 mm).

Cylinder boot

A cylinder boot is a hard rubber or plastic cover which fits over the base of a diving cylinder to protect the paint from abrasion and impact, to protect the surface the cylinder stands on from impact with the cylinder, and in the case of round bottomed cylinders, to allow the cylinder to stand upright on its base.

Cylinder net

A cylinder net is a tubular net which is stretched over a cylinder and tied on at top and bottom. The function is to protect the paintwork from scratching, and on booted cylinders it also helps drain the surface between the boot and cylinder, which reduces corrosion problems under the boot. Mesh size is usually about 6 millimetres (0.24 in). Some divers will not use boots or nets as they can snag more easily than a bare cylinder and constitute an entrapment hazard in some environments such as caves and the interior of wrecks.

Cylinder handle

A cylinder handle may be fitted, usually clamped to the neck, to conveniently carry the cylinder. This can also increase the risk of snagging in an enclosed environment.

A 15 litre, 232 bar cylinder with “Yoke” valve and cylinder handle

A 12 litre, 232 bar cylinder with DIN valve. The colour coding is the old UK standard for air prior to 2006

Face sealed isolation manifold on twin 12 l steel cylinders. The plastic discs are records of the latest internal inspection

Twinned cylinders showing cylinder boots and lower band

Cylinder capacity

12 litre and 3 litre steel diving cylinders: Typical Primary and Pony sizes

There are two commonly used conventions for describing the capacity of a diving cylinder. One is based on the internal volume of the cylinder. The other is based on nominal volume of gas stored.

Internal volume

The internal volume is commonly quoted in most countries. It can be measured easily by filling the cylinder with fresh water. This has resulted in the term ‘water capacity’ (WC) which is often marked on the cylinder shoulder. It’s almost always expressed as a volume but sometimes as weight of the water. Fresh water has a density close to one kilogram per litre so the numerical values will be similar.

The usual units are:

Volume in litres
Weight in kilograms
Pressure in bar.

Nominal volume of gas stored

The nominal volume of gas stored is commonly quoted in the USA. It’s a measure of the volume of gas that can be released from the cylinder at atmospheric pressure. Terms used for the volume include ‘free gas’ or ‘free gas equivalent’. It depends on the internal volume and the working pressure of a cylinder. If the working pressure is higher, the cylinder will store more gas for the same volume.

The working pressure is not necessarily the same as the actual pressure used. Some cylinders are permitted to exceed the nominal working pressure by 10% and this is indicated by a ‘+’ symbol. This extra pressure allowance is dependant on the cylinder passing the appropriate periodical hydrostatic test and is not generally valid for US cylinders exported to countries with differing standards.

For example, common Al80 cylinder is an aluminum cylinder which has a nominal ‘free gas’ volume of 80 cubic feet (2,300 L) when pressurised to 3,000 pounds per square inch (210 bar). It has an internal volume of 10.94 litres (0.386 cu ft).

Applications and configurations of diving cylinders

Technical diver with decompression gases in side mounted stage cylinders.

Divers may carry one cylinder or multiples, depending on the requirements of the dive. Where diving takes place in low risk areas, where the diver may safely make a free ascent, or where a buddy is available to provide an alternative air supply in an emergency, recreational divers usually carry only one cylinder. An example of this type is coral reef diving where it is possible to do an interesting dive without going deep or needing decompression. Where diving risks are higher, for example where the visibility is low or when recreational divers do deeper or decompression diving, divers routinely carry more than one gas source. An example of this type is north European diving where the temperature is often less than 15 °C (60 °F) and visibility less than 10 m (33 ft) and many interesting dive sites are shipwrecks in deeper water on the sea bed.

Diving cylinders may serve different purposes. One or two cylinders may be used as a primary breathing source which is intended to be breathed from for most of the dive. A smaller cylinder carried in addition to a larger cylinder is called a “pony bottle”. A cylinder to be used purely as an independent safety reserve is called a “bailout bottle”. A pony bottle is commonly used as a bailout bottle, but this would depend on the time required to surface.

Divers doing technical diving often carry different gases, each in a separate cylinder, for each phase of the dive:

“travel gas” is used during the descent and ascent. It is typically air or nitrox with an oxygen content between 21% and 40%. Travel gas is needed when the bottom gas is hypoxic and therefore is unsafe to breathe in shallow water.
“bottom gas” is only breathed at depth. It is typically a helium-based gas which is low in oxygen (below 21%) or hypoxic (below 17%).
“deco gas” is used at the decompression stops and is generally a nitrox with a high oxygen content, or pure oxygen, to accelerate decompression.
a “stage” is a cylinder holding reserve, travel or deco gas. They are usually carried “side slung”, clipped on either side of the diver to the harness of the backplate and wing or buoyancy compensator, rather than on the back. Commonly divers use aluminium stage cylinders because they are nearly neutrally buoyant in water and can be removed underwater with less effect on the diver’s overall buoyancy.

Rebreathers may use internal cylinders:

oxygen rebreathers have an oxygen cylinder
semi-closed circuit rebreathers have a cylinder which usually contains nitrox or a helium based gas.
closed circuit rebreathers have an oxygen cylinder and a “diluent” cylinder, which contains air, nitrox or a helium based gas

Rebreathers may also be supplied from “off-board” cylinders, which are not permanently plumbed into the rebreather, but connected to it by a flexible hose and coupling and usually carried side slung. Rebreather divers also often carry a bailout cylinder if the internal diluent cylinder is too small for safe use for bailout.

For safety, divers sometimes carry an additional independent scuba cylinder with its own regulator to mitigate out-of-air emergencies should the primary breathing gas supply fail. For much common recreational diving where a controlled emergency swimming ascent is acceptably safe, this extra equipment is not needed or used. This extra cylinder is known as a bail-out cylinder, and may be carried in several ways, and can be any size that can hold enough gas to get the diver safely back to the surface.

Open-circuit

For open-circuit divers, there are several options for the combined cylinder and regulator system:

Single cylinder or single aqualung: consists of a single large cylinder with one first-stage regulator, and usually two secondary regulator/mouthpieces. This configuration is simple and cheap but it is only a single system: it has no redundancy in case of failure. If the cylinder or first-stage regulator fails, the diver is totally out of air and faces an emergency. All training agencies train divers to rely on a buddy to assist them in this situation. The skill of gas sharing is required at the most basic scuba course. This equipment configuration, although common with entry-level divers and for most sport diving, is not recommended for any dive that is deeper than 30 m (100 ft) or where decompression stops are needed, or where there is an overhead environment (wreck diving, cave diving, or ice diving). Generally, these conditions, because they prevent immediate emergency ascent, may define technical diving.

Single cylinder with dual regulators: consists of a single large cylinder with two first-stage regulators, each with a second stage regulator/mouthpiece. This system is used for recreational diving where cold water makes redundancy required. It is common in continental Europe, especially Germany. The advantage is that a regulator failure can be solved underwater to bring the dive to a controlled conclusion without buddy breathing or gas sharing. However, it is hard to reach the valves, so there is some reliance on the dive buddy to help close the valves of the free-flowing regulator quickly.

Main cylinder plus a small independent cylinder: this configuration uses a larger, main cylinder along with an independent smaller cylinder, often called a “pony”. The diver has two independent systems, but the total ‘breathing system’ is now heavier, more expensive to buy and maintain.

The pony is typically a 2 to 5 litre cylinder. Its capacity determines the depth of dive and decompression duration for which it provides protection. Ponies are generally fixed to the diver’s buoyancy compensator (BC) or main cylinder behind the diver’s back. They can also be clipped to the BC at the diver’s side or chest. Ponies provided an acceptable emergency supply but are only useful if the diver trains to bail out, i.e. to use one.
Another type of separate small air source is a hand-held cylinder filled with about 85 litres (0.279 ft) of free air with a diving regulator directly attached, such as the Spare Air. This source provides only a few breaths of gas at depth and is mainly suitable as a shallow water bailout.

Independent twin set /doubles: this consists of two independent cylinders and two regulators. This system is heavier, more expensive to buy and maintain and more expensive to fill. Also the diver must swap demand valves during dive to preserve a safety reserve of air in each cylinder. If this is not done, then should a cylinder fail the diver may end up having no reserve. Independent twin sets do not work well with air-integrated computers – as they usually only monitor one cylinder. Many divers feel the complexity of switching regulators periodically to ensure both cylinders are evenly used is offset by the redundancy of two entirely separate breathing supplies. These will normally be mounted as a twin set on the diver’s back, but alternatively can be carried in a sidemount configuration where penetration of wrecks or caves requires it.

Manifolded twin set /doubles with a single regulator: two cylinders are joined at their pillar valves with a manifold but only one regulator is attached to the system. This makes it simple and cheap but means there is no redundant breathing system, only a double gas supply.

Manifolded twin set /doubles with two regulators: consist of two cylinders with their pillar valves joined with a manifold, with a valve that can isolate the two pillar valves. In the event of a problem with one cylinder the diver may close the isolator valve to preserve gas in the cylinder which has not failed. The pros of this configuration include a large gas supply, no requirement to change regulators underwater, automatic gas supply management, and in most failure situations the diver may close a failed valve or isolate a cylinder in order to leave himself with an emergency supply. On the down side the manifold is another potential point of failure, and there is a danger of losing all air if the manifold valve cannot be closed when a problem occurs. This configuration of cylinders is often used in Technical diving.

Sling bottles/cylinders: are a configuration of independent cylinder used for technical diving. They are independent cylinders with their own regulators and are carried clipped to the harness at the side of the diver. Their purpose may be to carry either stage, travel, decompression, or bailout gas while the back mounted cylinder(s) carry bottom gas. Stage cylinders carry gas to extend bottom time, travel gas is used to reach a depth where bottom gas may be safely used if it is hypoxic at the surface, and decompression gas is gas intended to be used during decompression to accelerate the elimination of inert gases. Bailout gas is an emergency supply intended to be used to surface if the main gas supply is lost.

Side mount cylinders: are sling cylinders mounted at the diver’s side which carry bottom gas when the diver does not carry back mount cylinders. They may be used in conjunction with other sling cylinders where necessary.

Closed-circuit

Diving cylinders are used in closed-circuit diving in two roles:

As part of the rebreather itself. The rebreather must have at least one source of fresh gas stored in a cylinder; many have two and some have more cylinders. Due to the lower gas consumption of rebreathers, these cylinders typically are smaller than those used for equivalent open-circuit dives. See the main article: rebreather.

In a bail out system: rebreather divers often carry one or more redundant gas sources should the rebreather fail:

Open-circuit: a simple diving cylinder and regulator. The number of open-circuit bail outs, their capacity and the breathing gases they contain depend on the depth and decompression needs of the dive. So on a deep, technical rebreather dive, the diver will need a bail out “bottom” gas and a bail out “decompression” gas for use. On such a dive, it is the capacity and duration of the bail out that limits the depth and duration of the dive – not the capacity of the rebreather.
Closed-circuit: a rebreather containing a diving cylinder and regulator. Using another rebreather as a bail out is possible but uncommon. Although the long duration of rebreathers seems compelling for a bail out, rebreathers are relatively bulky, complex, vulnerable to damage and require more time to start breathing from, than easy-to-use, instantly available, robust and reliable open-circuit equipment.

Long 9.2 litre aluminium cylinder rigged for sling mounting

15 litre, 232 bar, A clamp single cylinder open circuit breathing set

7 litre, 232 bar, DIN pillar valve independent twin set. The left cylinder shows manufacturer markings. The right cylinder shows test stamps

Manifolded twin 12 litre, 232 bar breathing set with two A-clamp pillar valves and two regulators

Two 3 litre, 232 bar, DIN cylinders inside an Inspiration Diving Rebreather closed circuit breathing set.

Gas calculations

Breathing gas endurance

A commonly asked question is ‘what is the underwater duration of a particular cylinder?’

There are two parts to this problem:

The cylinder’s capacity to store gas

Two features of the cylinder determine its gas carrying capacity:

working gas pressure : this normally ranges between 200 and 300 bars (2,900 and 4,400 psi)
internal volume : this normally ranges between 3 litres and 18 litres

To calculate the quantity of gas:

Volume of gas at atmospheric pressure = (cylinder volume) x (cylinder pressure) / (atmospheric pressure)

So a 12 litre cylinder at 232 bar would hold almost 2,784 litres (98.3 cu ft) of air at atmospheric pressure.

In the US and in many diving resorts you might find aluminum cylinders with an internal capacity of 0.39 cubic feet (11 L) filled to 3,000 psi (210 bar); Taking air pressure as 14.7 psi, this gives 0.39 x 3000 / 14.7 = 80 ft³ These cylinders would be described by US convention as “80 cubic foot cylinders”, (the common “aluminum-80”) as the US normally refers to cylinder capacity as free-air equivalent at its working pressure, rather than the internal volume of the cylinder, which is the measure commonly used in metric countries.

Up to 200 bar the ideal gas law remains valid and the relationship between the pressure, size of the cylinder and gas contained in the cylinder is linear; at higher pressures there is proportionally less gas in the cylinder. A 3 litre, 300 bar cylinder can only carry up to 810 litres (29 cu ft) of atmospheric pressure gas and not the 900 litres expected from the ideal gas law.

Diver gas consumption

There are three factors at work here:

breathing rate or respiratory minute volume (RMV) of the diver. In normal conditions this will be between 10 and 25 litres per minute (L/min) for recreational divers who are not working hard. At times of extreme high work rate, breathing rates can rise to 95 L/min. In the UK, a working breathing rate of 40 litres per minute is used for commercial diving, whilst a figure of 50 litres per minute is used for emergencies. (The Association of Diving Contractors)
time
ambient pressure: the depth of the dive determines this. The ambient pressure at the surface is 1 bar (15 psi). For every 10 metres (33 ft) in salt water the diver descends, the pressure increases by 1 bar (15 psi). As a diver goes deeper, the breathing gas is delivered at a pressure equal to ambient water pressure. Thus, it requires twice as much mass of gas to fill the same volume (the diver’s lungs) at 10 metres (33 ft) as it does at the surface, and three times as much at 20 metres (66 ft). If a given cylinder consumed at a constant rate would last a diver one hour at the surface, it would last 30 minutes at 10 metres (33 ft), 20 minutes at 20 metres (66 ft) and just 15 minutes at 30 metres (98 ft).

To calculate the quantity of gas consumed:

gas consumed = breathing rate × time × ambient pressure

Thus, a diver with a breathing rate of 20 L/min will consume at 30 meters (4 bar) the equivalent of 80 L/min at 1 bar (e.g. at the surface). If this diver only had a 10 litre 200 bar cylinder to breathe from, the gas in the cylinder would be exhausted after 2000/80 = 25 minutes.

Keeping this in mind, it is not hard to see why technical divers who do long deep dives require multiple cylinders or rebreathers.

Breathing time

For metric users:

Absolute maximum breathing time (BT) can be calculated as

BT = available air / rate of consumption

which, using the ideal gas law, is

BT = (available cylinder pressure × cylinder volume) / (rate of air consumption at surface) × (ambient pressure)

This may be written as

(1)

with

BT = Breathing Time (in minutes)

CP = Cylinder Pressure (in bars)

CS = Cylinder Size (in liters)

AP = Ambient Pressure (in bars)

BR = Breathing Rate (in liters per minute)

AP is deducted from CP, as the quantity of air represented by AP can in practice not be used for breathing by the diver as she needs it to overcome the pressure of the water (AP) when inhaling.

However, in normal diving usage, a reserve is always factored in. The reserve is a proportion of the cylinder pressure which a diver will not expect to use other than in case of emergency. The reserve may be a quarter or a third of the cylinder pressure or it may be a fixed pressure, common examples are 50 bar and 500 psi. The formula above is then modified to give the usable breathing time as

(2)

where RP is the reserve pressure.

