Scuba set

Scuba set

Diving with a recreational open circuit scuba set
Acronym	Scuba
Other names	Scuba gear Open circuit scuba Diving rebreather Aqualung Bailout set
Uses	Providing an underwater diver with an autonomous breathing gas supply

A scuba set is any breathing apparatus 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 has been used to refer to any diving equipment used by the scuba diver, though this would more commonly and accurately be termed scuba equipment. Scuba is overwhelmingly the most common underwater breathing system used by recreational divers and is also used in professional diving when it provides advantages, usually of mobility and range, over surface supplied diving systems, and is allowed by the relevant code of practice.

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 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.

The most immediate risk associated with scuba diving is drowning due to a failure of the breathing gas supply. This may be managed by diligent monitoring of remaining gas, adequate planning and provision of an emergency gas supply carried by the diver in a bailout cylinder or supplied by the diver's buddy.

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.^[1] Lambertsen first called the closed circuit rebreather apparatus he had invented "Laru", an (acronym for Lambertsen Amphibious Respiratory Unit) but, in 1952, rejected the term "Laru" for "SCUBA" ("Self-Contained Underwater Breathing Apparatus").^[2] Lambertsen's invention, for which he held several patents registered 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.^[3]

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,^[4] 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.

Application

A diver uses a self-contained underwater breathing apparatus (scuba) to breathe underwater. Scuba provides the diver with the advantages of mobility and horizontal range far beyond the reach of an umbilical hose attached to surface-supplied diving equipment (SSDE).^[5]

Unlike other modes of diving, which rely either on breath-hold or on breathing supplied under pressure from the surface, scuba divers carry their own source of breathing gas, usually compressed air,^[6] allowing them greater freedom of movement than with an air line or diver's umbilical and longer underwater endurance than breath-hold. Scuba diving may be done recreationally or professionally in a number of applications, including scientific, military and public safety roles, but most commercial diving uses surface supplied diving equipment for main gas supply when this is practicable. Surface supplied divers may be required to carry scuba as an emergency breathing gas supply to get them to safety in the event of a failure of surface gas supply.^[5][7][8]

There are 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 and regulated by national legislation.^[8]

Other specialist areas of scuba diving include military diving, with a long history of military frogmen in various roles. Their roles include 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.^[8]

There are also professional divers involved with underwater environment, such as underwater photographers or underwater videographers, who document the underwater world, or scientific diving, including marine biology, geology, hydrology, oceanography and underwater archaeology.^[7][8]

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 in commercial diving, may be restricted to surface supplied equipment by legislation and codes of practice.^[8][9]

Alternatives to scuba for diving

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

History

Early history

A scuba set is characterized by full independence from the surface during use, by providing breathing gas carried by 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^[10] Fréminet from Paris. He conceived an autonomous breathing machine equipped with a reservoir, dragged behind the diver or mounted on his back.^[11][12] 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.^[13][14]

The Frenchman Paul Lemaire d'Augerville built and used autonomous diving equipment in 1824,^[15] 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.^[16] 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.^[17][18]

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.^[19]

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

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 kilograms per square centimetre (2,100 psi; 150 bar). 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,^[20] 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.^[21] 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,^[22] 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.^[23]

In 1957, Eduard Admetlla i Lázaro used a modified version of the Aqua-Lung made by Nemrod and broke the world record by descending to a depth of 100 metres (330 ft).^[24]

Closed-circuit scuba

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 exhaled 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.^[25][26] 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.^[26][27] 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 5.5 metres (18 ft) 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 1,000 feet (300 m) 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.^[26]

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^[26][28] 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,^[28] and as an industrial breathing set.

