Scuba set

A scuba diver in recreational diving gear

A scuba set is any breathing set that is carried entirely by an underwater diver and provides the diver with breathing gas at the ambient pressure. (Scuba is an anacronym for self-contained underwater breathing apparatus.) Although strictly speaking the scuba set is only the diving equipment which is required for providing breathing gas to the diver, general usage includes the harness by which it is carried, and those accessories which are integral parts of the harness and breathing apparatus assembly, such as a jacket or wing style buoyancy compensator and instruments mounted in a combined housing with the pressure gauge, and in the looser sense it is used to refer to any diving equipment used by the scuba diver, though this would more commonly be termed scuba equipment. Scuba is overwhelmingly the most common underwater breathing equipment used by recreational divers. A scuba set is also used in professional diving when it provides advantages, usually of mobility and range, over surface supplied systems.

Two basic configurations of scuba are in general use:

• Open-circuit-demand scuba expels exhaled air to the environment, and requires each breath be delivered to the diver on demand by a diving regulator, which reduces the pressure from the storage cylinder and supplies it through the demand valve when the diver reduces the pressure in the demand valve slightly during inhalation.
• Rebreather scuba recycles the exhaled gas, removes carbon dioxide, and compensates for the used oxygen before the diver is supplied with gas from the breathing circuit. The amount of gas lost from the circuit during each breathing cycle depends on the design of the rebreather and depth change during the breathing cycle. Gas in the breathing circuit is at ambient pressure, and stored gas is provided through regulators or injectors, depending on design.

Etymology

The word SCUBA was coined in 1952 by Major Christian Lambertsen who served in the U.S. Army Medical Corps from 1944 to 1946 as a physician. From 1939 to 1944 Lambertsen first called breathing apparatus an invention of his own, a rebreather. Later he called it "Laru" (acronym for Lambertsen Amphibious Respiratory Unit) and finally, in 1952, rejected the term "Laru" to only retain "SCUBA" ("Self-Contained Underwater Breathing Apparatus"). Lambertsen's invention (patented by himself several times from 1940 to 1989) was a rebreather and is different from the open circuit diving regulator and diving cylinder assemblies also commonly referred to as scuba.

Open circuit demand scuba is a 1943 invention by the Frenchmen Émile Gagnan and Jacques-Yves Cousteau, but in the English language Lambertsen's acronym has become common usage and the name Aqua-Lung, (often spelled "aqualung"), coined by Cousteau for use in English-speaking countries, has fallen into secondary use. As with radar, the acronym scuba has become so familiar that it is generally not capitalized and is treated as an ordinary noun. For example, it has been translated into the Welsh language as sgwba.

History

Early history

File:Lethbridge diving machine.jpg

John Lethbridge's diving dress, the first enclosed diving suit, built in the 1710s.

A scuba set is characterized by full independence of the surface as a breathing device, by transporting breathable air or other breathing gas along with the diver. Early attempts to reach this autonomy from the surface were made in the 18th century by the Englishman John Lethbridge, who invented and successfully built his own underwater diving machine in 1715.

An early diving dress using a compressed air reservoir was designed and built in 1771 by Sieur Fréminet from Paris. He conceived an autonomous breathing machine equipped with a reservoir, dragged behind the diver or mounted on his back. Fréminet called his invention machine hydrostatergatique and used it successfully for more than ten years in the harbors of Le Havre and Brest, as stated in the explanatory text of a 1784 painting.

The Frenchman Paul Lemaire d'Augerville built and used autonomous diving equipment in 1824, as did the British William H. James in 1825. James' helmet was made of "thin copper or sole of leather" with a plate window, and the air was supplied from an iron reservoir. A similar system was used in 1831 by the American Charles Condert, who died in 1832 while testing his invention in the East River at only 20 feet (6 m) deep.

The oldest known oxygen rebreather was patented on June 17, 1808 by Sieur Touboulic from Brest, mechanic in Napoleon's Imperial Navy, but there is no evidence of any prototype having been manufactured. This early rebreather design worked with an oxygen reservoir, the oxygen being delivered progressively by the diver himself and circulating in a closed circuit through a sponge soaked in limewater.

After having travelled to England and discovered William James' invention, the French physician Manuel Théodore Guillaumet, from Argentan (Normandy), patented in 1838 the oldest known regulator mechanism. Guillaumet's invention was air-supplied from the surface and was never mass-produced due to problems with safety. The oldest practical rebreather relates to the 1849 patent from the Frenchman Pierre Aimable De Saint Simon Sicard.

First successful scuba equipment

None of those inventions solved the problem of high pressure when compressed air must be supplied to the diver (as in modern regulators); they were mostly based on a constant-flow supply of the air. The compression and storage technology was not advanced enough to allow compressed air to be stored in containers at sufficiently high pressures to allow useful dive times.

By the turn of the twentieth century, two basic templates for a scuba had emerged; open-circuit scuba where the diver's exhaled gas is vented directly into the water, and closed-circuit scuba where the diver's carbon dioxide is filtered from unused oxygen, which is then recirculated.

