Scuba is an apparatus (often called "scuba gear", "scuba system") utilizing a portable supply of compressed gas supplied at a regulated pressure and used for breathing while swimming underwater. Scuba is also the common name for the sport (also called "diving" or "scuba diving") which uses the apparatus for recreational diving.
There are two types of scuba diving equipment:
- The Aqua-lung - A tank (or cylinder) of compressed gas (usually air or a mixture similar to air) which is supplied to the diver through a regulator. The regulator adjusts the pressure of the gas to suit the ambient pressure for comfortable breathing. This kind of scuba system is called "open circuit" because the exhaled gas is released into the water and lost. The open circuit aqua-lung is the most common type of scuba system used by recreational divers.
- The Rebreather is also a tank (or cylinder) of compressed gas breathed by the diver through a regulator, but with other mechanisms that recycle the exhaled gas. This kind of scuba system is called "closed circuit". The economy of reusing and recycling exhaled gases allows a diver to remain submerged for longer periods. Less common, but becoming increasingly available, are closed and semi-closed rebreathers. Open-circuit sets vent off all exhaled gases, but rebreathers reprocess each exhaled breath for re-use by removing the carbon dioxide buildup and replacing the oxygen used by the diver. Rebreathers release few or no gas bubbles into the water, and use much less oxygen per hour because exhaled oxygen is recovered; this has advantages for research, military, photography, and other applications. Modern rebreathers are more complex and more expensive than sport open-circuit scuba, and need special training and maintenance to safely use.