Ambient pressure (AP) is the surrounding water pressure at a given depth and is made up of the sum of the water pressure and the air pressure at the surface. It is calculated as

(3)

+ atmospheric pressure

with

D = Depth (in meters)

g = Standard gravity (in meters per second squared)

ρ = Water Density (in kg per cube meter)

In practical terms, this formula can be approximated by

(4)

For example (using the first formula (1) for absolute maximum breathing time), a diver at a depth of 15 meters in water with an average density of 1020 kg / m³ (typical salt water), who breathes at a rate of 20 liters per minute, using a dive cylinder of 18 liters pressurized at 200 bars, can breathe for a period of 72 minutes before the cylinder and supply line pressure has fallen so low as to prevent her from inhaling. In most open circuit scuba systems this happens quite suddenly, from a normal breath to the next abnormal breath, a breath which typically cannot be fully drawn. (There is never any difficulty exhaling). In such circumstances there remains air under pressure in the cylinder, but the diver is unable to breathe it. Some of it can be breathed if the diver ascends, and even without ascent, in some systems a bit of air from the cylinder is available to inflate BCD devices even after it no longer has pressure enough to actuate the mouthpiece valve.

Using the same conditions and a reserve of 50 bar, the formula (2) for usable breathing time is worked thus:

Ambient pressure = water pressure + atmospheric pressure = 15/10 + 1 = 2.5 bar

Usable air = usable pressure * cylinder capacity = (200-50) * 18 = 2700 liters

Rate of consumption = surface air consumption * ambient pressure = 20 * 2.5 = 50 liters/min

Usable breathing time = 2700 liters / 50 liters/min = 54 min

This would give a dive time of 54 min at 15 m before reaching the reserve of 50 bar.

Reserves

It is strongly recommended that a portion of the usable gas of the cylinder be held aside as a safety reserve. The reserve is designed to provide gas for longer than planned decompression stops or to provide time to resolve underwater emergencies.

The size of the reserve depends upon the risks involved during the dive. A deep or decompression dive warrants a greater reserve than a shallow or a no stop dive. In recreational diving for example, it is recommended that the diver plans to surface with a reserve remaining in the cylinder of 500 psi, 50 bar or 25% of the initial capacity, depending of the teaching of the diver training organisation. This is because recreational divers practicing within “no-decompression” limits can normally make a direct ascent in an emergency. On technical dives where a direct ascent is either impossible (due to overhead obstructions) or dangerous (due to the requirement to make decompression stops), divers plan larger margins of safety using the rule of thirds: one third of the gas supply is planned for the outward journey, one third is for the return journey and one third is a safety reserve.

Some training agencies teach the concept of minimum gas and provide a simple calculation that allows a diver to work out an acceptable reserve to get two divers in an emergency to the surface. See DIR diving for more information.

Weight of gas consumed

The loss of the weight of the gas taken from the cylinder makes the cylinder and diver more buoyant. This can be a problem if the diver is unable to remain neutrally buoyant towards the end of the dive because most of the gas has been breathed from the cylinder.

Table showing the buoyancy of diving cylinders in water when empty and full of air.

Assumes 1 litre of air at atmospheric pressure and 10°C weighs 1.25g.

Cylinder			Air		Weight on land		Buoyancy
Material	Volume	Pressure	Volume	Weight	Empty	Full	Empty	Full
	(litre)	(bar)	(litre)	(kg)	(kg)	(kg)	(kg)	(kg)
Steel	12	200	2400	3.0	16.0	19.0	-1.2	-4.3
	15	200	3000	3.8	20.0	23.8	-1.4	-5.2
	16 (XS 130)	230	3680	4.7	19.5	23.9	-0.9	-5.3
	2×7	200	2800	3.5	19.5	23.0	-2.0	-5.6
	8	300	2400	3.0	13.0	16.0	-3.5	-6.5
	10	300	3000	3.8	17.0	20.8	-4.0	-7.8
	2×4	300	2400	3.0	15.0	18.0	-4.0	-7.0
	2×6	300	3600	4.6	21.0	25.6	-5.0	-9.6
Aluminium	9 (AL 63)	203	1826	2.3	12.2	13.5	+1.8	-0.5
	11 (AL 80)	203	2247	2.8	14.4	17.2	+1.8	-1.1
	13 (AL100)	203	2584	3.2	17.1	20.3	+1.4	-1.7

Filling cylinders

Diving cylinders should only be filled with air from diving air compressors or with other breathing gases using gas blending techniques. Both these services should be provided by reliable suppliers such as dive shops. Breathing industrial compressed gases can be lethal because the high pressure increases the effect of any impurities in them.

Special precautions need to be taken with gases other than air:

oxygen in high concentrations is a major cause of fire and rust.
oxygen should be very carefully transferred from one cylinder to another and only ever stored in containers that are certified and labeled for oxygen use.
gas mixtures containing proportions of oxygen other than 21% could be extremely dangerous to divers who are unaware of the proportion of oxygen in them. All cylinders should be labeled with their composition.
cylinders containing a high oxygen content must be cleaned for the use of oxygen and lubricated with oxygen service grease to reduce the chance of combustion.

Contaminated air at depth can be fatal. Common contaminants are: carbon monoxide a by-product of combustion, carbon dioxide a product of metabolism, oil and lubricants from the compressor.

Keeping the cylinder slightly pressurized at all times reduces the possibility of contaminating the inside of the cylinder with corrosive agents, such as sea water, or toxic material, such as oils, poisonous gases, fungi or bacteria.

The blast caused by a sudden release of the gas pressure inside a diving cylinder makes them very dangerous if mismanaged. The greatest risk of explosion exists at filling time and comes from thinning of the walls of the pressure vessel due to corrosion. Another cause of failure is damage or corrosion of the threads and neck of the cylinder where the pillar valve is screwed in. Aluminium cylinders have been observed occasionally to fail explosively, fragmenting the cylinder wall. Steel cylinders usually remain mostly intact, and tend to fail at the neck.

Manufacture and testing

Most countries require diving cylinders to be checked on a regular basis, see gas cylinder. This usually consists of an internal visual inspection and a hydrostatic test.

In the United States, a visual inspection is NOT required by the USA DOT every year though they do require a hydrostatic every five years. The visual inspection requirement is a diving industry standard based on observations made during a review by the National Underwater Accident Data Center.
In European Union countries a visual inspection is required every 2.5 years, and a hydrostatic every five years.
In Norway a hydrostatic (including a visual inspection) is required 3 years after production date, then every 2 years.
Legislation in Australia requires that cylinders are hydrostatically tested every twelve months, regardless.
In South Africa a hydrostatic test is required every 4 years, and visual inspection every year. Eddy current testing of neck threads must be done according to the manufacturer’s recommendations.

A hydrostatic test involves pressurising the cylinder to its test pressure (often 5/3 or 3/2 of the working pressure) and measuring its volume before and after the test. A permanent increase in volume above the tolerated level means the cylinder fails the test and is permanently removed from service.

When a cylinder is manufactured, its specification, including Working Pressure, Test Pressure, Data of Manufacture, Capacity and Weight are stamped on the cylinder.

After a cylinder passes the test, the test date, (or the test expiry date in some countries such as Germany), is punched into the shoulder of the cylinder for easy verification at fill time. Note: this is a European requirement. There is an international standard for the stamp format

Most compressor operators check these details before filling the cylinder and may refuse to fill non-standard or out-of-test cylinders. Note: this is a European requirement, a requirement of the USA DOT, and a South African requirement.

Safety

Before any cylinder is filled, verification of testing dates and a visual examination for external damage and corrosion are required by law in some jurisdictions, and are prudent even if not legally required at other places. In the United States, scuba tanks must be hydro-tested every five years and visually inspected every year. Test dates can be checked by looking at the visual inspection sticker and the hydro-test date is stamped on top of the cylinder.

Before use the user should verify the contents of the cylinder and check the function of the cylinder valve. Pressure and gas mixture are critical information for the diver, and the valve should open freely without sticking or leaks from the spindle seals. Sniffing air bled from a cylinder may also reveal unpleasant surprises better left on land than discovered in the water.

Cylinders should not be left standing unattended unless secured so that they can not fall in reasonable foreseeable circumstances as an impact could damage the cylinder valve mechanism, and cocievably fracture the valve at the neck threads. This is more likely with taper thread valves, and when it happens the energy of the compressed gas is released within a second, and can accelerate the cylinder to speeds which can causes severe injury or damage to the surroundings.

A neatly assembled setup, with regulators, gauges, and delicate computers butterflied inside the BCD, or clipped where they will not be walked on, and stowed under the boat bench or secured to a rack, is the practice of a competent diver.

As the scuba set is a life support system, one should not touch a fellow diver’s gear, even to move it, without their knowledge and approval.

Full cylinders should not be exposed to temperatures above 65°C and cylinders should not be filled to pressures greater than the developed pressure appropriate to the certified working pressure of the cylinder except by a test station performing a hydrostatic test.

Cylinders should be clearly labelled with their current contents. A generic “Nitrox” or “Trimix” label will alert the user that the contents may not be air, and must be analysed before use. In some parts of the world a label is required specifically indicating that the contents are air, and in other places a colour code without additional labels indicates by default that the contents are air.

Cases of lateral epicondylitis are also reported from the handling of diving cylinders.

Gas cylinder colour coding and labeling

A contents label for oxygen usage (UK)

European Union

In the European Union gas cylinders may be colour coded according to EN 1098-3. The “shoulder” is the top of the cylinder close to the pillar valve. For mixed gases, the colours can be either bands or “quarters”.

Air has either a white (RAL 9010) top and black (RAL 9005) band on the shoulder, or white (RAL 9010) and black (RAL 9005) “quartered” shoulders.
Heliox has either a white (RAL 9010) top and brown (RAL 8008) band on the shoulder, or white (RAL 9010) and brown (RAL 8008) “quartered” shoulders.
Nitrox, like Air, has either a white (RAL 9010) top and black (RAL 9005) band on the shoulder, or white (RAL 9010) and black (RAL 9005) “quartered” shoulders.
Pure oxygen has a white shoulder (RAL 9010).
Pure helium has a brown shoulder (RAL 9008).
Trimix has a white, black and brown segmented shoulder.

Note: As of the end of 2006, the quartered parts is obsolete, and new cylinders are now with the band, and the old system is repainted.

In the European Union breathing gas cylinders must be labeled with their contents. The label should state the type of breathing gas contained by the cylinder.

South Africa

Scuba cylinders are required to comply with the colours and markings specified in SANS 10019:2006.

Cylinder colour is Golden yellow with a French grey shoulder.
Cylinders containing gases other than air or medical oxygen must have a transparent adhesive label stuck on below the shoulder with the word NITROX or TRIMIX in green and the composition of the gas listed.
Cylinders containing medical oxygen must be black with a white shoulder.

In many recreational diving settings where air and nitrox are the widely used gases, nitrox cylinders are colour-coded with a green stripe on yellow bottom. The normal colour of aluminium diving cylinders is their natural silver. Steel diving cylinders are often painted, to reduce corrosion, mainly yellow or white to increase visibility. In some industrial cylinder identification colour tables, yellow shoulders means chlorine and more generally within Europe it refers to cylinders with Toxic and/or Corrosive contents; but this is of no significance in SCUBA since gas fittings would not be compatible.

Cylinders that are subject to gas blending with pure oxygen also need an “oxygen service certificate” label indicating they have been prepared for use in an oxygen-rich environment.

<!– amazon_ad_tag="pacicorn-20"; amazon_ad_width="468"; amazon_ad_height="60"; amazon_color_background="000000"; amazon_color_border="43EDD1"; amazon_color_logo="010320"; amazon_color_text="06F90E"; amazon_color_link="F9081F"; amazon_ad_logo="hide"; amazon_ad_link_target="new"; amazon_ad_border="hide"; amazon_ad_title="Diving Equipment"; //–>

Source :

http://en.wikipedia.org/wiki/Diving_cylinder

SWIMFINS

1 February 2013 04.24 / Leave a comment

Swimfins, swim fins, fins or flippers are worn on the foot or leg and made from finlike rubber or plastic, to aid movement through the water in water sports activities such as swimming, bodyboarding, bodysurfing, kneeboarding, riverboarding, and various types of underwater diving.

Scuba divers use fins to move through water efficiently, as human feet being very small provide relatively poor thrust, especially when the diver is carrying equipment that increases hydrodynamic drag. Very long fins and monofins are used by freedivers as a means of underwater propulsion that does not require high frequency leg movement.

History

Early inventors, including Leonardo da Vinci and Giovanni Alfonso Borelli, toyed with the concept of swimfins.

Benjamin Franklin made a pair of early swimfins when he was a young boy living in Boston, Massachusetts near the Charles River; they were two thin pieces of wood, about the shape of an art palette, which allowed him to move faster than he usually did in the water.

Modern swimfins are an invention from the Frenchman Louis de Corlieu, capitaine de corvette (Lieutenant Commander) in the French Navy. In 1914 De Corlieu made a practical demonstration of his first prototype for a group of navy officers, Yves le Prieur among them who, years later in 1926, invented an early model of scuba set. De Corlieu left the French Navy in 1924 to fully devote himself to his invention. In April 1933 he registered a patent (number 767013, which in addition to two fins for the feet included two spoon-shaped fins for the hands) and called this equipment propulseurs de natation et de sauvetage (which can be translated literally as “swimming and rescue propulsion device”).

After floundering for years, even producing his fins in his own flat in Paris, De Corlieu finally started mass production of his invention in France in 1939. The same year he issued a licence to Owen P. Churchill for mass production in the United States. To sell his fins in the USA Owen Churchill changed the French De Corlieu’s name (propulseurs) to “swimfins”, which is still the common English name. Churchill presented his fins to the US Navy, which decided to acquire them for its Underwater Demolition Team (UDT). American UDT and British COPP frogmen (COPP: Combined Operations Pilotage Parties) used the “Churchill fins” during all prior underwater deminings, thus enabling in 1944 the Normandy landings. During the years after World War II had ended, De Corlieu spent time and efforts struggling in civil procedures, suing others for patent infringement.

In Britain, Dunlop made frogman’s fins for World War II, but after the war saw no market for them in peacetime, and, after the first supply of war-surplus frogman’s kit was used up, the British public had no access to swimfins (except for home-made attempts such as gluing marine plywood to plimsolls), until Oscar Gugen began importing swimfins and swimming goggles from France.

In 1946 Lillywhites imported about 1100 pairs of swimfins; they all sold in under 3 months.

In 1948 Luigi Ferraro, collaborating with the Italian diving equipment company Cressi-sub, designed the first full-foot fin, the Rondine, named after the Italian word for swallow. A distinctive feature of Cressi’s continuing Rondine full-foot fin line is the embossed outline of the bird on the foot pockets and the blades.

In 1949 Ivor Howitt or a friend of his mailed to the Dunlop Rubber Company for swimfins; Dunlop answered that they had no plans to make swimfins and saw no use for them in peacetime. Howitt made his own swimfins with innertube rubber stretched across a frame of stiff rubber tube.

Types

Cressi-sub vented paddle fin

Beuchat Closed-heel Jetfins

Beuchat Open-heel adjustable Jetfins

Long bladed open heel fin with moderately stiff plastic blade

A freediver using a monofin

Types of fins have evolved to address the unique requirements of each community using them. Scuba divers, in particular, need large wide fins to overcome the water resistance caused by their diving equipment; snorkelers need lightweight flexible fins; ocean swimmers, bodysurfers, and lifeguards favor smaller designs that stay on their feet when moving through large surf and that make walking on the beach less awkward. In general there are two main groups of fins; full foot and open heel. Full foot fins fit like a shoe, and are designed to be worn over bare feet. If a larger size is chosen, however, full-foot fins can also be worn over socks and thin-soled booties. They are commonly used for surface swimming, and are in non adjustable sizes. Open heel have a foot pocket with an open heel area, and the fin is held to the foot by springs or straps. They can be worn over boots, and are common in diving. Many companies design fins with the same fin architecture but a choice of heel type. Other, more specific design trends are listed below.

