A scuba set, acronym for self-contained underwater breathing apparatus, 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 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 exhausts 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, dependent on design.
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 Aparatus").  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.
18th century 
The first diving dress using a compressed air reservoir was successfully designed and built in 1771 by Sieur (old French for "sir" or "Mister") Fréminet, a Frenchman from Paris. After having done research on surface-supplied diving he conceived an autonomous breathing machine equipped with a reservoir, dragged by and behind the diver, although Fréminet later put it 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.
19th century 
The Frenchman Paul Lemaire d'Augerville successfully built and used autonomous diving equipment in 1824, as did the British William H. James in 1825. James' helmet was "thin copper or sole of leather" with a plate window, and the air was supplied from an iron reservoir.
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. Touboulic called his invention Ichtioandre (Greek for "fish-man").
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. 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.
A more successful and safer regulator was mass-produced in France from 1865 to 1965 (although production was twice interrupted during that period): invented by Benoît Rouquayrol in 1860 for survival in flooded mines it was adapted to diving in 1864 with the help of French Navy officer Auguste Denayrouze. Even though it was independent from the surface for a very short duration, the Rouquayrol-Denayrouze apparatus reached worldwide celebrity after having been mentioned by Jules Verne in his adventure book Twenty Thousand Leagues Under the Sea; but Jules Verne wildly exaggerated its dive duration without external air supply. This equipment was the first reliable regulator to be mass-produced and was acquired as a standard breathing apparatus by the French Imperial Navy since 1865. The iron reservoir suffered from a lack of autonomy because the technology of that time was not yet able to provide reliable valves and pressure vessels when gases were highly compressed. That is why the Rouquayrol-Denayrouze reservoir, in the 1860s, was able to contain only 30 atmospheres of internal pressure, which translated into sufficient air to permit dives no longer than 30 minutes to a depth of no more than ten metres. The French divers of that time tended to prefer their well known diving dress. When used in surface-supplied configuration the Rouquayrol-Denayrouze air tank was used for bailout in the case of a hose failure. Rouquayrol-Denayrouze's mechanism was effective, but its autonomy depended too much on the weak high-pressure reservoirs of its time. For a longer and more secure autonomy from the surface technology had to wait until the 20th century had brought stronger and reliable compressed air cylinders.
20th century 
The first diving equipment that combined a high-pressure cylinder and a breathing device (although not a demand regulator as was the Rouquayrol-Denayrouze apparatus) was invented separately by the Japanese Ohgushi in 1918 and the Frenchmen Maurice Fernez and Yves le Prieur in 1926. Both were based on a constant-flow supply of the air. Ohgushi's invention was soon forgotten, but the Fernez-Le Prieur apparatus was mass-produced during the 1930s and adopted as a standard by the French Navy. It was the autonomus breathing device first used by the first scuba diving clubs in history (Racleurs de fond in California, 1933, founded by Glenn Orr, and Club des sous-l'eau in Paris, 1935, founded by Le Prieur himself). 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.
During the 1930s French pioneers Philippe Tailliez and Jacques-Yves Cousteau used and widely tested the Le Prieur apparatus before Émile Gagnan and Cousteau worked together on the invention of the modern regulator in 1943. It was during World War II when regulator and rebreather technologies were improved and took their currently known forms.
Among the things that prompted Cousteau to develop efficient air-breathing free-swimming diving gear, were two oxygen toxicity accidents that he had in 1939 with rebreathers, the first at 17 metres deep, but those accidents happened because he went too deep with pure oxygen. The invention of the modern diving regulator became possible after Cousteau met engineer Émile Gagnan. In 1942, in Paris, and following severe fuel restrictions due to the German occupation of France, Émile Gagnan, an Air Liquide employee, miniaturized and adapted a Rouquayrol-Denayrouze regulator (property of the Bernard Piel company in 1942) to gas generators. Gagnan's boss and owner of the Air Liquide company, Henri Melchior, decided to introduce Gagnan to Jacques-Yves Cousteau, his son-in-law, because he already knew that Cousteau was looking for an efficient and automatic demand regulator. Both men met in Paris in December 1942 and adapted Gagnan's regulator to a diving cylinder. After fixing some technical problems they patented the first modern demand regulator in 1943.
From 1934 to 1944 the Commeinhes family (René and his son Georges) from Alsace invented and successfully tested a demand regulator, but the main inventor, Georges, was killed in 1944 during the liberation of Strasbourg and Cousteau's regulator had no competitors immediately after the war. In July 1943 Georges Commeinhes had reached a depth of 53 metres off Marseille equipped with his GC42 breathing apparatus ("G" for Georges, "C" for Commeinhes and 42 for 1942). Not knowing this, Frédéric Dumas (a close friend of Cousteau) reached a depth of 62 metres off Les Goudes, not far from Marseille, in October 1943 with a Cousteau-Gagnan prototype. He felt then what is now called a nitrogen narcosis.
