Editing Scuba set (section)

==Application==
{{main|Scuba diving}}

A diver uses a self-contained underwater breathing apparatus (scuba) to breathe [[underwater]]. Scuba provides the diver with the advantages of mobility and horizontal range far beyond the reach of an umbilical hose attached to surface-supplied diving equipment (SSDE).<ref name="US Navy Diving Manual 2006" />

Unlike other modes of diving, which rely either on [[Free-diving|breath-hold]] or on [[surface-supplied diving|breathing gas supplied under pressure from the surface]], scuba divers carry their own source of [[breathing gas]], usually [[Air filter|filtered]] [[compressed air]],<ref name="Brubakk 2003"/> allowing them greater freedom of movement than with an [[air line]] or [[Umbilical cable#Diver|diver's umbilical]] and longer underwater endurance than breath-hold. Scuba diving may be done [[Recreational diving|recreationally]] or [[Professional diving|professionally]] in a number of applications, including scientific, military and public safety roles, but most commercial diving uses [[surface-supplied diving equipment]] for the main gas supply when this is practicable. Surface supplied divers may be required to carry scuba as an emergency breathing gas supply to get them to safety in the event of a failure of surface gas supply.<ref name="US Navy Diving Manual 2006" /><ref name="NOAA 4th Ed" /><ref name="Diving at Work Regulations 1997" />

There are 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 and regulated by national legislation.<ref name="Diving at Work Regulations 1997" />

Other specialist areas of scuba diving include [[military diving]], with a long history of military [[frogmen]] in various roles.  Their roles include 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 search and rescue diving teams may also be part of a [[fire department]], paramedical service or [[lifeguard]] unit, and may be classed as [[public safety diving]].<ref name="Diving at Work Regulations 1997" />

There are also professional divers involved with the [[underwater environment]], such as [[Underwater photography|underwater photographers]] or underwater videographers, who document the underwater world, or [[scientific diving]], including [[marine biology]], geology, [[hydrology]], [[oceanography]] and [[underwater archaeology]].<ref name="NOAA 4th Ed" /><ref name="Diving at Work Regulations 1997" />

The choice between scuba and surface supplied diving equipment is based on both legal and logistical constraints. Where the diver requires mobility and a large range of movement, scuba is usually the choice if safety and legal constraints allow. Higher risk work, particularly in commercial diving, may be restricted to surface supplied equipment by legislation and codes of practice.<ref name="Diving at Work Regulations 1997" /><ref name="SA Diving Regulations 2009" />

===Alternatives to scuba for diving===
There are alternative methods that a person can use to survive and function while underwater, currently including:
* [[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 (swimming)|snorkel]].
* [[surface-supplied diving]] – originally, and still used in [[professional diving]] for long or deep dives, where an umbilical cable provides [[breathing gas]], voice communication and sometimes warm water to heat the [[diving suit]] from the surface. Some tourist resorts offer a surface-supplied air line diving system, trademarked as [[Snuba]], as an introduction to diving for the inexperienced. Using the same type of demand valve as scuba diving, the diver breathes from a compressed air cylinder carried on a free-floating raft at the surface, through a simple hose limiting the diver to a depth of 20–30 feet (6–9 m).
* [[atmospheric diving suit]] – a rigid articulated pressure resistant suit that isolates the diver from the surrounding water pressure.

===Operation===
{{main|Scuba diving}}
{{see also|Scuba skills|Diving procedures}}
Breathing from scuba is mostly a straightforward matter. Under most circumstances it differs very little from normal surface breathing. In the case of a full-face mask, the diver may usually breathe through the nose or mouth as preferred, and in the case of a mouth held demand valve, the diver will have to hold the mouthpiece between the teeth and maintain a seal around it with the lips. Over a long dive this can induce jaw fatigue, and for some people, a gag reflex. Various styles of mouthpiece are available off the shelf or as customised items, and one of them may work better if either of these problems occur.

The frequently quoted warning against holding one's breath on scuba is a gross oversimplification of the actual hazard. The purpose of the admonition is to ensure that inexperienced divers do not accidentally hold their breath while surfacing, as the expansion of gas in the lungs could over-expand the lung air spaces and rupture the alveoli and their capillaries, allowing lung gases to get into the pulmonary return circulation, the pleura, or the interstitial areas near the injury, where it could cause dangerous medical conditions. Holding the breath at constant depth for short periods with a normal lung volume is generally harmless, providing there is sufficient ventilation on average to prevent carbon dioxide buildup, and is done as a standard practice by underwater photographers to avoid startling their subjects. Holding the breath during descent can eventually cause lung squeeze, and may allow the diver to miss warning signs of a gas supply malfunction until it is too late to remedy.

Skilled open circuit divers can and will make small adjustments to buoyancy by adjusting their average lung volume during the breathing cycle. This adjustment is generally in the order of a kilogram (corresponding to a litre of gas), and can be maintained for a moderate period, but it is more comfortable to adjust the volume of the buoyancy compensator over the longer term.

The practice of shallow breathing or skip breathing in an attempt to conserve breathing gas should be avoided as it tends to cause a carbon dioxide buildup, which can result in headaches and a reduced capacity to recover from a breathing gas supply emergency. The breathing apparatus will generally increase [[dead space (physiology)|dead space]] by a small but significant amount, and cracking pressure and flow resistance in the demand valve will cause a net work of breathing increase, which will reduce the diver's capacity for other work. Work of breathing and the effect of dead space can be minimised by breathing relatively deeply and slowly. These effects increase with depth, as density and friction increase in proportion to the increase in pressure, with the limiting case where all the diver's available energy may be expended on simply breathing, with none left for other purposes. This would be followed by a buildup in carbon dioxide, causing an urgent feeling of a need to breathe, and if this cycle is not broken, panic and drowning are likely to follow. The use of a low density inert gas, typically helium, in the breathing mixture can reduce this problem, as well as diluting the narcotic effects of the other gases.<ref name="Mitchell" />

Breathing from a rebreather is much the same, except that the work of breathing is affected mainly by flow resistance in the breathing loop. This is partly due to the carbon dioxide absorbent in the scrubber, and is related to the distance the gas passes through the absorbent material, and the size of the gaps between the grains, as well as the gas composition and ambient pressure. Water in the loop can greatly increase the resistance to gas flow through the scrubber. There is even less point in shallow or skip breathing on a rebreather as this does not even conserve gas, and the effect on buoyancy is negligible when the sum of loop volume and lung volume remains constant.{{Citation needed|date=December 2023}}

===Breathing gases for scuba===
{{further|Breathing gas#Breathing gases for diving and other hyperbaric use}}
Until [[Nitrox]], which contains more oxygen than air, was widely accepted in the late 1990s,<ref name="Lang 2000" /> almost all recreational scuba used simple compressed and filtered air. Other gas mixtures, typically used for deeper dives by [[Technical diving|technical]] divers, may substitute helium for some or all of the nitrogen (called [[Trimix (breathing gas)|Trimix]], or [[Heliox]] if there is no nitrogen), or use lower proportions of oxygen than air. In these situations divers often carry additional scuba sets, called stages, with gas mixtures with higher levels of oxygen that are primarily used to reduce decompression time in staged [[Decompression (diving)|decompression diving]].<ref name="Jablonski 2006" /> These gas mixes allow longer dives, better management of the risks of [[decompression sickness]], [[oxygen toxicity]] or lack of oxygen ([[Hypoxia (medical)|hypoxia]]), and the severity of [[nitrogen narcosis]]. Closed circuit scuba sets ([[Diving rebreather|rebreathers]]) provide a gas mix that is controlled to optimise the mix for the actual depth at the time.