Scuba set

SCUBA is an acronym for Self-Contained Underwater Breathing Apparatus. These initials originated in 1939 in the US Navy to refer to US military diver's rebreather sets. As with radar, the acronym has become so familiar that it is often not capitalised and is treated as an ordinary word: for example, it has been taken into the Welsh language as "sgwba". A scuba set provides a scuba diver with the breathing gas necessary to breathe underwater.

Scuba diving is swimming (or other underwater activities) using a scuba set. This entry is about the breathing sets only: For information about diving with scuba, see Scuba diving.

Scuba diver

Types of scuba set

Modern scuba sets are of two types:

  • open-circuit (or Aqua-Lung™, very often written "aqualung"). Here the diver breathes in from the set and out to waste. This type of equipment is relatively simple making it cheap and reliable. The duration of open-circuit dives is shorter than a dive with a rebreather, in proportion to the weight and bulk of the set. It is uneconomic in its use of expensive gas mixes such as heliox and trimix. The cylinder is nearly always worn on the back. "Twin sets" with two backpack cylinders were much more common in the 1960s than now. Submarine Products sold a sport air scuba with 3 backpack cylinders. Sometimes cave divers have cylinders slung at their sides instead.
  • closed-circuit (or rebreather). Here the diver breathes in from the set, and out back into the set where the exhaled gas is reprocessed to make it fit to breathe again. The mechanisms that control some types of rebreather are complex making them expensive and more difficult to use and maintain. They are not used by amateur divers. They use gas very economically making long dives easy and special mixes cheaper to use.

Both types consist of 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 rebreathers only have a constant-flow regulator like in blowtorches. Some divers use the word "scuba" to mean open-circuit sets only.

Open circuit scuba sets

Newspapers and television news often describe open circuit scuba wrongly as "oxygen" equipment. Open circuit scuba may supply various breathing gases; but rarely pure oxygen, usually while decompressing in technical diving.

Constant flow

Constant flow scuba sets do not have a demand regulator; the breathing gas flows at a constant rate unless the diver switches it on and off by hand. They run out of air quicker than aqualungs. There were attempts at designing and using these before 1939, for diving and for industrial use. Examples were "Ohgushi's Peerless Respirator", and Commandant le Prieur's breathing sets: see Timeline of underwater technology.

With a demand regulator

This type of set consists of one or more cylinders containing breathing gas at high pressure (typically 220-300 Bar) 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. See diving cylinder for more information about the cylinders and how they are arranged. Colloquially this type of breathing set is often called an aqualung, however, the word Aqua-Lung is correctly a tradename protected by the Cousteau-Gagnan patent.

Old-type "twin hose" Cousteau-type aqualung
Old-type "twin hose" Cousteau-type aqualung

"Twin-hose" open-circuit scuba

In this type of set the first and second stages of the regulator are in a large circular valve assembly mounted on top of the cylinder pack. One-stage and three-stage regulators were known.

It consists of two wide breathing tubes similar to those on many modern rebreathers. The return tube was not for rebreathing but because the air exhaust needed to be at the same depth as the regulator's second stage diaphragm to avoid pressure differences, which would cause a free-flow or resistance to breathing according to the diver's attitude in the water.

These sets came with a mouthpiece as standard, but fullface masks could be got for them.
One optional extra for them was a mouthpiece that also had a snorkel attached, and a valve to switch between aqualung and snorkel.

Note its layout in the image. In comics there have been thousands of drawings of two-cylinder twin-hose aqualungs shown wrongly with one wide breathing tube coming straight out of each cylinder top with no regulator, far more than of twin-hose aqualungs drawn correctly with a regulator.

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

A "single-hose" aqualung
A "single-hose" aqualung

"Single-hose" open-circuit scuba

Most modern open-circuit scuba sets have a diving regulator consisting of a first stage pressure reducing valve that is sealed over the diving cylinder’s output valve, and the second stage “demand valve” at the mouthpiece, with a thin pressure hose linking the two stages. This type is called "single hose". Many modern scuba sets have a spare second stage demand valve on its own hose, which is called an "octopus" or "alternate air source".