At the same time rebreathers also took a technological jump. During the 1930s and all through World War II, British, Japanese, Italians and Germans developed and extensively used oxygen rebreathers in order to fit out the first frogmen. The British used the Davis apparatus (invented in 1910 by Robert Henry Davis and mass-produced by Siebe Gorman) for submarine escape, but they adapted it to equip their frogmen during World War II. Germans developed the Dräger rebreathers as of 1912. They first adapted them to submarine escape sets and helmet suits and only adapted it to frogmen during World War II. Italians had 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 . He first called it breathing apparatus and offered it to the U.S. Navy, which rejected it. He then demonstrated it to the Office of Strategic Services (OSS). OSS then hired Major Lambertsen to lead the program and build-up the dive element of their maritime unit. 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. Lambertsen designed a series of rebreathers patented on 16 Dec 1940 and 2 May 1944.
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) and 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. Ted sold his first Porpoise Model CA single hose scuba early in 1952.
Before 1971 (when the Scubapro company commercialized the first stabiliser jacket) all scuba breathing sets came with a plain harness of straps with buckles like on a rucksack or spray-tank-pack. The buckles were usually quick-release. Many did not have a backpack plate, but the cylinders were held directly against the diver's back. Sport scuba usually had quick-release fastenings instead of ordinary buckles. The harnesses of many diving rebreathers made by Siebe Gorman included a large back-sheet of strong reinforced rubber.
In the beginning 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".
Notable early manufacturers 
Normalair is a firm that is now part of the Honeywell Corporation based in Yeovil (UK). They made an early make of single-hose aqualung that had a fullface mask as standard. Normalair provided the Deep-Dive 500 rebreather sets used by fictional secret agent James Bond 007 in the 1981 film For Your Eyes Only.
Captain Trevor Hampton in the 1950s or 1960s designed an early single-hose aqualung with a full-face mask with a circular window that was a very big, and thus very sensitive demand regulator diaphragm. However, when he patented it, the Navy requisitioned the patent, and by the time the Navy found no use in the patent and released it, the market had moved on and he got no use from it.
The second company to make single hose scuba was also in Melbourne. It was made by Jim Ager who owned Air Dive Pty., Ltd. His regulator was the Sea Bee (1955). Jim still makes scuba regulators and is the longest continuous maker of single hose scuba in the world.
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. In Britain it for a long time was often called an "aqualung".
- 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 
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, manifolded if applicable,
- a regulator mechanism to control gas flow and pressure, (two may be used for redundancy),
- a mouthpiece, full-face mask or helmet, if necessary with hoses, to deliver gas to the diver.
- an exhaust valve system to dispose of used gas,
- A harness to attach the set to the diver, or other method of carrying the set.
Additional components which when present are considered part of the scuba set are;
- external reserve valves and their control rods or levers, (uncommon)
- submersible pressure gauges, (almost ubiquitous) and
- secondary 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 nearly always worn on the back. "Twin sets" with two back-mounted cylinders connected by a high pressure manifold were much more common in the 1960s than now; although 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 with three backpack cylinders. Cave and wreck penetration divers sometimes carry cylinders slung at their sides instead, allowing them to swim through more confined spaces.
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 run out of air quicker than aqualungs. There were attempts at designing and using these for diving and for industrial use before the Cousteau-type aqualung started to be a common commercialized device (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 
This type of set consists of one or more diving cylinders containing breathing gas at high pressure, typically 200–300 bars (3,000–4,000 psi), connected to a diving regulator. The regulator supplies the diver with as much of the gas as needed, at a pressure suitable for breathing at the depth of the diver.
Colloquially this type of breathing set is sometimes (depending on the country of the English speaker) 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 
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 often had two cylinders.
In this type of set, the two (or occasionally one or three) stages of the regulator are in a large circular valve assembly mounted on top of the cylinder pack. This type has two wide bellows-like breathing tubes like those on many modern rebreathers, one for intake and one for exhaust. The return tube was not for rebreathing, but because the air exhaust needed to be as near as possible to the regulator's second stage diaphragm, to avoid pressure differences, which would cause a free-flow of gas, or extra resistance to breathing, according to the diver's orientation in the water — head-up, head-down, or level. In modern single-hose sets this problem is avoided by moving the second-stage regulator to the diver's mouthpiece. The twin-hose sets came with a mouthpiece as standard, but a full-face diving mask was an option. Another optional extra was a mouthpiece that also had a snorkel attached and a valve to switch between aqualung and snorkel.