Common types

Paddle fins

These are the most basic fins; a pair of simple stiff plastic, composite, or rubber blades that work as extensions of the feet while kicking. Some paddle fins have channels and grooves to improve power and efficiency though it has been shown that the desired effect does not occur. Paddle fins are widely believed to be the most versatile and have improved swimming economy in men. Tests in women showed a more flexible fin to be more economical, most likely due to lower leg power.

Vented fins

Vented fins were first designed in 1964 by Georges Beuchat and commercialised as Jetfins. The Jetfin tradename and design were sold to Scubapro in the 1970s. This style of fin is strongly favored by technical divers that use a frog kick allowing a high degree of control but sacrifice speed for low oxygen consumption. Vented fins are generally stiff paddle fins that have vents at the base of the foot pocket. The vents are intended to allow for the passage of water during the recovery stroke, but prevent passage during power strokes due to the blade angle, attempting to lessen effort during recovery and improve kick efficiency. A review and study by Pendergast et al in 2003 concluded that vented fins did not improve economy, implying that water does not pass through the vents. There is a risk of objects catching in the vents.

Split fins

Some swimfins have the end of the blade split. The manufacturers claim that split fins operate similarly to a propeller, by creating lift forces to move the swimmer forwards. The claim is that water flowing toward the center of the fin’s “paddle” portion also gains speed as it focuses, creating a “suction” force. A 2003 study by Pendergast et al called this into question by showing that there was no significant change in performance for a particular split fin design when the split was taped over. The technology used in most commercial split fin designs is patented by the industrial design firm Nature’s Wing, and is used under license.

Freediving fins

These are very similar to paddle fins, except they are far longer, and designed to work with slow stiff-legged kicks that claims to conserve energy. The vast majority are made in the “full-foot” design with very rigid footpockets, which serves to reduce weight and maximize power transfer from the leg into the fin. Freediving fins are commonly made of plastic, but are also often made from materials such as fiberglass and carbon fiber.

Monofins

A monofin is typically used in finswimming and free-diving. It consists of a single surface attached to footpockets for both the free-diver’s feet. Monofins can be made of glass fibre or carbon fibre. The diver’s muscle power and swimming style, and the type of activity the monofin is used for, determine the choice of size, stiffness, and materials.

Less usual types

Open and closed heel fins are predominant, but there is a range of fins that have specialised blade attachment architecture. These include (these names are tradenames):

Delfins

The Mor-Fin Corporation produces “delfins”, which are swimfins that end short and to the end is attached a shape like a fish’s forked homocercal caudal fin. The entire fin is based on the anatomy of various marine animals.

Force fins

“Force Fin” is the trademark for fins designed, developed, manufactured and distributed by Bob Evans Designs, Inc. They are distinguished by an open foot pocket, that encloses only the instep, leaving the toes free so the foot can flex.

Shinfins

These fins are attached to the ankle and rest against the (anatomically) upper side of the foot. The manufacturers claim this avoids leg cramps and reduces foot strain.

Flipfins

Flipfins in use by frogmen attacking in a harbor, for better mobility on land

Flipfins are an open-heel swimfin designed to allow easy walking on land. Its blade and foot part are separate: the blade hinges onto the foot part at each side, roughly on the level of the metatarsal heads, and when swimming is held in line by a clip on the front of the foot part. On land or when wading the blade can be unclipped and hinged vertically so it does not interfere with walking.

Breast stroke fins

Breast stroke fins are optimized for use with the breaststroke.

Swim fin strap attacment with simple rubber strap and wire buckle

Swim fin strap attachment with swivelling plastic buckle and clip

Aftermarket stainless steel spring fin strap attached with long D-shackles for security

Open heel fin with stainless steel spring strap with rubber padding

Training

Divers are initially taught to fin with legs straight, without excess bending of the knee, the action coming from the hips; a leg action with much knee bending like riding a bicycle is inefficient and is a common fault with divers who have not learned properly how to fin swim. Fins with differing characteristics (e.g. stiffness) may be preferred, depending on the application, and divers may have to learn a modified finning style to match.

The upper limit of a diver’s fin-kick thrust force using a stationary-swimming ergometer was shown to be 64 newtons (14 lbf). The maximum thrust averaged over 20 seconds against a strain gauge has been measured as high as 192 newtons (43 lbf). Resistive respiratory muscle training improves and maintains endurance fin swimming performance in divers.

MORE SWIMFINS

Source :

http://en.wikipedia.org/wiki/Swimfin

Swimfins

SNORKEL

31 January 2013 04.59 / Leave a comment

A swimmer’s snorkel is a tube typically about 30 centimeters long and with an inside diameter of between 1.5 and 2.5 centimeters, usually L- or J-shaped and fitted with a mouthpiece at the lower end, and constructed of rubber or plastic. It is used for breathing air from above the water surface when the wearer’s mouth and nose are submerged. The snorkel usually has a piece of rubber that attaches the snorkel to the outside of the strap of the diving mask. An older technique is pushing the snorkel between the mask-strap and the head, but this practice increases the chances the mask will leak.

Swimmers snorkel with splash guard on top, but with no sump valve.

The optimum design length of the snorkel tube is at most 40 centimetres (about 16 inches). A longer tube would not allow breathing when snorkelling deeper, since it would place the lungs in deeper water where the surrounding water pressure is higher. The lungs would then be unable to inflate when the snorkeler inhales, because the muscles that expand the lungs are not strong enough to operate against the higher pressure.

Snorkels also constitute respiratory dead space. When the user takes in a fresh breath, some of the previously exhaled air which remains in the snorkel is recycled into the lungs, reducing breathing efficiency and causing a build up of carbon dioxide in the blood, which can result in hypercapnia. The greater the volume of the tube, and the smaller the tidal volume of breathing, the more this problem is exacerbated. A smaller diameter tube reduces the dead volume, but also increases resistance to airflow and so increases the work of breathing. Occasional exhalation through the nose while snorkeling will reduce the build up of carbon dioxide, as well as helping to keep the mask clear of water.

Flooding and clearing

The most common type of snorkel is a simple tube that is allowed to flood when underwater. The snorkeler expels water from the snorkel either with a sharp exhalation on return to the surface (blast clearing) or by tilting the head back shortly before reaching the surface and exhaling until reaching or breaking the surface (displacement method) and facing forward again before inhaling the next breath. The displacement method expels water by displacing its presence in the snorkel with air; it is technique that takes practice but clears the snorkel with less effort, but only works when surfacing. Clearing splash water while at the surface requires blast clearing.

Snorkeler underwater, with snorkel’s sump valve apparent in foreground.

Some snorkels have a sump in the mouthpiece to allow a small volume of water to remain in the snorkel without being inhaled when the snorkeler breathes. Some also have a one-way output valve in the sump, which automatically drains the sump as it fills with water. A few snorkels have float-operated valves attached to the top end of the tube to keep water out when a wave passes, but these cause problems when diving as the snorkel must then be equalised, using part of the diver’s inhaled air supply. Some recent designs have a splash deflector on the top end that directs any water that splashes over the open tube to the outside of the tube, thereby keeping the tube relatively free from water.

A common problem with all assistive mechanical clearing mechanisms is their tendency to fail due to infrequent use, long periods of storage, and lack of maintenance, and also to fail due to environmental fouling.

Natural rubber slowly oxidizes and breaks down due to ultraviolet light exposure from the sun. It eventually loses its flexibility, becomes brittle and cracks. This causes one-way clearing valves to stick in the open or closed position, and float valves to leak water due to a failure of the valve seat to seal out water. It is unlikely that natural rubber is still used for any part of a snorkel. Silicone rubber is more resistant to degrading and so tends to have a longer service life. Application of a grease to the valve seats can aid in sealing, but this is mechanically washed away over time, and a heavy grease can cause valves to stick closed and trap grit, which will cause the valve to leak.

Environmental fouling can be caused by beach sand or loose floating plant or animal matter getting lodged in the valves and preventing proper opening or closing.

Although swimming with a snorkel is much easier than without, it is important that a novice swimmer also learns surface breathing and floating without a snorkel, in the event that these assistive clearing mechanisms fail. If either the sump clearing valve sticks open or the top float valve sticks closed, a snorkel is rendered useless as a breathing aid and the swimmer is forced to fall back on alternative surface breathing methods.

Some snorkels used to be made with small “ping pong” balls in a cage mounted to the open end of the tube to prevent water ingress, but these are no longer sold nor recommended to be used since they are considered hazardous to the snorkeler. Similarly, diving masks with a snorkel built into them are considered unsafe and obsolete.

The dive mask

Snorkelers normally wear the same kind of mask as those worn by scuba divers. By creating an airspace, the mask enables the snorkeler to see clearly underwater. All scuba diving masks consist of the lenses also known as a faceplate, a comfortable skirt, which also encloses the nose, and a head strap. There are different styles and shapes. These range from oval shaped models to lower internal volume masks and may be made from different materials; common choices are silicone and rubber.

The practice of snorkeling

Snorkeler with mask and snorkel.

Although donning a mask and snorkel and swimming in any body of water would technically constitute “snorkeling,” by and large it is generally accepted that a “snorkeler” would don such gear and practice such activity within the vicinity of a reef, wreck, or other submerged objects, either to observe aquatic organisms including fish, algae, etc. or to look at rock formations. Being non-competitive, snorkeling is considered more a leisure activity than a sport.

Snorkeling requires no special training, only the ability to swim and to breathe through the snorkel. However, for safety reasons, instruction and orientation from a fellow “experienced” snorkeler, tour guide, dive shop, or equipment-rental shop is recommended. Instruction generally covers equipment usage, basic safety, what to look for, and what to look out for, and conservation instructions (fragile organisms such as coral are easily damaged by divers and snorkelers). As with scuba-diving it is always recommended that one not snorkel alone, but rather with a “buddy”, a guide or a tour group.

Some commercial snorkeling locations require snorkelers to wear an inflatable vest, similar to a personal flotation device. They are usually bright yellow or orange and have a device that allows users to inflate or deflate the device to adjust their buoyancy. However these devices hinder and prevent a snorkeler from free diving to any depth. Especially in cooler water, a wetsuit of appropriate thickness and coverage may be worn; wetsuits do provide some buoyancy without as much resistance to submersion. In the tropics, snorkelers (especially those with pale skin) often wear a rashguard or a shirt and/or board shorts in order to help protect the skin of the back and upper legs against sunburn.

Experienced snorkelers often start to investigate amateur free-diving, which should be preceded by at least some training from a dive instructor or experienced free-diver.

Safety precautions

The greatest danger to snorkelers are inshore and leisure crafts such as jet skis, speed boats and the like. A snorkeler is often submerged in the water with only the tube visible above the surface. Since these crafts can ply the same areas snorkelers visit, the chance for accidental collisions exist. Sailboats and windsurfers are especially worrisome as their quiet propulsion systems indicates that a snorkeler may be unaware of their presence, unlike any motor-driven craft, as sound travels farther underwater. A snorkeler may surface underneath one and/or be struck by such vessels. Few places demarcate small craft areas from snorkelers, unlike for regular bathers who may have areas marked by buoys. Snorkelers may therefore choose to wear bright or highly reflective colors/outfits and/or to employ dive flags to utilize being spotted easily by boaters and others. On the other hand, some snorklers may use camouflage in order to surprise unsuspecting visitors.

Snorkelers’ backs can be exposed to the sun for extended periods and can burn badly (even if slightly submerged), without being noticed. Wearing appropriate covering such as a “rash guard” (in warmer waters), a t-shirt, a wetsuit and/or sunblock will mitigate the risk of sunburn.

Dehydration is another concern. Hydrating well before going in the water is recommended, especially if one intends to snorkel for several hours. Proper hydration also prevents cramps.

Snorkelers can experience hyperventilation, which can lead in turn to “shallow water blackout″; snorkeling with a buddy (and being aware of the buddy’s condition at all times) can help avoid this situation.

When snorkeling on or near coral reefs, care must be exercised to avoid contact with the delicate (and sometimes sharp and/or stinging) coral and its poisonous inhabitants, usually via protective gloves and by being careful of one’s environment. Booties and surf shoes are especially useful as they allow trekking over reefs exposed by low tide, to drop offs or deeper waters of the outer reef.

Also, for ecological reasons, contact with coral always should be avoided because even boulder corals are fragile. A soft touch can cause decades worth of growth to be undone in mere seconds, and the coral may never recover.

Snorkeling locations

Snorkelers observing fish in Cozumel, Mexico.

Snorkeling is possible in almost any body of water, but snorkelers are most likely to be found in locations where there are minimal waves, warm water, and something particularly interesting to see near the surface.

Generally shallow reefs ranging from sea level to 1 to 4 meters (3 to 12 feet) are favored by snorkelers. Deeper reefs are also good, but repeated breath holding to dive to those depths limit the number of practitioners and raises the bar on fitness and skill level.

More Snorkel

Source :

http://en.wikipedia.org/wiki/Snorkeling
Snorkel

DIVING MASK

30 January 2013 04.20 / Leave a comment

A diving mask (also dive mask or scuba mask) is an item of diving equipment that allows scuba divers, free-divers, and snorkelers to see clearly underwater. When the human eye is in direct contact with water as opposed to air, its normal environment, light entering the eye is refracted by a different angle and the eye is unable to focus the light. By providing an air space in front of the eyes, light enters normally and the eye is able to focus correctly.

Construction

Diving masks may have a single, durable, tempered glass plate in front of the eyes and a “skirt” of rubber or silicone to create a watertight seal with the diver’s face. Some masks may have two lenses, which in some cases allows the user to fit prescription lenses. In the case of freediving masks, which need to be low volume to minimise the change of pressure that occurs with depth, the lenses may be made of polycarbonate plastic. All masks have an elastic strap to keep them in position.

Masks that are used at depth must be constructed so that the diver can exhale through the nose into the mask to prevent the “squeeze” caused by increasing pressure during descent in water. The nose section of the mask also needs to be flexible to allow the diver to perform an eqalisation maneuver such as the valsalva maneuver or the frenzel maneuver when equalising pressure in the middle ear.

Some masks have a one-way purge valve under the nose to let water out. The diver simply holds the mask upright and exhales through the nose. While common in snorkeling, this feature is less favored by SCUBA divers because of the possibility of the valve failing at depth and leaving the user no means of clearing his mask. A simple and effective method for clearing a diving mask while underwater is to look down, place a finger on the top of the frame at either side, and slowly look up while exhaling through the nose.

Sometimes masks are sold in conjunction with snorkels and/or swimfins. Low quality snorkelling masks may have a low-quality plastic or glass faceplate, and are not recommended for anything but rare, casual use. Masks that have the snorkel built in are considered hazardous as well.

Divers often test whether a mask is a good fit by placing it on their face, without using the straps, and gently inhaling through their nose. If the mask stays on without any help this indicates that no air is being drawn in and that the fit is good.

Use

To prevent a mask from fogging up due to condensation on the glass plate many divers spit into the mask, wipe the spit around the inside of the plate and wash it out with a little water. There are commercial products that can be used as an alternative to the saliva method.

Refraction of light entering the mask makes objects in salt water appear about 34% bigger and 25% nearer when underwater, as shown in the underwater vision article. As the diver descends, the water acts as a colour filter eliminating the red end of the visible spectrum of the sunlight entering the water leaving only the blue end of the spectrum. Depending on the depth and clarity of the water, eventually all sunlight is blocked and the diver has to rely on artificial light sources to see underwater.

A variety of prescription lenses can be fitted inside the glass plate of the mask to correct some visual problems underwater. Divers may be able to use contact lenses inside the mask but they must keep their eyes closed if they remove the mask underwater to avoid losing the lenses. Double-dome masks restore natural sized underwater vision and field of view, while also correcting for a certain range of myopic vision. Mask removal and refitting is a basic skill that all divers are taught so that the diver can overcome floods or the mask being dislodged without panic.