Open-circuit scuba

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The Rouquayrol-Denayrouze apparatus was the first regulator to be mass-produced (from 1865 to 1965). In this picture the air reservoir presents its surface-supplied configuration.

The first important step for the development of scuba technology was the invention of the demand regulator. In 1864, the French engineers Auguste Denayrouze and Benoît Rouquayrol designed and patented their "Rouquayrol-Denayrouze diving suit" after adapting a pressure regulator and developing it for underwater use. This would be the first diving suit that could supply air to the diver on demand by adjusting the flow of air from the tank to meet the diver’s breathing and pressure requirements. The system still had to use surface supply, as the cylinders of the 1860s would not have been able to withstand the necessary high pressures.

The first open-circuit scuba system was devised in 1925 by Yves Le Prieur in France. Inspired by the simple apparatus of Maurice Fernez and the freedom it allowed the diver, he conceived an idea to make it free of the tube to the surface pump by using Michelin cylinders as the air supply, containing three litres of air compressed to 150 kg/cm2. The "Fernez-Le Prieur" diving apparatus was demonstrated at the swimming pool of Tourelles in Paris in 1926. The unit consisted of a cylinder of compressed air carried on the back of the diver, connected to a pressure regulator designed by Le Prieur adjusted manually by the diver, with two gauges, one for tank pressure and one for output (supply) pressure. Air was supplied continually to the mouthpiece and ejected through a short exhaust pipe fitted with a valve as in the Fernez design, however, the lack of a demand regulator and the consequent low endurance of the apparatus limited the practical use of LePrieur’s device.

Le Prieur's design was the first autonomous breathing device used by the first scuba diving clubs in history - Racleurs de fond founded by Glenn Orr in California in 1933, and Club des sous-l'eau founded by Le Prieur himself in Paris in 1935. Fernez had previously invented the noseclip, a mouthpiece (equipped with a one-way valve for exhalation) and diving goggles, and Yves le Prieur just joined to those three Fernez elements a hand-controlled regulator and a compressed-air cylinder. Fernez's goggles didn't allow a dive deeper than ten metres due to "mask squeeze", so, in 1933, Le Prieur replaced all the Fernez equipment (goggles, noseclip and valve) by a full face mask, directly supplied with constant flow air from the cylinder.

In 1942, during the German occupation of France, Jacques-Yves Cousteau and Émile Gagnan designed the first successful and safe open-circuit scuba, known as the Aqua-Lung. Their system combined an improved demand regulator with high-pressure air tanks. Émile Gagnan, an engineer employed by the Air Liquide company, miniaturized and adapted the regulator to use with gas generators, in response to constant fuel shortage that was a consequence of German requisitioning. Gagnan's boss, Henri Melchior, knew that his son-in-law Jacques-Yves Cousteau was looking for an automatic demand regulator to increase the useful period of the underwater breathing apparatus invented by Commander le Prieur, so he introduced Cousteau to Gagnan in December 1942. On Cousteau's initiative, the Gagnan's regulator was adapted to diving, and the new Cousteau-Gagnan patent was registered some weeks later in 1943.

In 1957, Eduard Admetlla i Lázaro used a modified version made by Nemrod of Aqua-Lung and beat the world record by descending to a depth of 100 metres (330 ft).

Closed-circuit scuba

Main article: Rebreather

Henry Fleuss (1851-1932) improved the rebreather technology.

The alternative concept, developed in roughly the same time frame was the closed-circuit scuba. The body consumes and metabolises only a small fraction of inhaled oxygen—the situation is even more wasteful of oxygen when the breathing gas is compressed as it is in scuba systems underwater. The rebreather therefore recycles the used oxygen, while constantly replenishing it from the supply so that the oxygen level does not get depleted. The apparatus also has to chemically remove the exhaled carbon dioxide, as a buildup of CO₂ levels would result in respiratory distress and hypercapnia.

The first commercially practical closed-circuit scuba was designed and built by the diving engineer Henry Fleuss in 1878, while working for Siebe Gorman in London. His self contained breathing apparatus consisted of a rubber mask connected to a breathing bag, with (estimated) 50-60% O₂ supplied from a copper tank and CO₂ scrubbed by rope yarn soaked in a solution of caustic potash; the system giving a duration of about three hours. Fleuss tested his device in 1879 by spending an hour submerged in a water tank, then one week later by diving to a depth of 5.5m in open water, upon which occasion he was slightly injured when his assistants abruptly pulled him to the surface.

His apparatus was first used under operational conditions in 1880 by the lead diver on the Severn Tunnel construction project, who was able to travel 1000 feet in the darkness to close several submerged sluice doors in the tunnel; this had defeated the best efforts of hard hat divers due to the danger of their air supply hoses becoming fouled on submerged debris, and the strong water currents in the workings.

