Spearfishing in Florida. Scuba diving is a form of underwater diving in which a diver uses a self contained underwater breathing apparatus (scuba) to breathe underwater. Unlike other modes of diving, which rely either on breath-hold or on air pumped from the surface, scuba divers carry their own source of breathing gas, (usually compressed air),[2] allowing them greater freedom of movement than with an air line or diver's umbilical and longer underwater endurance than breath-hold. Scuba equipment may be open circuit, in which exhaled gas is exhausted to the surroundings, or closed or semi-closed circuit, in which the breathing gas is scrubbed to remove carbon dioxide, and the oxygen used is replenished from a supply of feed gas before being re-breathed. A scuba diver usually moves around underwater by using fins attached to the feet, but external propulsion can be provided by a diver propulsion vehicle, or a sled pulled from the surface. By the early twentieth century, two basic templates for scuba, (self-contained underwater breathing apparatus), had emerged; open-circuit scuba where the diver's exhaust is vented directly into the water, and closed-circuit scuba where the diver's unused oxygen is filtered from the carbon dioxide and recirculated. Rebreathers The closed-circuit rebreathers were first developed for military use, due to their stealth advantages. The first commercially successful closed-circuit scuba was designed and built by English diving engineer, Henry Fleuss in 1878, while working for Siebe Gorman in London.[3][4] His self contained breathing apparatus consisted of a rubber mask connected to a breathing bag, with (estimated) 50-60% O2 supplied from a copper tank and CO2 scrubbed by rope yarn soaked in a solution of caustic potash; the system giving a duration of about three hours.[4][5] Sir Robert Davis, head of Siebe Gorman, perfected the oxygen rebreather in 1910[4][6] with his invention of the Davis Submerged Escape Apparatus, the first practical rebreather to be made in quantity.[7] Rebreathers have been increasingly used by civilians for recreation, especially since the end of Cold War. This reduced the perceived risk of attack by Communist Bloc forces, including by their combat divers. After that, the world's armed forces had less reason to requisition civilian rebreather patents, and automatic and semi-automatic recreational diving rebreathers started to appear. Open-circuit scuba The first commercially successful scuba sets were the Aqualung twin hose open-circuit units developed by Emile Gagnan and Jacques-Yves Cousteau, in which compressed air carried in back mounted cylinders is inhaled through a demand regulator and then exhaled into the water adjacent to the tank.[8] The single hose two stage scuba regulators trace their origins to Australia, where Ted Eldred developed the first example of this type of regulator, known as Porpoise scuba gear. This was developed, because patents protected the Aqualung's twin hose design. The single hose regulator separates the cylinder from the demand valve, giving the diver air at the pressure at their mouth, not that at the top of the cylinder. Etymology The term "SCUBA" (an acronym for "self-contained underwater breathing apparatus") originally referred to United States combat frogmen's oxygen rebreathers, developed during World War II by Christian J. Divers may be employed professionally to perform tasks underwater. Some of these tasks are suitable for scuba. Scuba divers are highly trained to encounter situations in which they may face vertigo or even get tangled in seaweed. To prepare for such events, Scuba divers face a very rigorous training process to handle themselves calmly. There are a fair number of divers who work, full or part-time, in the recreational diving community as instructors, assistant instructors, divemasters and dive guides. In some jurisdictions the professional nature, with particular reference to responsibility for health and safety of the clients, of recreational diver instruction, dive leadership for reward and dive guiding is recognised by national legislation.[12][13] Other specialist areas of diving include military diving, with a long history of military frogmen in various roles. They can perform roles including direct combat, infiltration behind enemy lines, placing mines or using a manned torpedo, bomb disposal or engineering operations. In civilian operations, many police forces operate police diving teams to perform "search and recovery" or "search and rescue" operations and to assist with the detection of crime which may involve bodies of water. In some cases diver rescue teams may also be part of a fire department, paramedical service or lifeguard unit, and may be classed as public service diving.

• published: 21 Sep 2014
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