When entering the water while wearing the mask, the diver normally needs to place a hand over the mask to disrupt fast water flow during entry. This prevents the mask from becoming dislodged or the glass damaged. Alternatively, a diver can enter the water with the mask off and then put it on or use an entry method such as the “forward roll”, where the diver rolls forward with head entering the water first, which does not result in fast water flow over the mask.

Mask should always be rinsed inside and out with clean, fresh water after each day’s use, and dried off after washing. A mask should not be stored in direct sunlight for long periods of time as ultraviolet light degrades the silicone. A well-maintained mask will last for many years.

Types of diving mask

Older diving mask with one big window

Disassembled single-window, low-volume dive mask

A two-window, soft-silicone dive mask without purge valve

A HydroOptix Double-Dome mask

Mask with bifocal lenses for reading instruments

A diving mask showing the retaining strap

Older diving masks had a single elliptical pane of glass. These masks have indentations underneath, either side of the nose. Divers can then put a thumb and forefinger in to pinch the nose, when performing a Valsalva maneuver to clear their ears. This design was improved by bringing the window closer to the face, reducing the volume of air inside the mask, thus making mask clearing easier. The window has a cutout to fit over the nose, which is covered by the rubber or silicone material of the mask. This facilitates pinching the nose when ear-clearing.

A further development is the mask with two windows, one window for each eye. It can have the windows closer to the face than the one-window type, and therefore contain even less space for the diver to have to blow water out of if the mask floods. These types are often called a “low-volume mask”.

Recent innovations have produced more complex designs, intended to provide extra features:

The double-dome mask. This was invented by HydroOptix. Double-dome masks allow a wider field of view and avoid the refraction error in perceived distance and size of objects. Underwater the curved mask windows make the diver’s vision effectively more hyperopic, or less myopic, and the diver must wear special contact lenses to compensate (unless his eyes are myopic to the right amount to compensate exactly for the refraction at the curved mask windows). The diver’s vision will become myopic when he puts his head out of water with the contact lenses in.
The “Data Mask”, developed by Oceanic, is an eyes-and-nose diving mask with a built-in LCD display which displays various dive and breathing set conditions including the function of a diving computer. It is currently very expensive.

Related equipment

There are several specialised types of diving headgear or outerwear:

full face diving mask – often worn by working divers who need underwater verbal communication ability.
diving helmet – often worn by divers using surface supplied diving equipment.
hard hat – part of the old fashioned standard diving dress.
fluid filled mask – the need to equilibrate the internal pressure in the mask by exhaling air through the nose reduces the freediver capacity to dive deep. Masks or swimming goggles with high power lenses (40-200 diopters) have been developed in this view: they are filled with water or saline fluid.

More Diving Mask

Source :

http://en.wikipedia.org/wiki/Diving_mask

Diving Mask

OCTOPUS

27 January 2013 17.10 / Leave a comment

The octopus (pron.:/ˈɒktəpʊs/; plural: octopuses, octopi, or octopodes) is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms and, like other cephalopods, they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms. Octopuses have no internal or external skeleton (although some species have a vestigial remnant of a shell inside their mantles), allowing them to squeeze through tight places. Octopuses are among the most intelligent and behaviorally flexible of all invertebrates.

The octopus inhabits many diverse regions of the ocean, including coral reefs, pelagic waters, and the ocean floor. They have numerous strategies for defending themselves against predators, including the expulsion of ink, the use of camouflage and deimatic displays, their ability to jet quickly through the water, and their ability to hide. An octopus trails its eight arms behind it as it swims. All octopuses are venomous, but only one group, the blue-ringed octopuses, is known to be deadly to humans.

Around 300 species are recognized, which is over one-third of the total number of known cephalopod species. The term ‘octopus’ may also be used to refer only to those creatures in the genus Octopus.

Biology

Schematic lateral aspect of octopod features

common octopus (Octopus vulgaris)

Octopuses are characterized by their eight arms, usually bearing suction cups. The arms of octopuses are often distinguished from the pair of feeding tentacles found in squid and cuttlefish. Both types of limbs are muscular hydrostats. Unlike most other cephalopods, the majority of octopuses – those in the suborder most commonly known, Incirrina – have almost entirely soft bodies with no internal skeleton. They have neither a protective outer shell like the nautilus, nor any vestige of an internal shell or bones, like cuttlefish or squid. A beak, similar in shape to a parrot’s beak, is the only hard part of their bodies. This enables them to squeeze through very narrow slits between underwater rocks, which is very helpful when they are fleeing from moray eels or other predatory fish. The octopuses in the less-familiar Cirrina suborder have two fins and an internal shell, generally reducing their ability to squeeze into small spaces. These cirrate species are often free-swimming and live in deep-water habitats, while incirrate octopus species are found in reefs and other shallower seafloor habitats.

An octopus moving between tide pools during low tide

Octopuses have a relatively short life expectancy, and some species live for as little as six months. Larger species, such as the giant pacific octopus, may live for up to five years under suitable circumstances. However, reproduction is a cause of death: males can only live for a few months after mating, and females die shortly after their eggs hatch. They neglect to eat during the (roughly) one-month period spent taking care of their unhatched eggs, eventually dying of starvation. In a scientific experiment, removal of both optic glands after spawning was found to result in cessation of broodiness, resumption of feeding, increased growth, and greatly extended lifespans.

Grimpoteuthis discoveryi, a finned octopus of the suborder Cirrina

Octopuses have three hearts. Two branchial hearts pump blood through each of the two gills, while the third is a systemic heart that pumps blood through the body. Octopus blood contains the copper-rich protein hemocyanin for transporting oxygen. Although less efficient under normal conditions than the iron-rich hemoglobin of vertebrates, in cold conditions with low oxygen pressure, hemocyanin oxygen transportation is more efficient than hemoglobin oxygen transportation. The hemocyanin is dissolved in the plasma instead of being carried within red blood cells, and gives the blood a bluish color. The octopus draws water into its mantle cavity, where it passes through its gills. As mollusks, their gills are finely divided and vascularized outgrowths of either the outer or the inner body surface.

Intelligence

Octopuses are highly intelligent, likely more so than any other order of invertebrates. The exact extent of their intelligence and learning capability is much debated among biologists, but maze and problem-solving experiments have shown evidence of a memory system that can store both short- and long-term memory. It is not known precisely what contribution learning makes to adult octopus behavior. Young octopuses learn almost no behaviors from their parents, with whom they have very little contact.

An octopus opening a container with a screw cap

An octopus has a highly complex nervous system, only part of which is localized in its brain. Two-thirds of an octopus’s neurons are found in the nerve cords of its arms, which have limited functional autonomy. Octopus arms show a variety of complex reflex actions that persist even when they have no input from the brain. Unlike vertebrates, the complex motor skills of octopuses are not organized in their brain using an internal somatotopic map of its body, using a nonsomatotopic system unique to large-brained invertebrates. Some octopuses, such as the mimic octopus, will move their arms in ways that emulate the shape and movements of other sea creatures.

In laboratory experiments, octopuses can be readily trained to distinguish between different shapes and patterns. They have been reported to practice observational learning, although the validity of these findings is widely contested on a number of grounds. Octopuses have also been observed in what some have described as play: repeatedly releasing bottles or toys into a circular current in their aquariums and then catching them. Octopuses often break out of their aquariums and sometimes into others in search of food. They have even boarded fishing boats and opened holds to eat crabs.

In some countries, octopuses are on the list of experimental animals on which surgery may not be performed without anesthesia. In the UK, cephalopods such as octopuses are regarded as ‘honorary vertebrates’ under the Animals (Scientific Procedures) Act 1986 and other cruelty to animals legislation, extending to them protections not normally afforded to invertebrates.

The octopus is the only invertebrate which has been shown to use tools. At least four specimens of the veined octopus (Amphioctopus marginatus) have been witnessed retrieving discarded coconut shells, manipulating them, and then reassembling them to use as shelter. This discovery was documented in the journal Current Biologyand has also been caught on video.

Defense

Greater blue-ringed octopus (Hapalochlaena lunulata)

An octopus’s main (primary) defense is to hide, either not to be seen at all, or not to be detected as an octopus. Octopuses have several secondary defenses (defenses they use once they have been seen by a predator). The most common secondary defense is fast escape. Other defenses include the use of ink sacs, camouflage, and autotomising limbs.

Most octopuses can eject a thick, blackish ink in a large cloud to aid in escaping from predators. The main coloring agent of the ink is melanin, which is the same chemical that gives humans their hair and skin color. This ink cloud is thought to reduce the efficiency of olfactory organs, which would aid an octopus’s evasion from predators that employ smell for hunting, such as sharks. Ink clouds of some species might serve as pseudomorphs, or decoys that the predator attacks instead.

Amphioctopus marginatus travels with shells it has collected for protection

An octopus’s camouflage is aided by certain specialized skin cells which can change the apparent color, opacity, and reflectiveness of the epidermis. Chromatophores contain yellow, orange, red, brown, or black pigments; most species have three of these colors, while some have two or four. Other color-changing cells are reflective iridophores, and leucophores (white). This color-changing ability can also be used to communicate with or warn other octopuses. The very venomous blue-ringed octopus becomes bright yellow with blue rings when it is provoked. Octopuses can use muscles in the skin to change the texture of their mantle to achieve a greater camouflage. In some species, the mantle can take on the spiky appearance of seaweed, or the scraggly, bumpy texture of a rock, among other disguises. However in some species skin anatomy is limited to relatively patternless shades of one color, and limited skin texture. It is thought that octopuses that are day-active and/or live in complex habitats such as coral reefs have evolved more complex skin than their nocturnal and/or sand-dwelling relatives.

When under attack, some octopuses can perform arm autotomy, in a similar manner to the way skinks and other lizards detach their tails. The crawling arm serves as a distraction to would-be predators.

A few species, such as the mimic octopus, have a fourth defense mechanism. They can combine their highly flexible bodies with their color-changing ability to accurately mimic other, more dangerous animals, such as lionfish, sea snakes, and eels.

Reproduction

When octopuses reproduce, the male uses a specialized arm called a hectocotylus to insert spermatophores (packets of sperm) into the female’s mantle cavity. The hectocotylus in benthic octopuses is usually the third right arm. Males die within a few months of mating. In some species, the female octopus can keep the sperm alive inside her for weeks until her eggs are mature. After they have been fertilized, the female lays about 200,000 eggs (this figure dramatically varies between families, genera, species and also individuals).

Sensation

Eye of Octopus vulgaris

Octopuses have keen eyesight. Like other cephalopods, they can distinguish the polarization of light. Color vision appears to vary from species to species, being present in O. aegina but absent in O. vulgaris. Attached to the brain are two special organs, called statocysts, that allow the octopus to sense the orientation of its body relative to horizontal. An autonomic response keeps the octopus’s eyes oriented so the pupil slit is always horizontal.

Octopuses also have an excellent sense of touch. An octopus’s suction cups are equipped with chemoreceptors so the octopus can taste what it is touching. The arms contain tension sensors so the octopus knows whether its arms are stretched out. However, it has a very poor proprioceptive sense. The tension receptors are not sufficient for the brain to determine the position of the octopus’s body or arms. (It is not clear whether the octopus brain would be capable of processing the large amount of information that this would require; the flexibility of an octopus’s arms is much greater than that of the limbs of vertebrates, which devote large areas of cerebral cortex to the processing of proprioceptive inputs.) As a result, the octopus does not possess stereognosis; that is, it does not form a mental image of the overall shape of the object it is handling. It can detect local texture variations, but cannot integrate the information into a larger picture.

The neurological autonomy of the arms means the octopus has great difficulty learning about the detailed effects of its motions. The brain may issue a high-level command to the arms, but the nerve cords in the arms execute the details. There is no neurological path for the brain to receive feedback about just how its command was executed by the arms; the only way it knows just what motions were made is by observing the arms visually.

Octopuses swim headfirst, with arms trailing behind.

Locomotion

Octopuses move about by crawling or swimming. Their main means of slow travel is crawling, with some swimming. Jet propulsion is their fastest means of locomotion, followed by swimming and walking.

They crawl by walking on their arms, usually on many at once, on both solid and soft surfaces, while supported in water. In 2005, some octopuses (Adopus aculeatus and Amphioctopus marginatus under current taxonomy) were found to walk on two arms, while at the same time resembling plant matter. This form of locomotion allows these octopuses to move quickly away from a potential predator while possibly not triggering that predator’s search image for octopus (food). A study of this behavior conducted by the Weymouth Sea Life Centre led to the suggestion that the two rearmost appendages may be more accurately termed ‘legs’ rather than ‘arms’. Some species of octopuses can crawl out of the water for a short period, which they may do between tide pools while hunting crustaceans or gastropods or to escape predators.

Octopuses swim by expelling a jet of water from a contractile mantle, and aiming it via a muscular siphon.

Prey

Bottom-dwelling octopuses eat mainly crabs, polychaete worms, and other molluscs such as whelks and clams. Open-ocean octopuses eat mainly prawns, fish and other cephalopods. They usually inject their prey with a paralysing saliva before dismembering it into small pieces with their beaks. Octopuses feed on shelled molluscs either by using force, or by drilling a hole in the shell, injecting a secretion into the hole, and then extracting the soft body of the mollusc.

Large octopuses have also been known to catch and kill some species of sharks.

Size

An adult giant Pacific octopus, Enteroctopus dofleini

The giant Pacific octopus, Enteroctopus dofleini, is often cited as the largest octopus species. Adults usually weigh around 15 kg (33 lb), with an arm span of up to 4.3 m (14 ft). The largest specimen of this species to be scientifically documented was an animal with a live mass of 71 kg (156.5 lb). The alternative contender is the seven-arm octopus, Haliphron atlanticus, based on a 61-kg (134-lb) carcass estimated to have a live mass of 75 kg (165 lb). However, a number of questionable size records would suggest E. dofleini is the largest of all octopus species by a considerable margin; one such record is of a specimen weighing 272 kg (600 lb) and having an arm span of 9 m (30 ft).

Etymology and pluralization

The term “octopus” is from Greek ὀκτάπους (oktapous, “eight-footed”), with traditional plural forms “octopuses” (pronounced /ˈɒktəpʊsɪz/) from English grammar and “octopodes” (pronounced /ɒkˈtɒpədiːz/) from the Greek. Currently, “octopuses” is the most common form in both the US and the UK. The term “octopod” (plural: “octopods” or “octopodes”) is taken from the taxonomic order Octopoda, but has no classical equivalent. The collective plural “octopus” is usually reserved for animals consumed for food.

Some authorities consider “octopi” an objectionable hypercorrection, feeling the form arose from the incorrect assumption that “octopus” is a Latin second-declension form. However, “octopus” is a scientific Latin third-declension noun with a plural of octopodes. Nevertheless, the Oxford English Dictionary (2008 Draft Revision) lists “octopuses”, “octopi”, and “octopodes” (in that order), labelling “octopodes” ‘rare’ and noting that “octopi” derives from the misapprehension that octōpusis a second-declension Latin noun. The book further maintains that if the word were native to Latin, it would be third declension octōpēs (plural: octōpedes) after the pattern of pēs (“foot”, plural pedēs). The original Latin word for octopus and other similar species is polypus, from Greek polýpous (πολύπους, “many-footed”); again, usually the inappropriate plural polypī is used instead of polypodēs.

Fowler’s Modern English Usage states, ‘the only acceptable plural in English is “octopuses”‘, that “octopi” is ‘misconceived’, and “octopodes” ‘pedantic’. Chambers 21st Century Dictionary and the Compact Oxford Dictionary list only “octopuses”, although the latter notes that “octopodes” is ‘still occasionally used’. The descriptivist Merriam-Webster 11th Collegiate Dictionary lists “octopuses” and “octopi” in that order; likewise, Webster’s New World College Dictionary lists in order “octopuses”, “octopi”, and “octopodes”.