Normalair ( is a firm that formerly were based at Yeovil (UK). They made an early make of single-hose aqualung that had a fullface mask as standard.

Captain Trevor Hampton in the 1950's or 1960's designed an early single-hose aqualung with a fullface mask with a big circular fullface window which was a very big and thus very sensitive demand regulator diaphragm. But 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 the patent.

Cryogenic open-circuit scuba

There have been designs for a cryogenic open-circuit scuba which has liquid-air tanks instead of cylinders.

  • Jordan Klein designed a cryogenic open-circuit scuba called Mako and made at least a prototype.
  • The Russian Kriolang (from Greek cryo- + 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.
An Inspiration™ rebreather seen from the front
An Inspiration™ rebreather seen from the front


With rebreathers, the diver breathes a breathing gas in and out of a breathing bag, which is also called a counterlung. The oxygen used is replaced, nearly always from a cylinder. The carbon dioxide breathed out is removed, nearly always in a canister full of absorbent. This type of scuba equipment is known as 'closed circuit'. Its economic use of gas allows dives of much longer duration than is possible with open circuit equipment. See rebreather for more information.

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: see Fluid_breathing#Use in diving. It is memorably portrayed in the film "The Abyss"

Artificial gills extracting oxygen from the water

These are science fiction only. In the real world they would have to process an unrealistically massive amount of water to extract enough oxygen to supply an active diver. See Artificial gills.

Duration of a dive

The duration of an open-circuit dive 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. An open circuit diver whose breathing rate at the surface (atmosphereic pressure) is 25 litres per minute will consume 100 litres of gas per minute at 30 metres. ( (30m / 10m per bar) + 1 bar atmospheric pressure ) 25 lpm = 100 lpm ). If a 15 litre cylinder filled to 200 bar is used until there is a reserve of 25% there is (150 15) = 2250 litres. At 100 lpm the dive will be a maximum of 22.5 minutes (2250/100).

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 and used gas vented. Although it uses gas more economically, the weight of the rebreathing equipment means the diver carries smaller cylinders. So, if the diver uses a 5 litre cylinder filled to 200 bar who leaves 25% in reserve will be able to do a 22.5 minute dive (10l 200 bar 0.75 / 33.33). A fully-closed circuit rebreather diver consumes about 1 litre of oxygen per minute and virtually no diluent. So, if the diver has a 3 litre cylinder filled to 200 bar who leaves 25% in reserve will be able to do a 450 minute dive (3l 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.

In practice, dive times are more often influenced by other factors such as water temperature and the requirement for safe ascent (see decompression sickness).

Alternatives to scuba

  • 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 - mainly 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. Technically, this is not a SCUBA, as the apparatus is not self-contained, but needs surface technology and support.
  • Atmospheric diving suit - an armored suit which protects the diver from the surrounding water pressure.


In modern scuba sets, a buoyancy compensator, such as a back-mounted wing or stabiliser jacket (otherwise 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 divers air supply, in order to provide easy inflation of the device, this can usually also be done manually via a mouthpiece.

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. For open-circuit divers, the two most common types of redundant configurations are the "twinset", consisting of two similar systems, and the "main plus pony", consisting of a large main gas source and a small "pony" set. 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 for decompression in shallow water. The heaviest cylinders are generally carried on the back supported from a backplate while others are side slung from strong points on the backplate.

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

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

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

Breathing sets used out of water

Breathing sets operating on the above principles are not only used underwater but in other situations where the atmosphere is dangerous (little oxygen, poisonous etc).

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 Momson lung was used as emergency escape gear by WWII submariners.

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 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 1960's inflatable diver's lifejackets (also called ABLJ for adjustable buoyancy life jacket) for aqualung-type scuba became available. The ABLJ is used for two purposes, one to adjust the buoyancy of the diver to compnesate for loss of buoyancy (chiefly due to compression of neoprene wet suit)and more importantly as a lifejacket 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. The use of an extra feed from the first stage regulator permits control of the life jacket as a buoyancy aid.

See Timeline of underwater technology.

See also

External links

pt:SCUBA ru:Акваланг fr:Scaphandre_autonome


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