Fleuss continually improved his apparatus, adding a demand regulator and tanks capable of holding greater amounts of oxygen at higher pressure. Sir Robert Davis, head of Siebe Gorman, perfected the oxygen rebreather in 1910 with his invention of the Davis Submerged Escape Apparatus, the first rebreather to be made in quantity. While intended primarily as an emergency escape apparatus for submarine crews, it was soon also used for diving, being a handy shallow water diving apparatus with a thirty-minute endurance, and as an industrial breathing set.

Davis Submerged Escape Apparatus being tested at the submarine escape test tank at HMS Dolphin, Gosport, 14 December 1942.

The rig comprised a rubber breathing/buoyancy bag containing a canister of barium hydroxide to scrub exhaled CO₂ and, in a pocket at the lower end of the bag, a steel pressure cylinder holding approximately 56 litres of oxygen at a pressure of 120 bar. The cylinder was equipped with a control valve and was connected to the breathing bag. Opening the cylinder's valve admitted oxygen to the bag and charged it to the pressure of the surrounding water. The rig also included an emergency buoyancy bag on the front of to help keep the wearer afloat. The DSEA was adopted by the Royal Navy after further development by Davis in 1927.

In 1912 the German firm Dräger developed their own version of standard diving dress without an umbilical. The air supply also came from a rebreather.

During the 1930s and all through World War II, the British, Italians and Germans developed and extensively used oxygen rebreathers in order to fit out the first frogmen. The British used the Davis apparatus for submarine escape, but they soon adapted it to equip their frogmen during World War II. Germans used the Dräger rebreathers, similarly first designed as submarine escape sets and only adapted for the use of frogmen during World War II.

Italians developed similar rebreathers for their own frogmen (the combat swimmers of the unit known as Decima Flottiglia MAS), especially the ARO, developed by the famous Pirelli society. In the USA Major Christian J. Lambertsen, who served in the U.S. Army Medical Corps from 1944 to 1946 as a physician, invented an underwater free-swimming oxygen rebreather in 1939, which was accepted by the Office of Strategic Services. In 1952 he patented a new modification of his apparatus, this time under the well known name of SCUBA. In spite of having coined the most common English word used for modern diving equipment, Lambertsen did not invent that equipment. After World War II, military frogmen of all countries continued to use rebreathers (since they do not make bubbles and thus are not visible from the surface).

Post WWII

Technical drawing of a Mistral Cousteau-type regulator (model of 1955) mounted on a diving cylinder. The regulator is formed by the ensemble of the mouthpiece and the regulator, joined on each of its sides by the two hoses. The rear of the regulator is connected to the high-pressure valve of the cylinder.

1. Hose
2. Mouthpiece
3. Valve
4. Harness
5. Backplate
6. Tank (also called cylinder)

Air Liquide started selling commercially the Cousteau-Gagnan regulator as of 1946 under the name of scaphandre Cousteau-Gagnan or CG45 ("C" for Cousteau, "G" for Gagnan and 45 for a new 1945 patent). The same year Air Liquide created a division called La Spirotechnique, to develop and sell regulators and other diving equipment. To sell his regulator in English-speaking countries Cousteau coined the Aqua-Lung label, which was first licensed to the U.S. Divers company (the American division of Air Liquide in the USA) and later sold alongside with La Spirotechnique and U.S. Divers to finally constitute the name of the company itself, Aqua-Lung/La Spirotechnique, nowadays sited in Carros, near Nice.

In 1948 the Cousteau-Gagnan patent was also licensed to Siebe Gorman of England, when Siebe Gorman was directed by Robert Henry Davis. Siebe Gorman was allowed to sell in Commonwealth countries, but had difficulty in meeting the demand and the U.S. patent prevented others from making the product. Ted Eldred of Melbourne, Australia, met this demand by developing the single hose regulator used today: the Porpoise. Ted sold his first Porpoise Model CA single hose scuba early in 1952. The first Porpoise scuba set design was a rebreather, but when a demonstration resulted in a diver passing out, Eldred began to develop the single-hose open-circuit scuba system. Its regulator's first stage and second stage had to be separated to avoid the Cousteau-Gagnan patent, which protected the double-hose scuba. In the process, Eldred also improved performance.

Before 1971 (when the Scubapro company commercialized the first stabiliser jacket), scuba sets were usually provided with a plain harness of shoulder straps and waist belt like on a rucksack. The waist belt buckles were usually quick-release, and shoulder straps sometimes had adjustable or quick release buckles. Many harnesses did not have a backpack plate, and the cylinders rested directly against the diver's back. The harnesses of many diving rebreathers made by Siebe Gorman included a large back-sheet of strong reinforced rubber.