In modern Greek, the word is χταπόδι (khtapódi; plural: χταπόδια, khtapódia), from Byzantine ὀκταπόδιον (oktapódion) derived from the Classical Greek variant ὀκτάπους (oktápous).

Relationship to humans

Moche Octopus (200 AD), Larco Museum Collection, Lima, Peru

Vase from a Mycenaean Greek cemetery at Prosymna, Argos, grave 2, circa 1500 BCE

a hoplite with an octopus image on his shield

Staatliche Antikensammlungen, Munich, Germany

An ancient Greek black-figure amphora, 530–520 BC

Ancient peoples of the Mediterranean were aware of the octopus, as evidenced by certain artworks and designs of prehistory. For example, a stone carving found in the archaeological recovery from Bronze Age Minoan Crete at Knossos has a depiction of a fisherman carrying an octopus.

Octopuses were often depicted in the art of the Moche people of ancient Peru, who worshipped the sea and its animals.

In mythology

The Hawaiian creation myth relates that the present cosmos is only the last of a series, having arisen in stages from the wreck of the previous universe. In this account, the octopus is the lone survivor of the previous, alien universe.

As a metaphor

Due to having numerous arms that emanate from a common center, the octopus is often used as a metaphor for a group or organization which is perceived as being powerful, manipulative or bent on domination. Use of this terminology is invariably negative and employed by the opponents of the groups or institutions so described.

As food

Octopus at Tsukiji fish market

Humans eat octopus in many cultures. The arms and sometimes other body parts are prepared in various ways, often varying by species.

Octopus is a common ingredient in Japanese cuisine, including sushi, takoyaki, and akashiyaki.

In Korea, some small species are sometimes eaten alive as a novelty food. A live octopus is usually sliced up, and it is eaten while still squirming.

Octopuses are “tickled” out of their holes in the Hawaiian Islands with three-pronged polespears

Raw octopus arms

Lightly boiled octopus arm that turned a bright purple

Octopus is eaten regularly in Hawaii, since many popular dishes are Asian in origin. Locally known by their Hawaiian or Japanese names (he’e and tako, respectively), octopus is also a popular fish bait.

Octopus is a common food in Mediterranean cuisine and Portuguese cuisine. In Galicia, polbo á feira(market fair style octopus) is a local delicacy. Restaurants which specialize or serve this dish are known as pulperías. On the Tunisian island of Djerba, local people catch octopuses by taking advantage of the animals’ habit of hiding in safe places during the night. In the evening, they put grey ceramic pots on the sea bed. The morning of the following day they check them for octopuses sheltered there. A common scene in the Greek islands is octopuses hanging in the sunlight from a rope, just like laundry from a clothesline. They are often caught by spear fishing close to the shore. The fisherman brings his prey to land and tenderizes the flesh by pounding the carcass against a stone surface. Thus treated, they are hung out to dry, and later will be served grilled, either hot or chilled in a salad. They are considered a superb meze, especially alongside ouzo.

According to the USDA Nutrient Database (2007), cooked octopus contains about 139 kilocalories (Calories) per three-ounce portion, and is a source of vitamin B₃, B₁₂, potassium, phosphorus, and selenium.

Care must be taken to boil the octopus properly, to rid it of slime, smell, and residual ink.

As pets

Though octopuses can be difficult to keep in captivity, some people keep them as pets. They often escape even from supposedly secure tanks, due to their problem-solving skills, mobility and lack of rigid structure.

The variation in size and lifespan among octopus species makes it difficult to know how long a new specimen can naturally be expected to live. That is, a small octopus may be just born or may be an adult, depending on its species. By selecting a well-known species, such as the California two-spot octopus, one can choose a small octopus (around the size of a tennis ball) and be confident it is young with a full life ahead of it.

Classification

Cirrothauma murrayi

Amphitretus pelagicus

Class Cephalopoda

Subclass Nautiloidea: nautilus
Subclass Coleoidea

Superorder Decapodiformes: squid, cuttlefish
Superorder Octopodiformes

Family †Trachyteuthididae
Order Vampyromorphida: vampire squid
Order Octopoda

Genus †Keuppia
Genus †Palaeoctopus
Genus †Paleocirroteuthis
Genus †Pohlsepia
Genus †Proteroctopus
Genus †Styletoctopus
Suborder Cirrina: finned deep-sea octopus

Family Opisthoteuthidae: umbrella octopus
Family Cirroteuthidae
Family Stauroteuthidae

Suborder Incirrina

Family Amphitretidae: telescope octopus
Family Bolitaenidae: gelatinous octopus
Family Octopodidae: benthic octopus
Family Vitreledonellidae: glass octopus
Superfamily Argonautoida

Family Alloposidae: seven-arm octopus
Family Argonautidae: argonauts
Family Ocythoidae: tuberculate pelagic octopus
Family Tremoctopodidae: blanket octopus

Source :

http://en.wikipedia.org/wiki/Octopus

Octopus

BARNACLE

24 January 2013 07.05 / Leave a comment

A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile (non-motile) suspension feeders, and have two nektonic (active swimming) larval stages. Around 1,220 barnacle species are currently known. The name “Cirripedia” is Latin, meaning “curl-footed”.

Ecology

Semibalanus balanoides feeding (also available at higher resolution)

Barnacles are encrusters, attaching themselves permanently to a hard substrate. The most common, “acorn barnacles” (Sessilia), are sessile, growing their shells directly onto the substrate. The order Pedunculata (“goose barnacles” and others) attach themselves by means of a stalk.

Most barnacles are suspension feeders; they dwell continually in their shell – which is usually constructed of six plates – and reach into the water column with modified legs. These feathery appendages beat rhythmically to draw plankton and detritus into the shell for consumption.

Other members of the class have quite a different mode of life. For example, members of the genus Sacculinaare parasitic, dwelling within crabs.

Although they have been found at water depths up to 600 m (2,000 ft), most barnacles inhabit shallow waters, with 75% of species living in water depths of less than 100 m (300 ft), and 25% inhabiting the intertidal zone. Within the intertidal zone, different species of barnacle live in very tightly constrained locations, allowing the exact height of an assemblage above or below sea level to be precisely determined.

Since the intertidal zone periodically desiccates, barnacles are well adapted against water loss. Their calcite shells are impermeable, and they possess two plates which they can slide across their aperture when not feeding. These plates also protect against predation.

Barnacles and limpets compete for space in the intertidal zone.

Barnacles are displaced by limpets and mussels, which compete for space. They also have numerous predators. They employ two strategies to overwhelm their competitors: “swamping” and fast growth. In the swamping strategy, vast numbers of barnacles settle in the same place at once, covering a large patch of substrate, allowing at least some to survive in the balance of probabilities. Fast growth allows the suspension feeders to access higher levels of the water column than their competitors, and to be large enough to resist displacement; species employing this response, such as the aptly named Megabalanus, can reach 7 cm (3 in) in length; other species may grow larger still (Austromegabalanus psittacus).

Competitors may include other barnacles, and there is (disputed) evidence that balanoid barnacles competitively displaced chthalamoid barnacles. Balanoids gained their advantage over the chthalamoids in the Oligocene, when they evolved a tubular skeleton. This provides better anchorage to the substrate, and allows them to grow faster, undercutting, crushing and smothering the latter group.

Among the most common predators on barnacles are whelks. They are able to grind through the calcareous exoskeletons of barnacles and feed on the softer inside parts. Mussels also prey on barnacle larvae. Another predator on barnacles is the starfish species Pisaster ochraceus.

Adult anatomy

Goose barnacles, with their cirriextended for feeding

Free-living barnacles are attached to the substratum by cement glands that form the base of the first pair of antennae; in effect, the animal is fixed upside down by means of its forehead. In some barnacles, the cement glands are fixed to a long muscular stalk, but in most they are part of a flat membrane or calcified plate. A ring of plates surrounds the body, homologous with the carapace of other crustaceans. These consist of the rostrum, two lateral plates, two carino-laterals and a carina. In sessile barnacles, the apex of the ring of plates is covered by an operculum, which may be recessed into the carapace. The plates are held together by various means, depending on species, in some cases being solidly fused.

Inside the carapace, the animal lies on its back, with its limbs projecting upwards. Segmentation is usually indistinct, and the body is more or less evenly divided between the head and thorax, with little, if any, abdomen. Adult barnacles have few appendages on the head, with only a single, vestigial, pair of antennae, attached to the cement gland. There are six pairs of thoracic limbs, referred to as “cirri”, which are feathery and very long, being used to filter food from the water and move it towards the mouth.

Barnacles have no true heart, although a sinus close to the oesophagus performs similar function, with blood being pumped through it by a series of muscles. The blood vascular system is minimal. Similarly, they have no gills, absorbing oxygen from the water through their limbs and the inner membrane of the carapace. The excretory organs of barnacles are maxillary glands.

The main sense of barnacles appears to be touch, with the hairs on the limbs being especially sensitive. The adult also has a single eye, although this is probably only capable of sensing the difference between light and dark. This eye is derived from the primary naupliar eye.

Parasitic barnacles

The anatomy of parasitic barnacles is generally simpler than that of their free-living relatives. They have no carapace or limbs, having only an unsegmented sac-like body. Such barnacles feed by extending thread-like rhizomes of living cells into the host’s body from their point of attachment.

Life cycle

Barnacles have two distinct larval stages, the nauplius and the cyprid, before developing into a mature adult.

Nauplius

Nauplius larva of Elminius modestus

A fertilised egg hatches into a nauplius: a one-eyed larva comprising a head and a telson, without a thorax or abdomen. This undergoes 6 months of growth, passing through five instars, before transforming into the cyprid stage. Nauplii are typically initially brooded by the parent, and released after the first moult as larvae that swim freely using setae.

Cyprid

The cyprid larva is the last larval stage before adulthood. It is a non-feeding stage whose role is to find a suitable place to settle, since the adults are sessile. The cyprid stage lasts from days to weeks. It explores potential surfaces with modified antennules; once it has found a potentially suitable spot, it attaches head-first using its antennules, and a secreted glycoproteinous substance. Larvae are thought to assess surfaces based upon their surface texture, chemistry, relative wettability, colour and the presence/absence and composition of a surface biofilm; swarming species are also more likely to attach near to other barnacles. As the larva exhausts its finite energy reserves, it becomes less selective in the sites it selects. It cements itself permanently to the substrate with another proteinacous compound, and then undergoes metamorphosis into a juvenile barnacle.

Adult

Typical acorn barnacles develop six hard calcareous plates to surround and protect their bodies. For the rest of their lives they are cemented to the ground, using their feathery legs (cirri) to capture plankton.

Once metamorphosis is over and they have reached their adult form, barnacles will continue to grow by adding new material to their heavily calcified plates. These plates are not moulted; however, like all ecdysozoans, the barnacle itself will still molt its cuticle.

Sexual reproduction

Most barnacles are hermaphroditic, although a few species are gonochoric or androdioecious. The ovaries are located in the base or stalk, and may extend into the mantle, while the testes are towards the back of the head, often extending into the thorax. Typically, recently molted hermaphroditic individuals are receptive as females. Self-fertilization, although theoretically possible, has been experimentally shown to be rare in barnacles.

The sessile lifestyle of barnacles makes sexual reproduction difficult, as the organisms cannot leave their shells to mate. To facilitate genetic transfer between isolated individuals, barnacles have extraordinarily long penises. Barnacles probably have the largest penis to body size ratio of the animal kingdom.

Fossil record

Miocene (Messinian) Megabalanus, smothered by sand and fossilised

The geological history of barnacles can be traced back to animals such as Priscansermarinus from the Middle Cambrian (on the order of 510 to 500 million years ago), although they do not become common as skeletal remains in the fossil record until the Neogene (last 20 million years). In part their poor skeletal preservation is due to their restriction to high-energy environments, which tend to be erosional – therefore it is more common for their shells to be ground up by wave action than for them to reach a depositional setting. Trace fossils of acrothoracican barnacle borings (Rogerella) are common in the fossil record from the Devonian to the Recent.

Barnacles can play an important role in estimating palæo-water depths. The degree of disarticluation of fossils suggests the distance they have been transported, and since many species have narrow ranges of water depths, it can be assumed that the animals lived in shallow water and broke up as they were washed down-slope. The completeness of fossils, and nature of damage, can thus be used to constrain the tectonic history of regions.

History of taxonomy

Balanus improvisus, one of the many barnacle taxa erected by Charles Darwin

Barnacles were originally classified by Linnaeus and Cuvier as Mollusca, but in 1830 John Vaughan Thompson published observations showing the metamorphosis of the nauplius and cypris larvae into adult barnacles, and noted how these larvae were similar to those of crustaceans. In 1834 Hermann Burmeister published further information, reinterpreting these findings. The effect was to move barnacles from the phylum of Mollusca to Articulata, showing naturalists that detailed study was needed to reevaluate their taxonomy.

Charles Darwin took up this challenge in 1846, and developed his initial interest into a major study published as a series of monographs in 1851 and 1854. Darwin undertook this study at the suggestion of his friend Joseph Dalton Hooker, in order to thoroughly understand at least one species before making the generalisations needed for his theory of evolution by natural selection.

In human culture

Barnacles are of economic consequence as they often attach themselves to man-made structures, sometimes to the structure’s detriment. Particularly in the case of ships, they are classified as fouling organisms.

Some barnacles are considered edible by humans, and goose barnacles (e.g. Pollicipes pollicipes), in particular, are a delicacy in Spain and Portugal. The resemblance of this barnacle’s fleshy stalk to a goose’s neck gave rise in ancient times to the notion that geese, or at least certain seagoing species of wild goose, literally grew from the barnacle. Indeed, the word “barnacle” originally referred to a species of goose, the Barnacle goose Branta leucopsis, whose eggs and young were rarely seen by humans because it breeds in the remote Arctic.

The picoroco barnacle is used in Chilean cuisine and is one of the ingredients in curanto.

Classification

Semibalanus balanoides (Thoracica: Sessilia) feeding

Some authorities regard Cirripedia as a full class or subclass, and the orders listed above are sometimes treated as superorders. In 2001, Martin and Davis placed Cirripedia as an infraclass of Thecostraca and divided it into six orders:

Infraclass Cirripedia Burmeister, 1834

Superorder Acrothoracica Gruvel, 1905

Order Pygophora Berndt, 1907
Order Apygophora Berndt, 1907

Superorder Rhizocephala Müller, 1862

Order Kentrogonida Delage, 1884
Order Akentrogonida Häfele, 1911

Superorder Thoracica Darwin, 1854

Order Pedunculata Lamarck, 1818
Order Sessilia Lamarck, 1818

Source :

http://en.wikipedia.org/wiki/Barnacle

SCUBA DIVING

17 January 2013 16.40 / Leave a comment

Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater.

Unlike earlier diving, which relied either on breath-hold or on air pumped from the surface, scuba divers carry their own source of breathing gas, (usually compressed air), allowing them greater freedom of movement than with an air line. Both surface supplied and scuba diving allow divers to stay underwater significantly longer than with breath-holding techniques as used in free-diving.

A scuba diver usually moves around underwater by using swimfins attached to the feet, but external propulsion can be provided by a diver propulsion vehicle, or a sled pulled from the surface.

History

Original Aqualung scuba set.
1: Air Hose, 2: Mouthpiece, 3: Regulator, 4: Harness, 5: Back plate, 6: Tank

The first commercially successful scuba sets were the Aqualung twin hose open-circuit units developed by Emile Gagnan and Jacques-Yves Cousteau, in which compressed air carried in back mounted cylinders is inhaled through a demand regulator and then exhaled into the water adjacent to the tank. The single hose two stage scuba regulators of today trace their origins to Australia, where Ted Eldred developed the first example of this typeof regulator, known as the Porpoise, which was developed because patents protected the Aqualung’s twin hose design. The single hose regulator separates the cylinder from the demand valve, giving the diver air at the pressure at his mouth, not that at the top of the cylinder.