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 (2.0 cu ft) of oxygen at a pressure of 120 bars (1,700 psi). 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 at ambient pressure. 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.^[29]

In 1912 the German firm Dräger developed their own version of standard diving dress with a self-contained gas supply from a rebreather with the circulation driven by an injector.^[30]

During the 1930s and all through World War II, the British, Italians and Germans developed and extensively used oxygen rebreathers to equip the first frogmen. The British used the Davis apparatus for submarine escape, but they soon adapted it for their frogmen during World War II. Germans used the Dräger rebreathers,^[31] which were also originally designed as submarine escape sets and only adapted for use by frogmen during World War II.

The Italians developed similar rebreathers for the combat swimmers of the Decima Flottiglia MAS, especially the Pirelli ARO.^[32] In the U.S. Major Christian J. Lambertsen invented an underwater free-swimming oxygen rebreather in 1939, which was accepted by the Office of Strategic Services.^[33] In 1952 he patented a modification of his apparatus, this time named SCUBA. Although he coined the most common English word used for autonomous open circuit diving equipment, Lambertsen did not invent that equipment.^[34] After World War II, military frogmen continued to use rebreathers since they do not make bubbles which would give away the presence of the divers.

Post WWII

Air Liquide started selling the Cousteau-Gagnan regulator commercially as of 1946 under the name of scaphandre Cousteau-Gagnan or CG45 ("C" for Cousteau, "G" for Gagnan and 45 for the 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 registered the Aqua-Lung trademark, which was first licensed to the U.S. Divers company (the American division of Air Liquide) and later sold with La Spirotechnique and U.S. Divers to finally become the name of the company, Aqua-Lung/La Spirotechnique, currently located in Carros, near Nice.^[35]

In 1948 the Cousteau-Gagnan patent was also licensed to Siebe Gorman of England,^[36] when Siebe Gorman was directed by Robert Henry Davis.^[37] 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. This demand was eventually met by Ted Eldred of Melbourne, Australia, who had been developing a rebreather called the Porpoise. When a demonstration resulted in a diver passing out, he began to develop the single-hose open-circuit scuba system, which separates the first and second stages by a low-pressure hose, and releases exhaled gas at the second stage. This avoided the Cousteau-Gagnan patent, which protected the twin-hose scuba regulator.^[38] In the process, Eldred also improved performance of the regulator. Eldred sold the first Porpoise Model CA single hose scuba early in 1952.

Early scuba sets were usually provided with a plain harness of shoulder straps and waist belt. The waist belt buckles were usually quick-release, and shoulder straps sometimes had adjustable or quick release buckles. Many harnesses did not have a backplate, 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 reinforced rubber.

Early scuba divers dived without any buoyancy aid.^[39] In emergency they had to jettison their weights. In the 1960s adjustable buoyancy life jackets (ABLJ) became available. One early make, since 1961, was Fenzy. The ABLJ is used for two purposes: to adjust the buoyancy of the diver to compensate for loss of buoyancy at depth, mainly due to compression of the neoprene wetsuit) and more importantly as a lifejacket that will hold an unconscious diver face-upwards at the surface, and that can be quickly inflated. It was put on before putting on the cylinder harness. The first versions were inflated with a small carbon dioxide cylinder, later with a small direct coupled air cylinder. An extra low-pressure feed from the regulator first-stage 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

Scuba sets are of two types:

• In open-circuit scuba the diver inhales from the equipment and all the exhaled gas is exhausted to the surrounding water. This type of equipment is relatively simple, economical and reliable.
• In closed-circuit or semi-closed circuit, also referred to as a rebreather, the diver inhales from the set, and exhales back into the set, where the exhaled gas is processed to make it fit to breathe again.

Both types of scuba set include a means of supplying air or other breathing gas, nearly always from a high pressure diving cylinder, and a harness to attach it to the diver. Most open-circuit scuba sets have a demand regulator to control the supply of breathing gas, and most rebreathers have a constant-flow injector, or an electronically controlled injector to supply fresh gas, but also usually have an automatic diluent valve (ADV), which functions in the same way as a demand valve, to maintain the loop volume during descent.

Open circuit

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 attached 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.