Early scuba divers dived without any buoyancy aid. In emergency they had to jettison their weights. In the 1960s adjustable buoyancy life jackets (ABLJ) for aqualung-type scuba became available; one early make, since 1961, was Fenzy. The ABLJ is used for two purposes: one to adjust the buoyancy of the diver to compensate for loss of buoyancy (chiefly due to compression of neoprene wetsuit) and more importantly as a lifejacket that can be quickly inflated even at depth. It was put on before putting on the cylinder harness. The first were inflated with a small carbon dioxide cylinder, later with a small air cylinder. An extra feed from the first-stage regulator lets the lifejacket be controlled as a buoyancy aid. This invention in 1971 of the "direct system," by ScubaPro, resulted in what was called a stabilizer jacket or stab jacket, and is now increasingly known as a buoyancy compensator [device], or simply "BCD".

Types

Modern scuba sets are of two types:

• open-circuit (examples are those invented in 1864 by Rouquayrol and Denayrouze, in 1926 by Yves le Prieur or the Aqua-Lung invented to extend duration with a demand regulator in 1942/43 by Jacques Yves Cousteau and Émile Gagnan). Here the diver breathes in from the equipment and all the exhaled gas goes to waste in the surrounding water. This type of equipment is relatively simple, making it cheaper and more reliable. The two-hose design originally used was the one designed by Cousteau and Gagnan. The single-hose design generally used today was invented in Australia by Ted Eldred.
• closed-circuit/semi-closed circuit (also referred to as a rebreather). Here the diver breathes in from the set, and breathes back into the set, where the exhaled gas is processed to make it fit to breathe again. These existed before the open-circuit sets and are still used, but less so than open-circuit sets.

Both types of scuba provide a means of supplying air or other breathing gas, nearly always from a high pressure diving cylinder, and a harness to strap it to the diver's body. Most open-circuit scuba and some rebreathers have a demand regulator to control the supply of breathing gas. Some "semi-closed" rebreathers only have a constant-flow regulator, or occasionally a set of constant-flow regulators of various outputs.

Open circuit

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A diving cylinder with its various components

Open circuit demand scuba exhausts exhaled air to the environment, and requires each breath to be delivered to the diver on demand by a diving regulator, which reduces the pressure from the storage cylinder and supplies it through the demand valve when the diver reduces the pressure in the demand valve slightly during inhalation.

The essential subsystems of an open circuit scuba set are;

• diving cylinders, with cylinder valves, which may be interconnected by a manifold,
• a regulator mechanism to control gas pressure,
• a demand valve with mouthpiece, full-face mask or helmet, with supply hose, to control flow and deliver gas to the diver.
• an exhaust valve system to dispose of used gas,
• A harness or other method to attach the set to the diver.

Additional components which when present are considered part of the scuba set are;

• external reserve valves and their control rods or levers, (currently uncommon)
• submersible pressure gauges, (almost ubiquitous) and
• secondary (backup) demand valves (common).

The buoyancy compensator is generally assembled as an integrated part of the set, but is not technically part of the breathing apparatus.

The cylinder is usually worn on the back. "Twin sets" with two low capacity back-mounted cylinders connected by a high pressure manifold were more common in the 1960s than now for recreational diving, although larger capacity twin cylinders ("doubles") are commonly used by technical divers for increased dive duration and redundancy. At one time a firm called Submarine Products sold a sport air scuba set with three manifolded back-mounted cylinders. Cave and wreck penetration divers sometimes carry cylinders slung at their sides instead, allowing them to swim through more confined spaces.

Newspapers and television news often describe open circuit scuba wrongly as "oxygen" equipment, possibly by false analogy to airplane pilots' oxygen cylinders.

Constant flow scuba

Main article: Diving regulator § Constant flow

Constant flow scuba sets do not have a demand regulator; the breathing gas flows at a constant rate, unless the diver switches it on and off by hand. They use more air than demand regulated scuba. There were attempts at designing and using these for diving and for industrial use before the Cousteau-type aqualung became commonly available circa 1950. Examples were Charles Condert dress in the USA (as of 1831), "Ohgushi's Peerless Respirator" in Japan (a hand-controlled regulator, as of 1918), and Commandant le Prieur's hand-controlled regulator in France (as of 1926); see Timeline of diving technology.

Open circuit demand scuba

Main article: Diving regulator

This system consists of one or more diving cylinders containing breathing gas at high pressure, typically , connected to a diving regulator. The demand regulator supplies the diver with as much gas as needed at the ambient pressure.

This type of breathing set is sometimes called an aqualung. The word Aqua-Lung, which first appeared in the Cousteau-Gagnan patent, is a trademark, currently owned by Aqua Lung/La Spirotechnique.

Twin-hose demand regulator

Main article: Diving regulator § Twin-hose open circuit demand scuba regulators

Classic twin-hose Cousteau-type aqualung

This is the first type of diving demand valve to come into general use, and the one that can be seen in classic 1960s television scuba adventures, such as Sea Hunt. They were often use with manifolded twin cylinders.