The open circuit compressed air systems were developed after Cousteau had a number of incidents of oxygen toxicity using an oxygen rebreather, in which exhaled oxygen is passed through an absorbent chemical to remove carbon dioxide before being breathed again. Modern versions of rebreather systems (both semi-closed circuit and closed circuit) are available, and form the second main type of scuba unit, mostly used for technical and military diving.

Etymology

The term “SCUBA” (an acronym for self-contained underwater breathing apparatus) originally referred to United States combat frogmen’s oxygen rebreathers, developed during World War II by Christian J. Lambertsen for underwater warfare.

“SCUBA” was originally an acronym, but is now generally used as a common noun or adjective, “scuba”. It has become acceptable to refer to “scuba equipment” or “scuba apparatus”—examples of the linguistic RAS syndrome.

Diving activities associated with scuba

Scuba diving may be performed for a number of reasons, both personal and professional. Recreational diving is performed purely for enjoyment and has a number of distinct technical disciplines to increase interest underwater, such as cave diving, wreck diving, ice diving and deep diving.

Divers may be employed professionally to perform tasks underwater. Some of these tasks are suitable for scuba.

There are a fair number of divers who work, full or part-time, in the recreational diving community as instructors, assistant instructors, divemasters and dive guides. In some jurisdictions the professional nature, with particular reference to responsibility for health and safety of the clients, of recreational diver instruction, dive leadership for reward and dive guiding is recognised by national legislation.

Other specialist areas of diving include military diving, with a long history of military frogmen in various roles. They can perform roles including direct combat, infiltration behind enemy lines, placing mines or using a manned torpedo, bomb disposal or engineering operations. In civilian operations, many police forces operate police diving teams to perform search and recovery or search and rescue operations and to assist with the detection of crime which may involve bodies of water. In some cases diver rescue teams may also be part of a fire department, paramedical service or lifeguard unit, and may be classed as public service diving.

Lastly, there are professional divers involved with the water itself, such as underwater photography or underwater filming divers, who set out to document the underwater world, or scientific diving, including marine biology, geology, hydrology, oceanography and underwater archaeology.

The choice between scuba and surface supplied diving equipment is based on both legal and logistical constraints. Where the diver requires mobility and a large range of movement, scuba is usually the choice if safety and legal constraints allow. Higher risk work, particularly commercial diving, may be restricted to surface supplied equipment by legislation and codes of practice.

Diving activities commonly associated with scuba may include:

Type of diving activity	Classification
aquarium maintenance in large public aquariums	commercial, scientific
boat and ship inspection, cleaning and maintenance	commercial, naval
cave diving	technical, recreational, scientific
diver training	professional
fish farm maintenance (aquaculture)	commercial
fishing, e.g. for abalones, crabs, lobsters, scallops, sea crayfish,	commercial
frogman, manned torpedo	military
media diving: making television programs, etc.	professional
mine clearance and bomb disposal, disposing of unexploded ordnance	military, naval
pleasure, leisure, sport	recreational
policing/security: diving to investigate or arrest unauthorized divers	police diving, military, naval
search and recovery diving	public safety, police diving
search and rescue diving	police, naval, public service
spear fishing	recreational
stealthy infiltration	military
surveys and mapping	scientific, recreational
scientific diving (marine biology, oceanography, hydrology, geology, palaeontology, diving physiology and medicine)	scientific
underwater archaeology (shipwrecks; harbors, and buildings)	scientific, recreational
underwater inspections and surveys (occasionally)	commercial, military
underwater photography	professional, recreational
underwater tour guiding	professional, recreational
underwater tourism	recreational

Breathing underwater

Scuba diver on reef

Water normally contains the dissolved oxygen from which fish and other aquatic animals extract all their required oxygen as the water flows past their gills. Humans lack gills and do not otherwise have the capacity to breathe underwater unaided by external devices. Although the feasibility of filling and artificially ventilating the lungs with a dedicated liquid (liquid breathing) has been established for some time, the size and complexity of the equipment allows only for medical applications with current technology.

Early diving experimenters quickly discovered it is not enough simply to supply air to breathe comfortably underwater. As one descends, in addition to the normal atmospheric pressure, water exerts increasing pressure on the chest and lungs—approximately 1 bar (14.7 pounds per square inch) for every 33 feet (10 m) of depth—so the pressure of the inhaled breath must almost exactly counter the surrounding or ambient pressure to inflate the lungs. It becomes virtually impossible to breathe unpressurised air through a tube below three feet under the water.

By always providing the appropriate breathing gas at ambient pressure, modern demand valve regulators ensure the diver can inhale and exhale naturally and without excessive effort, regardless of depth.

Because the diver’s nose and eyes are covered by a diving mask; the diver cannot breathe in through the nose, except when wearing a full face diving mask. However, inhaling from a regulator’s mouthpiece becomes second nature very quickly.

Open-circuit regulator

Aqualung Legacy regulator

Gekko dive computer with attached pressure gauge and compass

The most commonly used scuba set today is the “single-hose” open circuit 2-stage diving regulator, connected to a single high pressure gas cylinder, with the first stage connected to the cylinder valve and the second stage at the mouthpiece. This arrangement differs from Emile Gagnan’s and Jacques Cousteau’s original 1942 “twin-hose” design, known as the Aqua-lung, in which the cylinder pressure was reduced to ambient pressure in one or two stages which were all in the housing mounted to the cylinder valve or manifols. The “single-hose” system has significant advantages over the original system for most applications.

Aqualung 1st stage

Suunto pressure gauge close up

In the “single-hose” two-stage design, the first stage regulator reduces the cylinder pressure of up to about 240 bar (3000 psi) to an intermediate level of about 10 bar (145 psi) above ambient pressure. The second stage demand valve regulator, supplied by a low pressure hose from the first stage, delivers the breathing gas at ambient pressure to the diver’s mouth. The exhaled gases are exhausted directly to the environment as waste. The first stage typically has at least one outlet port delivering breathing gas at unreduced tank pressure. This is connected to the diver’s submersible pressure gauge or dive computer, to show how much breathing gas remains in the cylinder.

Rebreather

An Inspiration electronic fully closed circuit rebreather

Less common are closed circuit (CCR) and semi-closed (SCR) rebreathers, which unlike open-circuit sets that vent off all exhaled gases, process each exhaled breath for re-use by removing the carbon dioxide and replacing the oxygen used by the diver.

Rebreathers release little or no gas bubbles into the water, and use much less stored gas volume for an equivalent depth and time because exhaled oxygen is recovered; this has advantages for research, military, photography, and other applications. The first modern rebreather was the MK-19 that was developed at S-Tron by Ralph Osterhout and used the first electronic control system. Rebreathers are more complex and more expensive than open-circuit scuba, and special training and correct maintenance are required for them to be safely used, due to the larger variety of potential failure modes.

In a closed-circuit rebreather the oxygen partial pressure in the rebreather is controlled, so it can be increased to a safe continuous maximum, which reduces the inert gas (nitrogen and/or helium) partial pressure in the breathing loop. Minimising the inert gas loading of the diver’s tissues for a given dive profile reduces the decompression obligation. This requires continuous monitoring of actual partial pressures with time and for maximum effectiveness requires real-time computer processing by the diver’s decompression computer. Decompression can be much reduced compared to fixed ratio gas mixes used in other scuba systems and, as a result, divers can stay down longer or decompress faster. A semi-closed circuit rebreather injects a constant flow of a fixed nitrox mixture into the breathing loop, or changes a fixed percentage of the respired volume, so the partial pressure of oxygen at any time during the dive depends on the diver’s oxygen consumption or breathing rate. Planning decompression requirements requires a more conservative approach for a SCR than for a CCR, but decompression computers with a real time oxygen partial pressure input can optimise decompression for these systems.

Because rebreathers produce very few bubbles, they do not disturb marine life or make a diver’s presence known at the surface; this is useful for underwater photography, and for covert work.

Gas mixtures

Nitrox cylinder marked up for use showing maximum safe operating depth (MOD)

For some diving, gas mixtures other than normal atmospheric air (21% oxygen, 78% nitrogen, 1% trace gases) can be used, so long as the diver is properly trained in their use. The most commonly used mixture is Nitrox, also referred to as Enriched Air Nitrox (EAN), which is air with extra oxygen, often with 32% or 36% oxygen, and thus less nitrogen, reducing the likelihood of decompression sickness or allowing longer exposure to the same pressure for equal risk. The reduced nitrogen may also allow for no stops or shorter decompression stop times and a shorter surface interval between dives. A common misconception is that nitrox can reduce narcosis, but research has shown that oxygen is also narcotic.

Several other common gas mixtures are in use, and all need specialized training for safe use. The increased oxygen levels in nitrox help reduce the risk of decompression sickness; however, below the maximum operating depth of the mixture, the increased partial pressure of oxygen can lead to an unacceptable risk of oxygen toxicity. To displace nitrogen without the increased oxygen concentration, other diluents can be used, usually helium, when the resultant three gas mixture is called trimix, and when the nitrogen is fully substituted by helium, heliox.

For technical dives, some of the cylinders may contain different gas mixtures for the various phases of the dive, typically designated as Travel, Bottom, and Decompression gases. These different gas mixtures may be used to extend bottom time, reduce inert gas narcotic effects, and reduce decompression times.

Diver mobility

The diver needs to be mobile underwater. Streamlining dive gear will reduce drag and improve mobility. Personal mobility is enhanced by swimfins and Diver Propulsion Vehicles.

Controlling buoyancy underwater

Diver under the Salt Pier in Bonaire.

To dive safely, divers must control their rate of descent and ascent in the water. Ignoring other forces such as water currents and swimming, the diver’s overall buoyancy determines whether he ascends or descends. Equipment such as diving weighting systems, diving suits (wet, dry or semi-dry suits are used depending on the water temperature) and buoyancy compensators can be used to adjust the overall buoyancy. When divers want to remain at constant depth, they try to achieve neutral buoyancy. This minimizes gas consumption caused by swimming to maintain depth.

The buoyancy force on the diver is the weight of the volume of the liquid that he and his equipment displace minus the weight of the diver and his equipment; if the result is positive, that force is upwards. The buoyancy of any object immersed in water is also affected by the density of the water. The density of fresh water is about 3% less than that of ocean water. Therefore, divers who are neutrally buoyant at one dive destination (e.g. a fresh water lake) will predictably be positively or negatively buoyant when using the same equipment at destinations with different water densities (e.g. a tropical coral reef).

The removal (“ditching” or “shedding”) of diver weighting systems can be used to reduce the diver’s weight and cause a buoyant ascent in an emergency.

Diving suits made of compressible materials decrease in volume as the diver descends, and expand again as the diver ascends, causing buoyancy changes. Diving in different environments also necessitates adjustments in the amount of weight carried to achieve neutral buoyancy. The diver can inject air into dry suits to counteract the compression effect and squeeze. Buoyancy compensators allow easy and fine adjustments in the diver’s overall volume and therefore buoyancy. For open circuit divers, changes in the diver’s average lung volume during a breathing cycle can be used to make fine adjustments of buoyancy.

Neutral buoyancy in a diver is a metastable state. It is changed by small differences in ambient pressure caused by a change in depth, and the change has a positive feedback effect. A small descent will increase the pressure, which will compress the gas filled spaces and reduce the total volume of diver and equipment. This will further reduce the buoyancy, and unless counteracted, will result in sinking more rapidly. The equivalent effect applies to a small ascent, which will trigger an increased buoyancy and will result in accelerated ascent unless counteracted. The diver must continuously adjust buoyancy or depth in order to remain neutral. This is a skill which improves with practice until it becomes second nature.

Underwater vision

A diver wearing an Ocean Reef full face mask

Water has a higher refractive index than air – similar to that of the cornea of the eye. Light entering the cornea from water is hardly refracted at all, leaving only the eye’s crystalline lens to focus light. This leads to very severe hypermetropia. People with severe myopia, therefore, can see better underwater without a mask than normal-sighted people.

Diving masks and helmets solve this problem by providing an air space in front of the diver’s eyes. The refraction error created by the water is mostly corrected as the light travels from water to air through a flat lens, except that objects appear approximately 34% bigger and 25% closer in salt water than they actually are. Therefore total field-of-view is significantly reduced and eye–hand coordination must be adjusted.

(This also affects underwater photography: a camera seeing through a flat port in its housing is affected in the same way as its user’s eye seeing through a flat mask viewport, and so its operator must focus for the apparent distance to target, not for the real distance.)

Divers who need corrective lenses to see clearly outside the water would normally need the same prescription while wearing a mask. Generic and custom corrective lenses are available for some two-window masks. Custom lenses can be bonded onto masks that have a single front window or two windows.

A “double-dome-ported mask” has curved viewports in an attempt to cure these faults, but this causes a refraction problem of its own.

Commando frogmen concerned about revealing their position when light reflects from the glass surface of their diving masks may instead use special contact lenses to see underwater.

As a diver descends, he must periodically exhale through his nose to equalize the internal pressure of the mask with that of the surrounding water. Swimming goggles are not suitable for diving because they only cover the eyes and thus do not allow for equalization. Failure to equalise the pressure inside the mask may lead to a form of barotrauma known as mask squeeze.

Light underwater

Water preferentially absorbs red light, and to a lesser extent, yellow and green light, so the color that is least absorbed by water is blue light.

Table of Light Absorption in pure water
Color	Average wavelength	Approximate depth of total absorption
Ultraviolet	300 nm	25 m
Violet	400 nm	100 m
Blue	475 nm	275 m
Green	525 nm	110 m
Yellow	575 nm	50 m
Orange	600 nm	20 m
Red	685 nm	5 m
Infra-red	800 nm	3 m

Underwater communication

Two divers giving the sign that they are “OK” on a wreck in the Dominican Republic.

A diver cannot talk underwater unless he is wearing a full-face mask, but divers can communicate, using hand signals.

Table of Hand Signals

No.	Signal	Meaning	Comment
1.	Hand raised, fingers pointed up, palm to receiver.	STOP	Transmitted in the same way as a traffic police officer’s STOP
2.	Thumb extended downward from clenched fist.	GO DOWN or GOING DOWN
3.	Thumb extended upward from clenched fist.	GO UP or GOING UP
4.	Thumb and forefinger making a circle with three remaining fingers extended (if possible).	OK! or OK?	Divers wearing mittens may not be able to extend 3 remaining fingers distinctly.
5.	Two arms extended overhead with finger tips touching above head to make a large O shape.	OK! or OK?	A diver with only one free arm may make this signal by extending that arm overhead with finger tips touching top of head to make the O shape. Signal is for long-range use.
6.	Hand flat, fingers together, palm down, thumb sticking out, then hand rocking back and forth on axis of forearm.	SOMETHING IS WRONG	This is the opposite of OK! The signal does not indicate emergency.
7.	Hand waving over head (may also thrash hand on water).	DISTRESS	Indicates immediate aid required.
8.	Fist pounding on chest.	LOW ON AIR	Indicates signaler’s air supply is reduced.
9.	Hand slashing or chopping throat.	OUT OF AIR	Indicates that the signaler cannot breathe.
10.	Clenched fist on arm extended in direction of danger.	DANGER

All signals are to be answered by the receivers repeating the signal as sent. When answering signals 7 & 9, the receiver should approach to offer aid to signaler.

Hazards of scuba diving

According to a 1970 North American study, diving was (on a man-hours based criteria) 96 times more dangerous than driving an automobile. According to a 2000 Japanese study, every hour of recreational diving is 36 to 62 times riskier than automobile driving. A big difference between the risks of driving and diving is that the diver is less at risk from fellow divers than the driver is from other drivers.

Injuries due to changes in pressure

Divers must avoid injuries caused by changes in pressure. The weight of the water column above the diver causes an increase in pressure in proportion to depth, in the same way that the weight of the column of atmospheric air above the surface causes a pressure of 101.3 kPa (14.7 pounds-force per square inch) at sea level. This variation of pressure with depth will cause compressible materials and gas filled spaces to tend to change volume, which can cause the surrounding material or tissues to be stressed, with the risk of injury if the stress gets too high. Pressure injuries are called barotrauma and can be quite painful, even potentially fatal – in severe cases causing a ruptured lung, eardrum or damage to the sinuses. To avoid barotrauma, the diver equalizes the pressure in all air spaces with the surrounding water pressure when changing depth. The middle ear and sinus are equalized using one or more of several techniques, which is referred to as clearing the ears.

