The word SCUBA is an acronym for Self-Contained Underwater Breathing Apparatus. These initials originated in 1939 in the United States Navy to refer to their military diver's rebreather sets. As with radar, the acronym has become so familiar that it is often not capitalized and is treated as an ordinary noun; for example, it has been taken into the Welsh language as "sgwba".
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.
Some divers use the word "scuba" to mean open-circuit sets only.
Newspapers and television news often describe open circuit scuba wrongly as "oxygen" equipment, probably by false analogy to airplane pilots' oxygen cylinders. Until Enriched Air Nitrox was widely accepted in the late 1990s, almost all sport scuba used simple compressed air. This allowed the scuba industry in the U.S. to avoid regulation by the U.S. Food and Drug Administration (FDA), which defines non-air gas mixtures intended to prevent or treat diseases as "drugs". Exotic gas mixtures presently used in scuba are intended to prevent decompression illness in diving, but officially, the FDA appears to continue to believe that scuba divers all use compressed air.
At partial pressures over about 1.6 atmospheres, oxygen becomes toxic. Open-circuit scuba sets may supply various breathing gases, but rarely pure oxygen, except during decompression stops in technical diving.
Some divers use Enriched Air Nitrox, which has a higher percentage of oxygen, usually 32% or 36% (EAN32 and EAN36, respectively). This lets them stay underwater longer, because less nitrogen is absorbed into the body's tissues. The most common Nitrox blending method needs an oxygen service cylinder, which is a cylinder that has had any non-oxygen-compatible grease or rubber removed, by cleaning and replacing parts.
Colloquially this type of breathing set is sometimes (depending on the country of the English speaker) often called an aqualung; however, the word "Aqua-Lung" (note spelling) is a tradename protected by the Cousteau-Gagnan patent. The trademark is now owned by U.S. Divers.
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.
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 exhalation. 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, 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.
Most modern open-circuit scuba sets have a diving regulator consisting of a first-stage pressure-reducing valve fastened over the diving cylinder's output valve. This valve cuts the pressure from the cylinder, which may be up to 300 bar, to a constant lower pressure, often about 10 bar above the ambient pressure, which is used in the "low pressure" part of the system. A relatively thin low-pressure hose links this with the second-stage regulator, or "demand valve," which is located in the mouthpiece. Exhalation occurs out of a one-way diaphragm in the chamber of the demand valve, directly into the water quite close to the diver's mouth. This configuration type is called "single hose". The first make of this sort of scuba 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).
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.
Most modern scuba sets have a spare second-stage demand valve on a separate hose, a configuration called an "octopus", because it often has two or more hoses for other purposes coming out of the primary regulator on the cylinder top. This separate second-stage regulator and hose, or "alternate air source", "safe secondary" or "safe-second" for short, is typically yellow in color, 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 on a diver's chest, easily available to be grabbed by, or offered to, a second diver short of air. In so doing, this second mouthpiece eliminates the need for two divers who need to share a cylinder to "buddy-breathe," by trading off the same mouthpiece. Diving instructors continue to teach buddy-breathing; then they show the new method that has superseded it. The original octopus idea was conceived by cave-diving pioneer Sheck Exley as a way for single-file-swimming cave divers to share air in a narrow tunnel, but has now become the standard in recreational diving. Modern "octopus" type primary-stage regulators also typically feature high-pressure ports for use by dive-computer pressure sensors, and additional ports for additional low-pressure hoses for inflation of dry suits and BC devices.
Increasingly, in the 21st century, "safety" secondary mouthpieces have been combined with the inflator and exhaust assembly of BC devices. This combination eliminates the need for a separate low pressure hose for the BC. Some diving schools now suggest that a diver offer another diver in trouble their primary mouthpiece, i.e., the one in their mouth, before going to their own safe-secondary. The idea here is that the diver not in trouble has much more time to sort things out with his/her own equipment after temporarily losing ability to breathe.
The Russian Kriolang (from Greek cryo- (= "frost") + 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.
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 20 inches square.
With rebreathers, the gas the diver exhales is stored between breaths in a "counterlung". In some rebreathers, one-way valves direct the gas through a "loop". In other rebreathers, 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 from a cylinder. The exhaled carbon dioxide generated by the diver is removed by passing the gas through a "scrubber" — a canister full of soda lime, making the gas fit to be re-inhaled. This type of scuba equipment is known as closed circuit.
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 expense of more complicated technology and more experience and longer training. There are three variants of rebreather — oxygen rebreathers, semi-closed circuit rebreathers, and fully closed circuit rebreathers.
The rebreather's economic use of gas, typically 1.6 litres of oxygen per minute, allows dives of much longer duration than is possible with open circuit equipment where gas consumption is typically ten times higher. Oxygen rebreathers have a maximum operating depth of around 6 metres (18 feet), but several types of fully closed circuit rebreathers, when using a helium-based diluent, can dive deeper than 100 metres (330 feet). The main limiting factors on rebreathers are the duration of the carbon dioxide scrubber, which is generally at least 3 hours, and 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.
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.
A semi-closed circuit rebreather dive is about three times the length of the equivalent open circuit dive; 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).
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.
Air cylinders used for scuba diving come in various sizes and materials and are typically designated by material — aluminium, steel, high-pressure steel, etc. 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.), while in Europe the size is given as their internal volume (10 liter, 12 liter, etc.). The most common is the "Aluminum 80", which will give an average experienced diver from 40 to 60 minutes of dive time under common dive conditions.
Air cylinder pressure will vary according to the type of material used, ranging from up to bar.
Aluminium cylinders are less expensive than steel and have been known to last for 20 years with standard regular maintenance. The drawback is that aluminium cylinders are neutrally buoyant when full, and positively buoyant when nearing empty. This results in having to monitor buoyancy during the dive more closely so as not to experience the "express elevator to the surface". Aluminium cylinders also need the diver to carry more weight. Divers often prefer to use steel cylinders, as they are negatively buoyant when full and neutral when empty. Many steel cylinders also accept higher pressure fills, giving more capacity for a longer dive for the same size of cylinder.
Compressed air diving cylinders are sometimes colloquially called "tanks", although the proper technical term for them is "cylinder".
These breathing sets are nowadays called SCBA (Self Contained Breathing Apparatus) (The initials SCBA have had other meanings). The first open-circuit industrial breathing sets 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.
A predecessor to scuba gear, the Momsen lung, was used as emergency escape gear by interwar and World War II American submariners.
Jacques-Yves Cousteau and Emile Gagnan invented the first commercially successful open circuit type of SCUBA diving equipment, the Aqua-Lung (often spelled "aqualung") in 1943. Among the things that prompted Cousteau to develop efficient air-breathing diving free-swimming diving gear, were two oxygen toxicity accidents that he had with rebreathers. The Cousteau Gagnan patent was licensed to Siebe Gorman of England. 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 in early in 1952.
Before 1971 all breathing sets including scuba 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 for aqualung-type scuba became available; one early make 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 life jacket that can be rapidly 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 life jacket be controlled as a buoyancy aid.
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.
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.