All the stages of this type of regulator are in a large valve assembly mounted directly to the cylinder valve or manifold, behind the diver's neck. Two large bore corrugated rubber breathing hoses connect the regulator with the mouthpiece, one for supply and one for exhaust. The exhaust hose is used to return the exhaled air to the regulator, to avoid pressure differences due to depth variation between the exhaust valve and final stage diaphragm, which would cause a free-flow of gas, or extra resistance to breathing, depending on the diver's orientation in the water. In modern single-hose sets this problem is avoided by moving the second-stage regulator to the diver's mouthpiece. The twin-hose regulators came with a mouthpiece as standard, but a full-face diving mask was an option.

Single-hose regulator

Main article: Diving regulator § Single hose two-stage open circuit demand regulators

A single-hose regulator with 2nd stage, gauges, BC attachment, and dry suit hose mounted on a cylinder

Most modern open-circuit scuba sets have a diving regulator consisting of a first-stage pressure-reducing valve connected to the diving cylinder's output valve or manifold. This regulator reduces the pressure from the cylinder, which may be up to 300 bars (4,400 psi), to a lower pressure, generally between about 9 and 11 bar above the ambient pressure. A low-pressure hose links this with the second-stage regulator, or "demand valve", which is mounted on the mouthpiece. Exhalation occurs through a rubber one-way mushroom valve in the chamber of the demand valve, directly into the water quite close to the diver's mouth. Some early single hose scuba sets used full-face masks instead of a mouthpiece, such as those made by Desco and Scott Aviation (who continue to make breathing units of this configuration for use by firefighters).

Modern regulators typically feature high-pressure ports for pressure sensors of dive-computers and submersible pressure gauges, and additional low-pressure ports for hoses for inflation of dry suits and BC devices.

Secondary demand valve on a regulator

Main article: Diving regulator § Secondary demand valve (Octopus)

File:Plongee-StabilisateurDorsal 20090220 PlaqueLacasse.jpeg

Scuba harness with backplate and back mounted "wing" buoyancy compensator

• 1) DV/Regulator first stage
• 2) Cylinder valve
• 3) Shoulder straps
• 4) Buoyancy compensator bladder
• 5) Buoyancy compensator relief and lower manual dump valve
• 6) DV/Regulator second stages (primary and “octopus”)
• 7) Console (pressure gauge, depth gauge & compass)
• 8) Dry-suit inflator hose
• 9) Backplate
• 10) Buoyancy compensator inflator hose and inflation valve
• 11) Buoyancy compensator mouthpiece and manual dump valve
• 12) Crotch strap
• 13) Waist straps

Most recreational scuba sets have a secondary second-stage demand valve on a separate hose, a configuration called a "secondary", or "octopus" demand valve, "alternate air source", "safe secondary" or "safe-second". It is frequently yellow in color for high visibility, signaling that it is an emergency or backup device. It is often worn secured into a clip on the buoyancy compensator (BC) or a special friction plug attached in the diver's chest area, easily available to be grabbed by, or offered to, a second diver short of air. Other divers secure it while diving by sliding a loop of the hose into the shoulder strap cover of a jacket style BC, or suspend it under the chin on a break-away bungee loop known as a necklace. These methods also allow easy access and keep the secondary from dangling in the mud or snagging on the bottom, which is common when it is left to hang at the end of the hose. Some divers will store it in a BC pocket, but this reduces availability in an emergency. By providing a secondary demand valve the need to alternately breathe off the same mouthpiece when sharing air is eliminated. This reduces the stress on divers who are already in a stressful situation, and this in turn reduces air consumption during the rescue. Some diving instructors continue to teach single demand valve buddy-breathing as an obsolete but still useful technique; then they show the method that has superseded it, since availability of two second stages per diver is now assumed as standard in recreational scuba.

The original octopus idea was conceived by cave-diving pioneer Sheck Exley as a way for cave divers to share air while swimming single-file in a narrow tunnel, but has now become the standard in recreational diving.

Occasionally, the secondary second-stage is combined with the inflation and exhaust valve assembly of the buoyancy compensator device. This combination eliminates the need for a separate low pressure hose for the BC, though the low pressure hose for the combined use must be larger than standard BC inflation hoses, because demand on it will be higher if it is used for breathing.

No matter which configuration of secondary demand valve is used, many diver training agencies now suggest that a diver routinely offer another diver in trouble their "primary" demand valve, i.e., the one in their mouth, then switch to their own secondary demand valve. The idea behind this technique is that the primary demand valve is known to be working, and the diver donating the air has more time to sort out his/her own equipment after temporarily losing ability to breathe. In many instances, panicked out-of-air divers have grabbed the primary regulators out of the mouths of other divers, so changing breathing regulators suddenly in an out-of-air emergency becomes necessary for the rescue diver, in any case. With integrated DV/BC inflator designs, the secondary demand valve is at the end of the even shorter BC inflation hose than is the case with the conventional octopus demand valve hose, and the donor must retain access to it for buoyancy control, so deliberate use of the primary regulator and hose to help another diver becomes even more appropriate, and almost essential, with this configuration.

Cryogenic

There have been designs for a cryogenic open-circuit scuba which has liquid-air tanks instead of cylinders. Underwater cinematographer Jordan Klein, Sr. of Florida co-designed such a scuba in 1967, called "Mako", and made at least a prototype.