The scuba mask (half-mask) is equalized during descent by periodically exhaling through the nose. During ascent it will automatically equalise by leaking excess air round the edges. A helmet or full face mask will automatically equalise as any pressure differential will either vent through the exhaust valve or open the demand valve and release air into the low pressure space.

If a drysuit is worn, it must be equalized by inflation and deflation, much like a buoyancy compensator. Most dry suits are fitted with an auto-dump valve, which, if set correctly, and kept at the high point of the diver by good trim skills, will automatically release gas as it expands and retain a virtually constant volume during ascent. During descent the dry suit must be inflated manually.

Although there are many dangers involved in scuba diving, divers can decrease the risks through proper procedures and appropriate equipment. The requisite skills are acquired by training and education, and honed by practice. Open-water certification programs highlight diving physiology, safe diving practices, and diving hazards, but do not provide the diver with sufficient practice to become truly adept.

Effects of breathing high pressure gas

Decompression sickness

The prolonged exposure to breathing gases at high partial pressure will result in increased amounts of non-metabolic gases, usually nitrogen and/or helium, (referred to in this context as inert gases) dissolving in the bloodstream as it passes through the alveolar capillaries, and thence carried to the other tissues of the body, where they will accumulate until saturated. This saturation process has very little immediate effect on the diver. However when the pressure is reduced during ascent, the amount of dissolved inert gas that can be held in stable solution in the tissues is reduced. This effect is described by Henry’s Law.

As a consequence of the reducing partial pressure of inert gases in the lungs during ascent, the dissolved gas will be diffused back from the bloodstream to the gas in the lungs and exhaled. The reduced gas concentration in the blood has a similar effect when it passes through tissues carrying a higher concentration, and that gas will diffuse back into the bloodsteam, reducing the loading of the tissues.

As long as this process is gradual, all will go well and the diver will reduce the gas loading by diffusion and perfusion until it eventually re-stabilises at the current saturation pressure. The problem arises when the pressure is reduced more quickly than the gas can be removed by this mechanism, and the level of supersaturation rises sufficiently to become unstable. At this point, bubbles may form and grow in the tissues, and may cause damage either by distending the tissue locally, or blocking small blood vessels, shutting off blood supply to the downstream side, and resulting in hypoxia of those tissues.

This effect is called decompression sickness or ‘the bends’, and must be avoided by reducing the pressure on the body slowly while ascending and allowing the inert gases dissolved in the tissues to be eliminated while still in solution. This process is known as “off-gassing”, and is done by restricting the ascent (decompression) rate to one where the level of supersaturation is not sufficient for bubbles to form. This is done by controlling the speed of ascent and making periodic stops to allow gases to be eliminated. The procedure of making stops is called staged decompression, and the stops are called decompression stops. Decompression stops that are not computed as strictly necessary are called safety stops, and reduce the risk of bubble formation further. Dive computers or decompression tables are used to determine a relatively safe ascent profile, but are not completely reliable. There remains a statistical possibility of decompression bubbles forming even when the guidance from tables or computer has been followed exactly.

Decompression sickness must be treated as soon as practicable. Definitive treatment is usually recompression in a recompression chamber with hyperbaric oxygen treatment. Exact details will depend on severity and type of symptoms, response to treatment, and the dive history of the casualty. Administering enriched-oxygen breathing gas or pure oxygen to a decompression sickness stricken diver on the surface is a good form of first aid for decompression sickness, although death or permanent disability may still occur.

Nitrogen narcosis

Nitrogen narcosis or inert gas narcosis is a reversible alteration in consciousness producing a state similar to alcohol intoxication in divers who breathe high pressure gas at depth. The mechanism is similar to that of nitrous oxide, or “laughing gas,” administered as anesthesia. Being “narced” can impair judgment and make diving very dangerous. Narcosis starts to affect some divers at 66 feet (20 m). At this depth, narcosis manifests itself as a slight giddiness. The effects increase drastically with the increase in depth. Almost all divers are able to notice the effects by 132 feet (40 meters). At these depths divers may feel euphoria, anxiety, loss of coordination and lack of concentration. At extreme depths, hallucinogenic reaction and tunnel vision can occur. Jacques Cousteau famously described it as the “rapture of the deep”. Nitrogen narcosis occurs quickly and the symptoms typically disappear during the ascent, so that divers often fail to realize they were ever affected. It affects individual divers at varying depths and conditions, and can even vary from dive to dive under identical conditions. However, diving with trimix or heliox dramatically reduces the effects of inert gas narcosis.

Oxygen toxicity

Oxygen toxicity occurs when oxygen in the body exceeds a safe partial pressure (PPO₂). In extreme cases it affects the central nervous system and causes a seizure, which can result in the diver spitting out his regulator and drowning. While the exact limit is idiomatic, it is generally recognized that Oxygen toxicity is preventable if one never exceeds an oxygen partial pressure of 1.4 bar. For deep dives—generally past 180 feet (55 m), divers use “hypoxic blends” containing a lower percentage of oxygen than atmospheric air. For more information, see oxygen toxicity.

Hazards of the diving environment

Loss of body heat

Dry suit for reducing exposure

Water conducts heat from the diver 25 times better than air, which can lead to hypothermia even in mild water temperatures. Symptoms of hypothermia include impaired judgment and dexterity, which can quickly become deadly in an aquatic environment. In all but the warmest waters, divers need the thermal insulation provided by wetsuits or drysuits.

In the case of a wetsuit, the suit is designed to minimize heat loss. Wetsuits are usually made of neoprene that has small closed gas cells, generally nitrogen, trapped in it during the manufacturing process. The poor thermal conductivity of this expanded cell neoprene means that wetsuits reduce loss of body heat by conduction to the surrounding water. The neoprene, and to a larger extent the nitrogen gas, in this case acts as an insulator. The effectiveness of the insulation is reduced when the suit is compressed due to depth, as the nitrogen filled bubbles are then smaller and conduct heat better.

The second way in which wetsuits reduce heat loss is to trap a thin layer of water between the diver’s skin and the insulating suit itself. Body heat then heats the trapped water. Provided the wetsuit is reasonably well-sealed at all openings (neck, wrists, ankles zippers and overlaps with other suit components), this reduces flow of cold water over the surface of the skin, and thereby reduces loss of body heat by convection, which helps keep the diver warm (this is the principle employed in the use of a “Semi-Dry” wetsuit)

Spring suit (short legs and sleeves) and steamer (full legs and sleeves)

In the case of a drysuit, it does exactly what the name implies: keeps a diver dry. The suit is waterproof and sealed so that frigid water cannot penetrate the suit. Drysuit undergarments are usually worn under a drysuit to keep a layer of air inside the suit for better thermal insulation. Some divers carry an extra gas bottle dedicated to filling the dry suit. Usually this bottle contains argon gas, because of its better insulation as compared with air. Dry suits should not be inflated with gases containing helium as it is a good thermal conductor.

Drysuits fall into two main categories: neoprene and membrane; both systems have their good and bad points but generally their thermal properties can be reduced to:

Membrane or Shell drysuits: usually a trilaminate construction; owing to the thinness of the material (around 1 mm), these require an undersuit, usually of high insulation value if diving in cooler water.
Neoprene drysuits: a similar construction to wetsuits; these are often considerably thicker (7–8 mm) and have sufficient insulation to allow a lighter-weight undersuit (or none at all); however on deeper dives the neoprene can compress to as little as 2 mm thus losing a proportion of its insulation. Compressed or crushed neoprene may also be used (where the neoprene is pre-compressed to 2–3 mm) which avoids the variation of insulating properties with depth. These drysuits function more like a membrane suit.

Injuries due to contact with the solid surroundings

Diving suits also help prevent the diver’s skin being damaged by rough or sharp underwater objects, marine animals, coral, or metal debris commonly found on shipwrecks.

Hazards inherent in the diver

Diver behaviour and competence

Inadequate learning or practice of critical safety skills may result in the inability to deal with minor incidents, which consequently may develop into major incidents.

Overconfidence can result in diving in conditions beyond the diver’s competence, with high risk of accident due to inability to deal with known environmental hazards.

Inadequate strength or fitness for the conditions can result in inability to compensate for difficult conditions even though the diver may be well versed at the required skills, and could lead to over-exertion, overtiredness, stress injuries or exhaustion.

Peer pressure can cause a diver to dive in conditions where he may be unable to deal with reasonably predictable incidents.

Diving with an incompetent buddy can result in injury or death while attempting to deal with a problem caused by the buddy.

Overweighting can cause difficulty in neutralising and controlling buoyancy, and this can lead to uncontrolled descent, inability to establish neutral buoyancy, inefficient swimming, high gas consumption, poor trim, kicking up silt, difficulty in ascent and inability to control depth accurately for decompression.

Underweighting can cause difficulty in neutralising and controlling buoyancy, and consequent inability to achieve neutral buoyancy, particularly at decompression stops.

Diving under the influence of drugs or alcohol, or with a hangover may result in inappropriate or delayed response to contingencies, reduced ability to deal timeously with problems, leading to greater risk of developing into an accident, increased risk of hypothermia and increased risk of decompression sickness.

Use of inappropriate equipment and/or configuration can lead to a whole range of complications, depending on the details.

Diving longer and deeper safely

There are a number of techniques to increase the diver’s ability to dive deeper and longer:

Technical diving – diving deeper than 40 metres (130 ft), using mixed gases, and/or entering overhead environments (caves or wrecks)
Surface supplied diving – use of umbilical gas supply and diving helmets.
Saturation diving – long-term use of underwater habitats under pressure and a gradual release of pressure over several days in a decompression chamber at the end of a dive.

Scuba diver training and certification agencies

Diving lessons in Monterey Bay,California

Recreational scuba diving does not have a centralized certifying or regulatory agency, and is mostly self regulated. There are, however, several large diving organizations that train and certify divers and dive instructors, and many diving related sales and rental outlets require proof of diver certification from one of these organizations prior to selling or renting certain diving products or services.

The largest international certification agencies that are currently recognized by most diving outlets for diver certification include:

American Canadian Underwater Certifications (ACUC) (formerly Association of Canadian Underwater Councils) – originated in Canada in 1969 and expanded internationally in 1984
British Sub Aqua Club (BSAC) – based in the United Kingdom, founded in 1953 and is the largest dive club in the world
European Committee of Professional Diving Instructors (CEDIP) based in Europe since 1992
Confédération Mondiale des Activités Subaquatiques (CMAS), the World Underwater Federation
National Association of Underwater Instructors (NAUI) – based in the United States
Professional Diving Instructors Corporation (PDIC) – based in the United States
Professional Association of Diving Instructors (PADI) – based in the United States, largest recreational dive training and certification organization in the world
Scottish Sub Aqua Club (SSAC or ScotSAC) the National Governing Body for the sport of diving in Scotland.
International Training SDI, TDI & ERDi – based in the United States, TDI is the world’s largest technical diving agency, SDI is the recreational division focusing on new methods and online courses, and ERDi is the public safety component.
Scuba Schools International (SSI) – based in the United States with 35 Regional Centers and Area Offices around the globe.
YMCA Scuba – based in the United States, provided by Young Men’s Christian Association (YMCA) of the USA; discontinued on 31 December 2008.

Endurance Records

The current record for the longest continuous submergence using SCUBA gear was set by Mike Stevens of Birmingham, UK at the National Exhibition Centre, Birmingham, UK during the annual National Boat, Caravan and Leisure Show between February 14 and February 23, 1986. Mike Stevens was continuously submerged for 212.5 hours beating his own previous record of 121.5 hours. The record was ratified by the Guinness Book of Records. Mike used a standard regulator and mask and wore only a t-shirt and swim shorts and an 8 pound weight belt, he had no surface breaks during the 212.5 hours. A team of divers attended Mike throughout the dive. The team was led by Diving Officer Trevor Parkes. The dive raised £10,000 for the Birmingham Children’s Hospital from donations by the public.

Source :

http://en.wikipedia.org/wiki/Scuba_diving

Scuba diving

SEA URCHINS

11 January 2013 08.17 / Leave a comment

Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. There are c. 950 species of echinoids inhabiting all oceans from the intertidal to 5000 meters deep. Their shell, or “test”, is round and spiny, typically from 3 to 10 cm (1.2 to 3.9 in) across. Common colors include black and dull shades of green, olive, brown, purple, and red. They move slowly, feeding mostly on algae. Sea otters, wolf eels, triggerfish, and other predators feed on them. Their “roe” (actually the gonads) is a delicacy in many cuisines.

The name “urchin” is an old name for the round spiny hedgehogs that sea urchins resemble.

Taxonomy

Sea urchins are members of the phylum Echinodermata, which also includes sea stars, sea cucumbers, brittle stars, and crinoids. Like other echinoderms, they have fivefold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive “tube feet”. The symmetry is not obvious in the living animal, but is easily visible in the dried test. Echinodermatemeans “spiny skin” in Greek.

Specifically, the term “sea urchin” refers to the “regular echinoids”, which are symmetrical and globular. The term includes several different taxonomic groups: the order Echinoida, the order Cidaroida or “slate-pencil urchins”, which have very thick, blunt spines, and others. Besides sea urchins, the class Echinoidea also includes three groups of “irregular” echinoids: flattened sand dollars, sea biscuits, and heart urchins.

Together with sea cucumbers (Holothuroidea), they make up the subphylum Echinozoa, which is characterized by a globoid shape without arms or projecting rays. Sea cucumbers and the irregular echinoids have secondarily evolved diverse shapes. Although many sea cucumbers have branched tentacles surrounding the oral opening, these have originated from modified tube feet and are not homologous to the arms of the crinoids, sea stars, and brittle stars.

Anatomy

Urchins typically range in size from 6 to 12 cm (2.4 to 4.7 in), although the largest species can reach up to 36 cm (14 in).

Fivefold symmetry

Like other echinoderms, sea urchins are bilaterans. Their early larvae have bilateral symmetry, but they develop fivefold symmetry as they mature. This is most apparent in the “regular” sea urchins, which have roughly spherical bodies, with five equally sized parts radiating out from their central axes. Several sea urchins, however, including the sand dollars, are oval in shape, with distinct front and rear ends, giving them a degree of bilateral symmetry. In these urchins, the upper surface of the body is slightly domed, but the underside is flat, while the sides are devoid of tube feet. This “irregular” body form has evolved to allow the animals to burrow through sand or other soft materials.

Organs and test

The lower half of a sea urchin’s body is referred to as the oral surface, because it contains the mouth, while the upper half is the aboral surface. The internal organs are enclosed in a hard test composed of fused plates of calcium carbonate covered by a thin dermis and epidermis. The test is rigid, and divides into five ambulacral grooves separated by five interambulacral areas. Each of these areas consists of two rows of plates, so the test includes 20 rows in total. The plates are covered in rounded tubercles, to which the spines are attached. The inner surface of the test is lined by peritoneum.

Feet

Urchins have tube feet, which arise from the five ambulacral grooves. Tube feet are moved by a water vascular system. This water vascular system works through hydraulic pressure, allowing the Sea Urchin to pump water into and out of the tube feet, enabling it to locomote.

Mouth/anus

The mouth lies in the centre of the oral surface in regular urchins, or towards one end in irregular urchins. It is surrounded by lips of softer tissue, with numerous small, bony pieces embedded in it. This area, called the peristome, also includes five pairs of modified tube feet and, in many species, five pairs of gills. On the upper surface, opposite the mouth, is a region termed the periproct, which surrounds the anus. The periproct contains a variable number of hard plates, depending on species, one of which contains the madreporite.