Note the correct layout of this type, in the image to the right. There have been many incorrect depictions in comics of two-cylinder twin-hose aqualungs, showing one wide breathing tube coming directly out of each cylinder top with no regulator: see Diving regulator#Twin-hose without visible regulator valve (fictional).
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. 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, which is used in the "low pressure" part of the system. A low-pressure hose links this with the second-stage regulator, or "demand valve," which is mounted on in the mouthpiece. Exhalation occurs through a one-way diaphragm valve in the chamber of the demand valve, directly into the water quite close to the diver's mouth. The first make of this configuration of scuba regulator was the Porpoise, which was made in Melbourne, Australia by Ted Eldred. 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 ports for low-pressure hoses for inflation of dry suits and BC devices.
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.
Secondary demand valve on a regulator 
Most modern scuba sets have a secondary second-stage demand valve on a separate hose, a configuration variously called a "secondary", or "octopus" demand valve, "alternate air source", "safe secondary" or "safe-second". It is frequently yellow in color, signaling that it is an emergency or backup device, and making it easier to see. 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 the secondary 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 the 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 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 regulator is combined with the inflator and exhaust assembly of the BC 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 dedicated BC inflation hoses, because demand on it will be higher if it is used for breathing). In this configuration, the secondary demand valve is integral with the BC, rather than a separate hose and demand valve.
No matter which configuration of secondary demand valve is used, many diving schools now suggest that a diver routinely offer another diver in trouble their "primary" mouthpiece, i.e., the one in their mouth, then using their own secondary demand valve. The idea behind this technique is that the primary mouthpiece 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 a great 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 regulator/BC inflator designs, the secondary demand valve is at the end of an even shorter hose (the BC mouthpiece/exhaust) than is the case with the conventional octopus demand valve, so deliberate use of the primary regulator and hose to help another diver becomes even more appropriate, and almost essential, with the BC-integrated-regulator configuration.
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.
|This section may require copy-editing. (June 2012)|
Exhaled gas is stored between breaths in a "counterlung" of the rebreather. In some rebreathers, one-way valves direct the gas through a "loop", while in others the inhaled and exhaled gas goes back and forth along a single tube: this is called the pendulum system. The oxygen consumed by the diver is replaced, nearly always by adding gas from a cylinder, and carbon dioxide exhaled by the diver is chemically removed by passing the gas through a "scrubber" — a canister full of soda lime or similar reagent. This type of scuba equipment is known as closed circuit, as the gas is recirculated through the system, and only lost to the surroundings when it expands due to pressure change or when more is added than the circuit can hold.
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 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 is typically 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 dive 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, that the scrubber gets less efficient at depth because the scrubber's inside is more crowded with diluent molecules, hindering the carbon dioxide molecules from reaching the absorbent as quickly. and the requirement to be able to safely bail out at any point of the dive.
Comparison between open and closed circuit scuba 
The duration of open-circuit dives is shorter than a rebreather dive, in proportion to the weight and bulk of the set.
Breathing gases for scuba 
Until nitrox was widely accepted in the late 1990s, almost all recreational scuba used simple compressed and filtered air. Exotic gas mixtures presently used in scuba are intended to prevent decompression illness and nitrogen narcosis.
Some divers use nitrox, which usually has a higher percentage of oxygen, 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 limit their exposure to partial pressures of less than 1.6 bar, by limiting the maximum operating depth for the mixture.
Gas endurance of a scuba set 
Gas endurance of a scuba set is the 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 capcity 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.
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 how much air 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 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".
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 (human) - these are mostly science fiction. In the real world they have to process a massive amount 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).
Breathing devices used out of water 
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.
- Other jobs out of water, e.g., welding in a confined space
- Mining, especially mine rescue
- Operations in enclosed or poorly ventilated areas, e.g., large fluid or gas containers.
- SCAMP (Supercritical Air Mobility Pack) is an out-of-water liquid-air open-circuit breathing set designed by NASA by adapting space suit technology. Its maker claims that a man wearing it can crawl through a hole 50 centimetres (20 in) square.
These devices are called Self-Contained Breathing Apparatus (SCBA). (The initials SCBA have had other meanings.) 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.
In 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 scuba set 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. 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 BCD's 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.
See also 
- Timeline of diving technology
- List of diver training organizations
- See Frogman#Mistakes in fiction for common mistakes in depicting scuba gear.
- Vann, Richard D (2004). "Lambertsen and O2: beginnings of operational physiology". Undersea Hyperb Med 31 (1): 21–31. PMID 15233157. Retrieved 2013-04-20.
- See Lambertsen's homage by the Passedaway.com website.