The Russian Kriolang (from Greek cryo- (= "frost" taken to mean "cold") + English "lung") was copied from Jordan Klein's "Mako" cryogenic open-circuit scuba. Janwillem Bech's rebreather site shows pictures of a Kriolang that was made in 1974. Its diving duration is likely several hours. It would have to be filled immediately before use.

Rebreathers

File:Inspiration front.JPg

An Inspiration rebreather seen from the front

Main article: Rebreather

A rebreather recirculates the breathing gas already used by the diver after replacing oxygen used by the diver and removing the carbon dioxide metabolic product. Rebreather diving is used by recreational, military and scientific divers where it can have advantages over open circuit scuba.

Since 80% or more of the oxygen remains in normal exhaled gas, and is thus wasted, rebreathers use gas very economically, making longer dives possible and special mixes cheaper to use at the cost of more complicated technology and more possible failure points. More stringent and specific training and greater experience is required to compensate for the higher risk involved. There are two main variants of rebreather — semi-closed circuit rebreathers, and fully closed circuit rebreathers, which include the subvariant of oxygen rebreathers.

The rebreather's economic use of gas, typically of oxygen per minute, allows dives of much longer duration for an equivalent gas supply than is possible with open circuit equipment where gas consumption may be ten times higher.

Oxygen rebreathers have a maximum safe operating depth of around 6 metres (20 ft), but several types of fully closed circuit rebreathers, when using a helium-based diluent, can be used deeper than 100 metres (330 ft). The main limiting factors on rebreathers are the duration of the carbon dioxide scrubber, which is generally at least 3 hours, increased work of breathing at depth, reliability of gas mixture control, and the requirement to be able to safely bail out at any point of the dive.

Rebreathers are generally used for scuba applications, but are also occasionally used for bailout systems for surface supplied diving.

The possible endurance of a rebreather dive is longer than an open-circuit dive, for similar weight and bulk of the set, if the set is bigger than the practical lower limit for rebreather size, and a rebreather can be more economical when used with expensive gas mixes such as heliox and trimix, but this may require a lot of diving before the break-even point is reached, due to the high initial and running costs of most rebreathers, and this point will be reached sooner for deep dives where the gas saving is more pronounced.

Breathing gases for scuba

Main article: Breathing gas

See also: Air, Nitrox, Trimix (breathing gas), Heliox, and Decompression (diving)

Until nitrox was widely accepted in the late 1990s, almost all recreational scuba used simple compressed and filtered air. Other gas mixtures currently used in scuba are intended to reduce the risk of decompression sickness and the severity of nitrogen narcosis.

Some divers use nitrox, which usually has a higher percentage of oxygen than air, often 32% or 36% in EAN32 and EAN36, respectively. This lets them stay underwater longer for the same decompression requirement as for air, because less nitrogen is absorbed into the body's tissues. The drawback to the higher oxygen content is that at higher than normal partial pressures, oxygen becomes toxic, so scuba divers generally limit their exposure to oxygen partial pressures of less than 1.6 bar, by limiting the maximum operating depth for the mixture.

Open-circuit scuba sets may supply various breathing gases, but rarely pure oxygen, except during shallow decompression stops in technical diving.

Diving cylinders

Main article: Diving cylinder

Gas cylinders used for scuba diving come in various sizes and materials and are typically designated by material — usually aluminium or steel, and size. In the U.S. the size is designated by their nominal capacity of the gas they contain when expanded to 1 atmosphere, 80, 100, 120 cubic feet, etc., with the most common being the "Aluminum 80". In most of the rest of the world the size is given as the actual internal volume of the cylinder, sometimes referred to as water capacity, as that is how it is measured and marked (WC) on the cylinder (10 liter, 12 liter, etc.).

Cylinder working pressure will vary according to the standard of manufacture, generally ranging from 200 bar (2,900 psi) up to 300 bar (4,400 psi).

An aluminium cylinder is thicker and bulkier than a steel cylinder of the same capacity and working pressure, as suitable aluminium alloys have lower tensile strength than steel, and is more buoyant although actually heavier out of the water, which means the diver would need to carry more ballast weight. Steel is also more often used for high pressure cylinders, which carry more air for the same internal volume.

The common method of blending nitrox by partial pressure requires that the cylinder is in "oxygen service", which means that the cylinder and cylinder valve have had any non-oxygen-compatible components replaced and any contamination by combustible materials removed by cleaning.

Diving cylinders are sometimes colloquially called "tanks", "bottles" or "flasks" although the proper technical term for them is "cylinder".

Harness configuration

See also: Backplate and wing and Side-mount

File:Bcd - stab.JPG

Stabilisor jacket harness

The scuba set can be carried by the diver in several ways. Most common for recreational diving is the stabilisor jacket harness, in which a single cylinder, or occasionally twins, is strapped to the jacket style buoyancy compensator which is used as the harness. Some jacket style harnesses allow a bailout or decompression cylinder to be sling mounted from D-rings on the harness. A bailout cylinder can also be strapped to the side of the main back-mounted cylinder.