Constant flow scuba

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 bite-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

This system consists of one or more diving cylinders containing breathing gas at high pressure, typically 200–300 bars (2,900–4,400 psi), 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.^[40]

Twin-hose demand regulator

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

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 ^[41] and Scott Aviation ^[42] (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

Most recreational scuba sets have a backup 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". The 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. 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 and frees the donor's hand.

Some diver training agencies recommend that a diver routinely offer their primary demand valve to a diver requesting to share air, and then switch to their own secondary demand valve.^[43] The idea behind this technique is that the primary demand valve is known to be working, and the diver donating the gas is less likely to be stressed or have a high carbon dioxide level, so has more time to sort out their own equipment after temporarily suspending the ability to breathe. In many instances, panicked divers have grabbed the primary regulators out of the mouths of other divers, so changing to the backup as a routine reduces stress when it is necessary in an emergency.

In technical diving donation of the primary demand valve is commonly the standard procedure, and the primary is connected to the first stage by a long hose, typically around 2 m, to allow gas sharing while swimming in single file in a narrow space as might be required in a cave or wreck. In this configuration the secondary is generally held under the chin by a loose bungee loop around the neck, supplied by a shorter hose, and is intended for backup use by the diver donating gas.^[43] The backup regulator is usually carried in the diver's chest area where it can be easily seen and accessed for emergency use. It may be worn secured by a breakaway clip on the buoyancy compensator, plugged into a soft friction socket attached to the harness, secured by sliding a loop of the hose into the shoulder strap cover of a jacket style BC, or suspended under the chin on a break-away bungee loop known as a necklace. These methods also keep the secondary from dangling below the diver and being contaminated by debris or snagging on the surroundings. Some divers store it in a BC pocket, but this reduces availability in an emergency.

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 connector for combined use must have a larger bore than for standard BC inflation hoses, because it will need to deliver a higher flow rate if it is used for breathing. This combination unit is carried in the position where the inflator unit would normally hang on the left side of the chest. With integrated DV/BC inflator designs, the secondary demand valve is at the end of the shorter BC inflation hose, and the donor must retain access to it for buoyancy control, so donation of the primary regulator to help another diver is essential with this configuration.

The secondary demand valve is often partially yellow in color, and may use a yellow hose, for high visibility, and as an indication that it is an emergency or backup device.

When a side-mount configuration is used, the usefulness of a secondary demand valve is greatly reduced, as each cylinder will have a regulator and the one not in use is available as a backup. This configuration also allows the entire cylinder to be handed off to the receiver, so a long hose is also less likely to be needed.

Some diving instructors continue to teach buddy-breathing from a single demand valve as an obsolescent but still occasionally useful technique, learned in addition to the use of the backup DV, since availability of two second stages per diver is now assumed as standard in recreational scuba.

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. and were made until at least 1974.^[44] It would have to be filled a short time before use.

Rebreathers

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. The rebreather's economic use of gas, typically 1.6 litres (0.06 cu ft) 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.

There are two main variants of rebreather — semi-closed circuit rebreathers, and fully closed circuit rebreathers, which include the subvariant of oxygen rebreathers. 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,^[45] and a rebreather can be more economical when used with expensive gas mixes such as heliox and trimix,^[45] 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

Until Nitrox, which contains more oxygen than air, was widely accepted in the late 1990s,^[46] almost all recreational scuba used simple compressed and filtered air. Other gas mixtures, typically used for deeper dives by technical divers, may substitute helium for some or all of the nitrogen (called Trimix, or Heliox if there is no nitrogen), or use lower proportions of oxygen than air. In these situations divers often carry additional scuba sets, called stages, with gas mixtures with higher levels of oxygen that are primarily used to reduce decompression time in staged decompression diving.^[43] These gas mixes allow longer dives, better management of the risks of decompression sickness, oxygen toxicity or lack of oxygen (hypoxia), and the severity of nitrogen narcosis. Closed circuit scuba sets (rebreathers) provide a gas mix that is controlled to optimise the mix for the actual depth at the time.

