SCUBA is an acronym for Self-Contained Underwater Breathing Apparatus. These initials originated in 1939 in the US Navy to refer to US frogman's rebreathers. As with radar, the acronym has become so familiar that it is often not capitalised. A scuba set provides a scuba diver with the breathing gas necessary to stay underwater for longer periods.

Scuba diving is swimming (or occasionally walking) underwater breathing from a scuba set worn by the diver. This page is about the breathing sets used: For information about diving with scuba, see Scuba diving.

Contents

1 Types 2 Accessories 3 Open circuit scuba sets 3.1 Constant flow 3.2 With a demand regulator 3.2.1 "Twin-hose" open-circuit scuba 3.2.2 "Single-hose" open-circuit scuba 3.2.3 Cryogenic open-circuit scuba 4 Rebreathers 5 Alternatives to scuba 6 Breathing sets used out of water 7 History

Types

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.
• 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 re-breathable.

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.

Accessories

In modern scuba sets, a buoyancy compensator, such as a wings or stabiliser jacket, is built into the scuba set harness, although strictly speaking it is not a part of the breathing apparatus. 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, 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. An excess of tubes and connections passing through the water tend to decrease diving performance by causing hydrodynamic drag in swimming.

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 pure oxygen is used rarely, generally as a decompression gas in volumes 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 set consists one or more cylinders containing a high pressure breathing gas connected to a diving regulator that 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. Until about the 1980s or 1990s, that sort of breathing set was often called an aqualung in Britain; the word Aqua-Lung is correctly a tradename protected by the Cousteau-Gagnan patent but colloquially and in writing it was often used as a generic.

"Twin-hose" open-circuit scuba

These sets had one or more (usually two) cylinders lengthwise on the back. The first and second stages of the regulator were in a large circular valve assembly mounted on top of the cylinder pack. One-stage and three-stage regulators were known. It had two wide breathing tubes like 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 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.

"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 (http://www.divingheritage.com/normalair.htm) 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. One type is the Russian Kriolang (from Greek cryo- + English lung), which was copied from Jordan Klein's "Mako" cryogenic open-circuit scuba. Janwillem Bech's rebreather site (http://www.therebreathersite.nl/cryo_pjotrr.htm) shows pictures of a Kriolang that was made in 1974. Its diving duration is likely several hours. It would have to be filled immediately before use.

Rebreathers

With rebreathers, the diver breathes a breathing gas in and out of a breathing bag, 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.

Alternatives to scuba

• Atmospheric diving suit - an armored suit which protects the diver from the surrounding water pressure.
• 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.

Breathing sets used out of water

Breathing sets operating on these principles are used not only underwater. Adapted versions of them are used for human activity in various conditions where the atmosphere is unsuitable for breathing, e.g. during firefighting, mining and mine rescue, internal inspection and maintenance of large fluid or gas containers, etc. Such breathing sets are then called SCBA – Self Contained Breathing Apparatus. 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.

History

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 (sometimes nowadays called ABLJ) for aqualung-type scuba became available. 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.

See Timeline of underwater technology.