File:Diving - scubadiver.JPG

Backplate and wing harness

Another popular configuration is the backplate and wing arrangement, which uses a back inflation buoyancy compensator bladder sandwiched between a rigid backplate and the main gas cylinder or cylinders. This arrangement is particularly popular with twin or double cylinder sets, and can be used to carry larger sets of three or four cylinders and most rebreathers. Additional cylinders for decompression can be sling mounted at the diver's sides.

File:Top view of sidemount diver DSC 0078 Photo by Pete Nawrocky.JPG

Top view of diver with sidemount harness

Side-mount harnesses support the cylinders by clipping them to D-rings at chest and hip on either or both sides, and the cylinders hang roughly parallel to the diver's torso when underwater. The harness usually includes a buoyancy compensator bladder. It is possible for a skilled diver to carry up to 3 cylinders on each side with this system.

File:Scuba set with integral bag PA111753.JPG

Diving with a scuba set with integral storage and transport bag

An unusual configuration which does not appear to have become popular is the integrated harness and storage container. These units comprise a bag which contains the buoyancy bladder and the cylinder, with a harness and regulator components which are stored in the bag and unfolded to the working position when the bag is unzipped. Some military rebreathers such as the Interspiro DCSC also store the breathing hoses inside the housing when not in use.

It is also possible to use a plain backpack harness to support the set, either with a horse-collar buoyancy compensator, or without any buoyancy compensator. This was the standard arrangement before the introduction of the buoyancy compensator, and is still used by dome recreational and professional divers when it suits the diving operation.

• Scuba set in integral carry bag PA081710.JPG

  Scuba set in integral carry bag

• Scuba set with integral bag PA111713.JPG

  Diver wearing scuba set with integral bag

Gas endurance of a scuba set

Main article: Scuba gas planning

See also: Diving cylinder § Gas calculations

Gas endurance of a scuba set is the time that the gas supply will last during a dive. This is influenced by the type of scuba set and the circumstances in which it is used.

Open circuit

The gas endurance of open circuit demand scuba depends on factors such as the capacity (volume of gas) in the diving cylinder, the depth of the dive and the breathing rate of the diver, which is dependent on exertion, fitness, physical size of the diver, and experience among other factors. New divers frequently consume all the air in a standard "aluminum 80" cylinder in 30 minutes or less on a typical dive, while experienced divers frequently dive for 60 to 70 minutes at the same average depth, using the same capacity cylinder, as they have learned more efficient diving techniques.

An open circuit diver whose breathing rate at the surface (atmospheric pressure) is 15 litres per minute will consume 3 x 15 = 45 litres of gas per minute at 20 metres. [(20 m/10 m per bar) + 1 bar atmospheric pressure] × 15 L/min = 45 L/min). If an 11-litre cylinder filled to 200 bar is used until there is a reserve of 17% there is (83% × 200 × 11) = 1826 litres. At 45 L/min the dive at depth will be a maximum of 40.5 minutes (1826/45). These depths and times are typical of experienced sport divers leisurely exploring a coral reef using 200 bar aluminum cylinders rented from a commercial sport diving operation in most tropical island or coastal resorts.

Semi-closed rebreather

A semi-closed circuit rebreather may have an endurance of about 3 to 10 times that of the equivalent open circuit dive, and is less affected by depth; gas is recycled but fresh gas must be constantly injected to replace at least the oxygen used, and any excess gas from this must be vented. Although it uses gas more economically, the weight of the rebreathing equipment means the diver carries smaller cylinders. Still, most semi-closed systems allow at least twice the duration of open circuit systems (around 2 hours) and are often limited by scrubber endurance.

Closed circuit rebreathers

An oxygen rebreather diver or a fully closed circuit rebreather diver consumes about 1 litre of oxygen per minute. Except during ascent or descent, the fully closed circuit rebreather that is operating correctly uses no or very little diluent. So, a diver with a 3-litre oxygen cylinder filled to 200 bar who leaves 25% in reserve will be able to do a 450-minute = 7.5 hour dive (3 L × 200 bar × 0.75 / 1). The life of the soda lime scrubber is likely to be less than this and so will be the limiting factor of the dive.

In practice, dive times for rebreathers are more often influenced by other factors, such as water temperature and the need for safe ascent (see Decompression (diving)), and this is generally also true for large capacity open circuit sets.