- Authentic photographed SCUBA sets, images provided by Guardian Spies: The Story of the U.S. Coast Guard and OSS in World War II, a specialized website. Notice that no bubbles are produced upon immersion.
- "Aqua-lung". Massachusetts Institute of Technology.
- Fréminet's invention mentioned in the Musée du Scaphandre website (a diving museum in Espalion, south of France)
- Alain Perrier, 250 réponses aux questions du plongeur curieux, Éditions du Gerfaut, Paris, 2008, ISBN 978-2-35191-033-7 (p.46, in French)
- French explorer and inventor Jacques-Yves Cousteau mentions Fréminet's invention and shows this 1784 painting in his 1955 documentary Le Monde du silence.
- In 1784 Fréminet sent six copies of a treatise about his machine hydrostatergatique to the chamber of Guienne (nowadays called Guyenne). On April 5, 1784, the archives of the Chamber of Guienne (Chambre de Commerce de Guienne) officially recorded: Au sr Freminet, qui a adressé à la Chambre six exemplaires d'un précis sur une « machine hydrostatergatique » de son invention, destinée à servir en cas de naufrage ou de voie d'eau déclarée.
- Daniel David, Les pionniers de la plongée - Les précurseurs de la plongée autonome 1771-1853, 20X27 cm 170 p, first published in 2008
- Davis p. 563
- Avec ou sans bulles ? (With or without bubbles?), an article (in French) by Eric Bahuet, published in the specialized Web site plongeesout.com.
- Ichtioandre's technical drawing.
- James, Augerville, Condert and Saint Simon Sicard as mentioned by the Musée du Scaphandre Web site (a diving museum in Espalion, south of France)
- Description of the Rouquayrol-Denayrouze apparatus in the Musée du Scaphandre website (a diving museum in Espalion, south of France)
- Histoire de la plongée ("history of diving"), by Mauro Zürcher, 2002
- Jacques-Yves Cousteau & Frédéric Dumas, Le Monde du silence, Éditions de Paris, Paris, 1953, Dépôt légal 1er Trimestre 1954 - Édition N° 228 - Impression N° 741 (pp. 21-22, in French)
- The Musée du Scaphandre website (a diving museum in Espalion, south of France) mentions the contributions of different inventors: Guillaumet, Rouquayrol and Denayrouze, Le Prieur, René and Georges Commheines, Gagnan and Cousteau (in French)
- Brief history of diving by the Club aquatique Cellois (in French)
- See page 52 in Capitaine de frégate PHILIPPE TAILLIEZ, Plongées sans câble, Arthaud, Paris, January 1954, Dépôt légal 1er trimestre 1954 - Édition N° 605 - Impression N° 243 (in French)
- Jacques-Yves Cousteau & Frédéric Dumas, Le Monde du silence, Éditions de Paris, Paris, 1953, Dépôt légal 1er Trimestre 1954 - Édition N° 228 - Impression N° 741 (pp. 35-37, in French)
- Drägerwerk page in Divingheritage.com, a specialised website.
- The Pirelli Aro and other postwar italian rebreathers in therebreathersite.nl
- Shapiro, T. Rees (2011-02-19). "Christian J. Lambertsen, OSS officer who created early scuba device, dies at 93". The Washington Post.
- 1944 Lambertsen's breathing appartus patent in Google Patents
- Laurent-Xavier Grima, Aqua Lung 1947-2007, soixante ans au service de la plongée sous-marine ! (in French)
- The Siebe Gorman tadpole set, the one licensed from La Spirotechnique, is here described by a French collector.
- Rediscovering The Adventure Of Diving From Years Gone By, an article by Andrew Pugsley.
- cf. The Silent World, a film shot in 1955, before the invention of buoyancy control devices: in the film, Cousteau and his divers are permanently using their fins.
- Yves Paul Gaston Le Prieur, Premier de plongée ('First on Diving'), Éditions France Empire, Paris, 1956
- J. Y. Cousteau & Frédéric Dumas, The Silent World, Hamish Hamilton, London, 1953
- Shreeves, K and Richardson, D (2006). "Mixed-Gas Closed-Circuit Rebreathers: An Overview of Use in Sport Diving and Application to Deep Scientific Diving". In: Lang, MA and Smith, NE (eds.). Proceedings of Advanced Scientific Diving Workshop. Smithsonian Institution, Washington, DC. ISBN 20060725 Check
|isbn=value (help). Retrieved 17 June 2012.
- Lang, Michael A, ed. (2001). DAN nitrox workshop proceedings. Durham, NC: Divers Alert Network. p. 195. Retrieved 2008-09-20.
External images 
- www.divingmachines.com — Vintage aqualungs including three-cylinder types