Scuba diving is a form of underwater diving in which the diver uses a Self-Contained Underwater Breathing Apparatus (SCUBA) to breathe underwater. Unlike other modes of diving, which rely either on breath-hold techniques or on breathing gas pumped from the surface, scuba divers carry their own source of breathing gas, usually in the form of compressed air. This allows them greater freedom of movement compared to an air-line and longer underwater endurance than the breath-hold technique. Scuba equipment may be open circuit, in which the exhaled gas is expelled to the surroundings or a closed or semi-closed circuit re-breather, in which the breathing gas is scrubbed to remove the carbon dioxide, and the oxygen used is replenished from a supply of feed gas before being re-breathed.
By the early twentieth century, two basic templates for SCUBA had emerged. One was the open-circuit scuba where the diver’s exhaust could be vented directly into the water, and the other one was the closed-circuit scuba where the diver’s unused oxygen could be filtered from the carbon dioxide and re-circulated. The first commercially successful scuba sets were the Aqualung twin hose open-circuit units developed by Emile Gagnan and Jacques-Yves Cousteau in 1943. In these units compressed air could be carried in back mounted cylinders, inhaled through a demand regulator and then exhaled into the water adjacent to the tank.
Scuba diving may be performed for a number of reasons, both personal and professional. Recreational diving is performed purely for enjoyment and has a number of distinct technical disciplines to increase interest underwater, such as cave diving, wreck diving, ice diving and deep diving. Divers may be employed professionally to perform tasks underwater. Some of these tasks are suitable for scuba. Professional scuba divers are trained to manage situations in which they may face vertigo or entanglement. There are divers who work full or part-time in the recreational diving community as instructors, assistant instructors, dive masters and dive guides. In some jurisdictions the professional nature of recreational diver instruction, with particular reference to responsibility of the health and the safety of the clients, dive leadership for reward and dive guiding is recognized and regulated by national legislation.
Other specialist areas of scuba 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 manned torpedoes, bomb disposal units or engineering operations. In civilian operations, many police forces operate police diving teams to perform “search and recovery“ or “search and rescue“ operations and/or 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. Lastly, there are professional divers involved with underwater environment, such as underwater photography or underwater videography divers, who document the underwater world, or scientific diving, including marine biology, geology, hydrology, oceanography and underwater archaeology.
The depth range applicable to scuba diving depends on the application and training, but most recreational divers go up to a maximum of 30 meters (100 feet). Recreational dives are limited to no-stop dives or relatively short planned decompression stops to minimize the risk of decompression sickness. Recreational scuba is generally limited to depths of no more than 42 meters (140 feet) by US-based agencies such as PADI and NAUI, or 50 meters (160 feet) by UK-based agencies such as BSAC and SAA. Professional diving also limits the allowed planned decompression depending on the code of practice, operational directives or statutory restrictions. Depth limits depend on the jurisdiction, and maximum depths allowed range from 30 meters (100 feet) to more than 50 meters (160 feet), depending on the breathing gas used and the availability of a decompression chamber nearby or on-site. Technical diving may involve exploring the logistical and physiological limits, and in these cases a higher risk is accepted by the diver.
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