Endoskeleton

The sea urchin builds its spicules, the sharp crystalline “bones” that constitute the animal’s endoskeleton, in the larval stage. The fully formed spicule is composed of a single crystal with an unusual morphology. It has no facets, and within 48 hours of fertilization assumes a shape that looks very much like the Mercedes-Benz logo.

In other echinoderms, the endoskeleton is associated with a layer of muscle that allows the animal to move its arms or other body parts. This is entirely absent in sea urchins, which are unable to move in this way.

Spines

Living Sea Urchin in Natural Habitat

The spines, long and sharp in some species, protect the urchin from predators. They inflict a painful wound when they penetrate human skin, but are not dangerous. It is not clear if the spines are venomous (unlike the pedicellariae between the spines, which are venomous).

Typical sea urchins have spines that are 1 to 3 cm (0.39 to 1.2 in) in length, 1 to 2 mm (0.039 to 0.079 in) thick, and not terribly sharp. Diadema antillarum, familiar in the Caribbean, has thin, potentially dangerous spines that can reach 10 to 30 cm (3.9 to 12 in) long.

Reproductive organs

Male flower sea urchin (Toxopneustes roseus) releasing milt, November 1, 2011 Lalo Cove, Sea of Cortez

Sea urchins are dioecious, having separate male and female sexes, although distinguishing the two is not easy, except for their locations on the sea bottom. Males generally choose an elevated and exposed location, so their milt can be broadcast by sea currents. Females generally choose a low-lying location in sea bottom crevices, presumably so the tiny larvae can have better protection from predators. Indeed, very small sea urchins are found hiding beneath rocks. Regular sea urchins have five gonads, lying underneath the interambulacral regions of the test, while the irregular forms have only four, with the hindmost gonad being absent. Each gonad has a single duct rising from the upper pole to open at a gonopore lying in one of the genital plates surrounding the anus. The gonads are lined with muscles underneath the peritoneum, and these allow the animal to squeeze its gametes through the duct and into the surrounding sea water where fertilization takes place.

Physiology

Digestion

The mouth of most sea urchins is made up of five calcium carbonate teeth or jaws, with a fleshy, tongue-like structure within. The entire chewing organ was known as Aristotle’s lantern (image), from Aristotle’s description in his History of Animals:

…the urchin has what we mainly call its head and mouth down below, and a place for the issue of the residuum up above. The urchin has, also, five hollow teeth inside, and in the middle of these teeth a fleshy substance serving the office of a tongue. Next to this comes the esophagus, and then the stomach, divided into five parts, and filled with excretion, all the five parts uniting at the anal vent, where the shell is perforated for an outlet… In reality the mouth-apparatus of the urchin is continuous from one end to the other, but to outward appearance it is not so, but looks like a horn lantern with the panes of horn left out. (Tr. D’Arcy Thompson)

However, this has recently been proven to be a mistranslation. Aristotle’s lantern is actually referring to the whole shape of sea urchins, which look like the ancient lamps of Aristotle’s time.

Recent research has shown the sea urchin’s teeth are self-sharpening; it can chew through stone.

Heart urchins are unusual in not having a lantern. Instead, the mouth is surrounded by cilia that pull strings of mucus-containing food particles towards a series of grooves around the mouth.

The lantern, where present, surrounds both the mouth cavity and the pharynx. At the top of the lantern, the pharynx opens into the esophagus, which runs back down the outside of the lantern, to join the small intestine and a single caecum. The small intestine runs in a full circle around the inside of the test, before joining the large intestine, which completes another circuit in the opposite direction. From the large intestine, a rectum ascends towards the anus. Despite the names, the small and large intestines of sea urchins are in no way homologous to the similarly named structures in vertebrates.

Digestion occurs in the intestine, with the caecum producing further digestive enzymes. An additional tube, called the siphon, runs beside much of the intestine, opening into it at both ends. It may be involved in resorption of water from food.

Circulation

Sea urchins possess both a water vascular system and a hemal system, the latter containing blood. However, the main circulatory fluid fills the general body cavity, or coelom. This fluid contains phagocytic coelomocytes, which move through the vascular and hemal systems. The coelomocytes are an essential part of blood clotting, but also collect waste products and actively remove them from the body through the gills and tube feet.

Respiration

Most sea urchins possess five pairs of external gills, located around the mouth. These thin-walled projections of the body cavity are the main organs of respiration in those urchins that possess them. Fluid can be pumped through the gills’ interiors by muscles associated with the lantern, but this is not continuous, and occurs only when the animal is low on oxygen. Tube feet can also act as respiratory organs, and are the primary sites of gas exchange in heart urchins and sand dollars, both of which lack gills.

Nervous system

The nervous system of sea urchins has a relatively simple layout. There is no true brain. The center is a large nerve ring encircling the mouth just inside the lantern. From the nerve ring, five nerves radiate underneath the radial canals of the water vascular system, and branch into numerous finer nerves to innervate the tube feet, spines, and pedicellariae.

Senses

Sea urchins are sensitive to touch, light, and chemicals. Although they do not have eyes or eye spots, recent research suggests their entire body might function as one compound eye. They also have statocysts, called spheridia, located within the ambulacral plates to help the animal remain upright.

Development

Ingression of primary mesenchyme cells

Sea urchin blastula

During early development, the sea urchin embryo undergoes 10 cycles of cell division, resulting in a single epithelial layer enveloping a blastocoel. The embryo must then begin gastrulation, a multipart process which involves the dramatic rearrangement and invagination of cells to produce the three germ layers.

The first step of gastrulation is the epithelial-to-mesenchymal transition and ingression of primary mesenchyme cells into the blastocoel. Primary mesenchyme cells, or PMCs, are located in the vegetal plate specified to become mesoderm. Prior to ingression, PMCs exhibit all the features of other epithelial cells that comprise the embryo. Cells of the epithelium are bound basally to a laminal matrix and apically to an extraembryonic matrix. The apical microvilli of these cells reach into the hyaline layer, a component of the extraembryonic matrix. Neighboring epithelial cells are also connected to each other through apical junctions, protein complexes containing adhesion molecules, such as cadherins, linked to catenins.

Prospective PMCs at vegetal plate

As PMCs begin to undergo an epithelial-to-mesenchymal transition, the lamina which binds them dissolves to begin the mechanical release of the cells. Expression of the membrane protein that binds laminin, integrin, also becomes irregular at the beginning of ingression. The microvilli which secure PMCs to the hyaline layer shorten, as the cells reduce their affinity for the extraembryonic matrix. These cells concurrently increase their affinity for other components of the basal matrix, such as fibronectin, in part driving the movement of cells inward. The apical junctions which bind PMCs to their neighboring epithelial cells become disrupted during this transition, and are absent in cells that have fully ingressed into the blastocoel. Because staining for cadherins and catenins in ingressing cells decreases and develops as intracellular accumulations, apical junctions are thought to be cleared by endocytosis during ingression.

Once the PMCs disrupt all attachment to their former location, the cells themselves change their morphology by contracting their apical surfaces, apical constriction, and enlarging their basal surfaces, thus acquiring a “bottle cell” phenotype. Cytoskeletal rearrangements mediate the shape changes of PMCs; though the cytoskeleton assists in the mechanics of ingression, other mechanisms drive the process. Experimentally disrupting microtubule dynamics in the species Strongylocentrotus pupuratus by applying colchicine stalls the ingression of PMCs, but does not inhibit it. Similarly, experimentally disrupting actin-myosin contraction using inhibitors slows down ingression, but does not arrest the process.

Epithelial-to-mesenchymal transition and ingression of PMCs

The morphogenetic movements of the PMCs are an autonomous cellular behavior. Experimentally grafting PMCs into heterotopic tissue does not prevent the cells from ingressing. In studies where PMCs are cultured in insolation, the cells were observed to gain affinity for fibronectin and simultaneously lose affinity for extraembryonic matrix, independent of the embryonic environment.

Life history

At first glance, sea urchins often appear sessile, i.e. incapable of moving. Sometimes, the most visible life sign is the spines, which attach to ball-and-socket joints and can point in any direction. In most urchins, touch elicits a prompt reaction from the spines, which converge toward the touch point. Sea urchins have no visible eyes, legs, or means of propulsion, but can move freely over hard surfaces using adhesive tube feet, working in conjunction with the spines.

Sea urchin off the coast of Veracruz, Mexico

Reproduction

In most cases, the female Sea Urchin’s eggs float freely in the sea, but some species hold onto them with their spines, affording them a greater degree of protection. The fertilized egg, once met with the free floating sperm released by males, develops into a free-swimming blastula embryo in as little as 12 hours. Initially a simple ball of cells, the blastula soon transforms into a cone-shaped echinopluteus larva. In most species, this larva has 12 elongated arms. The arms are lined with bands of cilia that capture food particles and transport them to the mouth. In a few species, the blastula contains supplies of nutrient yolk and lacks arms, since it has no need to feed.

It may take several months for the larva to complete its development, which begins with the formation of the test plates around the mouth and anus. Soon the larva sinks to the bottom and metamorphoses into adult form in as little as one hour. In some species, adults reach their maximum size in about five years.

Ecology

Echinothrix calamaris, a species of sea urchin: The sphere in the middle of a sea urchin is its anus.

Sea urchins feed mainly on algae, but can also feed on sea cucumbers and a wide range of invertebrates, such as mussels, polychaetes, sponges, brittle stars and crinoids. Population densities vary by habitat, with more dense populations being found in barren areas as compared to kelp stands. Even in these barren areas, greatest densities are also found in shallow water. Populations are also generally found in deeper water if wave action is present. Densities also decrease in winter when storms cause them to seek protection in cracks and around larger underwater structures. The shingle urchin (Colobocentrotus atratus), which lives on exposed shorelines, is particularly resistant to wave action.

Sea urchins are some of the favorite foods of sea otters, and are also the main source of nutrition for wolf eels. Left unchecked, urchins devastate their environments, creating what biologists call an urchin barren, devoid of macroalgae and associated fauna. Sea otters have re-entered British Columbia, dramatically improving coastal ecosystem health.

Evolutionary history

Fossil heart urchin Lovenia woodsi from the Pliocene of Australia

The earliest echinoid fossils date to the upper part of the Ordovician period (circa450 MYA), and the taxon has survived to the present as a successful and diverse group of organisms. Spines may be present in well-preserved specimens, but usually only the test remains. Isolated spines are common as fossils. Some echinoids (such as Tylocidaris clavigera, from the Cretaceous period’s English Chalk Formation) had very heavy, club-shaped spines that would be difficult for an attacking predator to break through and make the echinoid awkward to handle. Such spines simplify walking on the soft sea floor.

Cretaceous heart urchins from Castle Hayne quarry, North Carolina, USA

Most of the fossil echinoids from the Paleozoic era are incomplete, consisting of isolated spines and small clusters of scattered plates from crushed individuals, mostly in Devonian and Carboniferous rocks. The shallow-water limestones from the Ordovician and Silurian periods of Estonia are famous for echinoids. Paleozoic echinoids probably inhabited relatively quiet waters. Because of their thin tests, they would certainly not have survived in the wave-battered coastal waters inhabited by many modern echinoids. During the upper part of the Carboniferous period, a marked decline in echinoid diversity occurred, and this trend continued to the Permian period. They neared extinction at the end of the Paleozoic era, with just six species known from the Permian period. Only two lineages survived this period’s massive extinction and into the Triassic: the genus Miocidaris, which gave rise to modern cidaroida (pencil urchins), and the ancestor that gave rise to the euechinoids. By the upper part of the Triassic period, their numbers began to increase again. Cidaroids have changed very little since the Late Triassic and are today considered to be living fossils.

Two saddle wrasses, Thalassoma duperrey, feeding on a sea urchin

The euechinoids, on the other hand, diversified into new lineages throughout the Jurassic and into the Cretaceous periods, and from them emerged the first irregular echinoids (superorder Atelostomata) during the early Jurassic, and later the other superorder (Gnathostomata) of irregular urchins, which evolved independently. These superorders today represent 47% of all extant species of echinoids because of their adaptive breakthroughs, which allowed them to exploit habitats and food sources unavailable to regular echinoids. During the Mesozoic and Cenozoic eras, the echinoids flourished. Most echinoid fossils are often abundant in the restricted localities and formations where they occur. An example of this is Enallaster, which exists by the thousands in certain outcrops of limestone from the Cretaceous period in Texas. Many fossils of the Late Jurassic Plesiocidaris still have the spines attached.

Some echinoids, such as Micraster, which is found in the Cretaceous period Chalk Formation of England and France, serve as zone or index fossils. Because they evolved rapidly, they aid geologists in dating the surrounding rocks. However, most echinoids are not abundant enough and are of too limited range to serve as zone fossils.

In the early Tertiary (circa 65 to 1.8 MYA), sand dollars (order Clypeasteroida) arose. Their distinctive, flattened tests and tiny spines were adapted to life on or under loose sand. They form the newest branch on the echinoid tree.

Relation to humans

In biology

Sea urchins are traditional model organisms in developmental biology. This use originated in the 1800s, when their embryonic development became easily viewed by microscopy. Sea urchins were the first species in which sperm cells were proven to fertilize ova.

The recent sequencing of the sea urchin genome established homology between sea urchin and vertebrate immune system-related genes. Sea urchins code for at least 222 toll-like receptor genes and over 200 genes related to the nod-like-receptor family found in vertebrates. This increases its usefulness as a valuable model organism for studying the evolution of innate immunity.

Sea urchin (uni) served Japanese style as sashimi, with a dab of wasabi

Japanese uni-ikura don, sea urchin egg and salmon egg donburi

As food

The gonads of both male and female sea urchins, usually called sea urchin roe or corals, are culinary delicacies in many parts of the world.

In cuisines around the Mediterranean, Paracentrotus lividus is often eaten raw, with lemon., and known as ricci on Italian menus where it is sometimes used in pasta sauces. It can also flavour omelettes, scrambled eggs, fish soup, mayonnaise, béchamel sauce for tartlets, the boullie for a soufflé, or Hollandaise sauce to make a fish sauce. In Chilean cuisine, it is served raw with lemon, onions, and olive oil.

Though the edible Strongylocentrotus droebachiensis is found in the North Atlantic, it is not widely eaten. However, sea urchins (called uutuk in Alutiiq) are commonly eaten by the Alaska Native population around Kodiak Island. It is commonly exported, mostly to Japan. It was formerly a delicacy in the Orkney Islands, used instead of butter.

In the West Indies, slate pencil urchins are eaten.

On the Pacific Coast of North America, Strongylocentrotus franciscanus was praised by Euell Gibbons; Strongylocentrotus purpuratus is also eaten.

In New Zealand, Evechinus chloroticus, known as kina in Maori, is a delicacy, traditionally eaten raw. Though New Zealand fishermen would like to export them to Japan, their quality is too variable.

In Japan, sea urchin is known as uni (ウニ^?), and its roe can retail for as much as A$450/kg; it is served raw as sashimi or in sushi, with soy sauce and wasabi. Japan imports large quantities from the United States, South Korea, and other producers. Japanese demand for sea urchin corals has raised concerns about overfishing.

Aquaria

Some species of sea urchins, such as the slate pencil urchin (Eucidaris tribuloides), are commonly sold in aquarium stores. Some species are effective at controlling hair algae, and they make good additions to an invertebrate tank.

Source :

http://en.wikipedia.org/wiki/Sea_urchin

Sea urchins

PACIFIC RAYA

Yearly Archives: 2013

DIVING REGULATOR

PEARL

DIVING CYLINDER

SWIMFINS

SNORKEL

DIVING MASK

OCTOPUS

BARNACLE

SCUBA DIVING

SEA URCHINS

Post navigation

Yearly Archives: 2013

Bagikan ini:

Bagikan ini:

Bagikan ini:

Bagikan ini:

Bagikan ini:

Bagikan ini:

Bagikan ini:

Bagikan ini:

Bagikan ini:

Bagikan ini:

Post navigation