Underwater alternatives to scuba

There are alternative methods that a person can use to survive and function while underwater, including:

• free-diving - swimming underwater on a single breath of air.
• snorkeling - a form of free-diving where the diver's mouth and nose can remain underwater when breathing, because the diver is able to breathe at the surface through a short tube known as a snorkel.
• surface-supplied diving - originally used in professional diving for long or deep dives where an umbilical line connects the diver with the surface providing breathing gas, and sometimes warm water to heat the diving suit, and usually nowadays voice communications. Some tourist resorts now offer a surface-supplied diving arrangement, trademarked as Snuba, as an introduction to diving for the inexperienced. Using the same type of equipment as scuba diving, the diver breathes from compressed air cylinders, which float on a free floating raft at the surface, allowing the diver only 20–30 feet (6–9 m) of depth to travel.
• Atmospheric diving suit - an armored suit which protects the diver from the surrounding water pressure.
• Liquid breathing - so far, in the real world, liquid breathing for humans is only laboratory experiments, and (one lung at a time) medical treatment. It has possibilities of being used for very deep diving. It is memorably portrayed in the film The Abyss.
• Artificial gills - these are hypothetical. They would have to process a huge volume of water to extract enough oxygen to supply an active diver, and processing this much water takes a great deal of energy (possible for cold-blooded fish, but harder for humans with higher metabolic rates).

Self contained breathing apparatus used out of water

Main article: Self-contained breathing apparatus

Breathing devices operating on the above principles are not only used underwater, but in other situations where the atmosphere is dangerous — little oxygen, poisonous, etc. These devices are called Self-Contained Breathing Apparatus (SCBA).

• Firefighting
• Mining, especially mine rescue
• Operations in enclosed or poorly ventilated areas, e.g., large fluid or gas containers, and welding in a confined space.

The first open-circuit industrial breathing devices were designed by modifying the design of the Cousteau aqualung. Industrial rebreathers have been used since soon after 1900. Rebreather technology is also used in space suits.

Accessories

In most modern scuba sets, a buoyancy compensator (BC) or buoyancy control device (BCD), such as a back-mounted wing or stabilizer jacket (also known as a "stab jacket"), is built into the harness. Although strictly speaking this is not a part of the breathing apparatus, it is usually connected to the diver's air supply, in order to provide easy inflation of the device. This can usually also be done manually via a mouthpiece, in order to save air while on the surface, or in case of a malfunction of the pressurized inflation system. The bladders inside the BCD inflate with air from the "direct feed" to increase the volume of the SCUBA equipment and cause the diver to float. Another button deflates the BCD and decreases the volume of the equipment and causes the diver to sink. Certain BCDs allow for integrated weight, meaning that the BCD has special pockets for the weights that can be dumped easily in case of an emergency. The aim of using the BCD, whilst underwater, is to keep the diver neutrally buoyant, i.e., neither floating up or sinking. The BCD is used to compensate for the compression of a wet suit, and to compensate for the decrease of the diver's mass as the air from the cylinder is breathed away.

Diving weighting systems, ranging from 2 to 15 kilograms, increase density of the scuba diver to compensate for the buoyancy of diving equipment, allowing the diver to fully submerge underwater with ease by obtaining neutral or slightly negative buoyancy. While weighting systems originally consisted of solid lead blocks attached to a belt around the diver's waist, some modern diving weighting systems are now incorporated into the BCD. These systems use small nylon bags of lead shot pellets which are distributed throughout the BCD, allowing a diver to gain a better overall weight distribution leading to a more horizontal position in the water. There are cases of lead weights being threaded on the straps holding the cylinder into the BCD.

Many modern rebreathers use advanced electronics to monitor and regulate the composition of the breathing gas.

Some scuba sets incorporate attached extra stage cylinders, as bailout in case the main breathing gas supply is used up or malfunctions, or containing another gas mixture. If these extra cylinders are small, they are sometimes called "pony cylinders". They often have their own demand regulators and mouthpieces, and if so, they are technically distinct extra scuba sets.

The diver may carry two or more sets of breathing equipment to provide redundant alternative gas systems in the event that the other fails or is exhausted. Modern recreational rigs most often have two regulators connected to a single cylinder, in case the primary regulator fails or another diver runs out of air. Some divers instead connect their backup regulator to a smaller "pony cylinder" for extra safety, and there are also emergency systems which mount a simple regulator directly to the top of a small cylinder. Rebreather divers often carry a side-slung open-circuit "bail out" to be used in the event the rebreather fails.

In technical diving, the diver may carry different equipment for different phases of the dive; some breathing gas mixes may only be used at depth, such as trimix and others, such as pure oxygen, which only may be used during decompression stops in shallow water. The heaviest cylinders are generally carried on the back supported from a backplate while others are side slung from strong points on the backplate.

When the diver carries many diving cylinders, especially those made of steel, lack of buoyancy becomes a problem. High-capacity BCs are used to allow the diver to control his or her depth.

An excess of tubes and connections passing through the water tend to decrease diving performance by causing hydrodynamic drag in swimming.

Some diver training organizations and groups of divers teach techniques, such as DIR diving for configuring diving equipment.

See also

• Timeline of diving technology
• List of diver certification organizations
• See Frogman#Mistakes in fiction for common mistakes in depicting scuba gear.

External images

• www.divingmachines.com — Vintage aqualungs including three-cylinder types