Diving cylinders

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, the volume of the gas they contain when expanded to normal atmospheric pressure. Common sizes include 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.^[47] Diving cylinders are sometimes colloquially called "tanks", "bottles" or "flasks" although the proper technical term for them is "cylinder".

Harness configuration

The scuba set can be carried by the diver in several ways. Most common for recreational diving is the stabilizer 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.

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.

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.

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 some recreational and professional divers when it suits the diving operation.

Accessories

In most 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, 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 BCD inflates with air from the low pressure inflator hose to increase the volume of the scuba equipment and cause the diver gain buoyancy. Another button opens a valve to deflate the BCD and decrease the volume of the equipment and causes the diver to lose buoyancy. Some 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 function of the BCD, while 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 increase the average density of the scuba diver and equipment to compensate for the buoyancy of diving equipment, particularly the diving suit, allowing the diver to fully submerge with ease by obtaining neutral or slightly negative buoyancy. Weighting systems originally consisted of solid lead blocks attached to a belt around the diver's waist, but some diving weighting systems are incorporated into the BCD or harness. These systems may use small nylon bags of lead shot or small weights which are distributed around the BCD, allowing a diver to gain a better overall weight distribution leading to a more horizontal trim in the water. Tank weights can be attached to the cylinder or threaded on the cambands holding the cylinder into the BCD.

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

Rebreather divers and some open circuit scuba divers carry extra diving cylinders for bailout in case the main breathing gas supply is used up or malfunctions. If the bailout cylinder is small, they may be called "pony cylinders". They have their own demand regulators and mouthpieces, and are technically distinct extra scuba sets. In technical diving, the diver may carry different equipment for different phases of the dive. Some breathing gas mixes, such as trimix, may only be used at depth, and others, such as pure oxygen, may only be used during decompression stops in shallow water. The heaviest cylinders are generally carried on the back supported by a backplate while others are side slung from strong points on the harness.

When the diver carries many diving cylinders, especially those made of steel, lack of buoyancy can be a problem. High-capacity BCs may be needed to allow the diver to effectively control buoyancy.

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

A diffuser is a component fitted over the exhaust outlet to break up the exhaled gas into bubbles small enough not to be seen above the surface the water, and make less noise (see acoustic signature). They are used in combat diving, to avoid detection by surface observers or by underwater hydrophones, Underwater mine disposal operations conducted by clearance divers, to make less noise,^[48] to reduce the risk of detonating acoustic mines, and in marine biology, to avoid disruption of fish behavior.^[49]

Designing an adequate diffuser for a rebreather is much easier than for open-circuit scuba, as the gas flow rate is generally much lower. An open circuit diffuser system called the "scuba muffler" was prototyped by Eddie Paul in the early 1990s for underwater photographers John McKenney and Marty Snyderman which had two large filter stones mounted on the back of the cylinder with a hose connected to the exhaust ports of the second stage regulator. The filter stones were mounted on a hinged arm to float 1 to 2 feet (30 to 60 cm) above the diver, to set up a depth-pressure-differential suction effect to counteract the extra exhalation pressure needed to breathe out through the diffuser. The scuba muffler cut the exhalation noise by 90%.^[50] Closed circuit rebreathers proved more useful in letting divers get near sharks.^[51]

Gas endurance of a scuba set

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 to be used until there is a reserve of 17% there is (83% × 200 × 11) = 1826 litres available. 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 recreational divers leisurely exploring a coral reef using standard 200 bar "aluminum 80" cylinders as may be 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 corrected to atmospheric pressure per minute. Except during ascent or descent, the fully closed circuit rebreather that is operating correctly uses very little or no diluent. 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 litres × 200 bar × 0.75 litres per minute = 450 minutes). This endurance is independent of depth. 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.

See also

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

References

Bibliography

External images

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