Mode of underwater diving

Ambient pressure diving modes are those in which the diver is exposed to the ambient pressure in the water due to hydrostatic and local atmospheric pressure.^[1] These include:

Freediving

The ability to dive and swim underwater while holding one's breath is considered a useful emergency skill, an important part of water sport and navy safety training, and an enjoyable leisure activity.^[4] Underwater diving without breathing apparatus can be categorised as underwater swimming, snorkelling and freediving. These categories overlap considerably. Several competitive underwater sports are practised without breathing apparatus.^[5]^[6]^[7]^[8]^[9]

Freediving excludes the use of underwater breathing devices, and relies on the ability of divers to hold their breath until resurfacing. The technique ranges from simple breath-hold diving to competitive apnea dives. Fins and a diving mask are often used in free diving to improve vision and provide more efficient propulsion. A short breathing tube called a snorkel allows the diver to breathe at the surface while the face is immersed. Snorkelling on the surface with no intention of diving is a popular water sport and recreational activity.^[4]^[10]

Scuba diving

Scuba diving in open circuit and rebreather modes

Recreational scuba divers on open circuit

Explosive ordnance disposal divers using rebreathers

Scuba diving is diving with a self-contained underwater breathing apparatus, which is completely independent of surface supply. Scuba gives the diver mobility and horizontal range far beyond the reach of an umbilical hose attached to surface-supplied diving equipment (SSDE).^[11] Scuba divers engaged in armed forces covert operations may be referred to as frogmen, combat divers or attack swimmers.^[12]

A scuba diver moves underwater primarily by using fins attached to the feet;^[13] external propulsion can be provided by a diver propulsion vehicle, or a towboard pulled from the surface. Other equipment includes a diving mask to improve underwater vision, a protective diving suit, equipment to control buoyancy, and equipment related to the specific circumstances and purpose of the dive.^[14] Scuba divers are trained in the procedures and skills appropriate to their level of certification by instructors affiliated to the diver certification organisations which issue these diver certifications. These include standard operating procedures for using the equipment and dealing with the general hazards of the underwater environment, and emergency procedures for self-help and assistance of a similarly equipped diver experiencing problems. A minimum level of fitness and health is required by most training organisations, and a higher level of fitness may be needed for some applications.^[15]

Open circuit scuba

Open circuit scuba systems discharge the breathing gas into the environment as it is exhaled, and consist of one or more diving cylinders containing breathing gas at high pressure which is supplied to the diver through a diving regulator. They may include additional cylinders for decompression gas or emergency breathing gas.^[16]

Scuba rebreather

Closed-circuit or semi-closed circuit rebreather scuba systems allow recycling of exhaled gases. The volume of gas used is reduced compared to that of open circuit, so a smaller cylinder or cylinders may be used for an equivalent dive duration. They greatly extend the time spent underwater as compared to open circuit for the same gas consumption. Rebreathers produce fewer bubbles and less noise than scuba which makes them attractive to covert military divers to avoid detection, scientific divers to avoid disturbing marine animals, and media divers to avoid bubble interference.^[17]

Open-water diving

Open-water diving is diving in an open water environment, where the diver has unrestricted access by way of a direct vertical ascent to the breathable air of the atmosphere. Other environmental hazards may exist which do not affect the classification. In some contexts it also implies that there is no decompression obligation hindering direct ascent.^[18] Blue-water diving is open-water diving done in mid-water where the bottom is out of sight of the diver and there may be no fixed visual reference. The techniques of blue-water diving have been developed over the years to suit the conditions and address the hazards of an environment which is functionally bottomless, and has no fixed visible positional references.^[19]

Penetration diving

Penetration diving is diving under a physical overhead of any kind, where for a significant part of the dive it is not possible to ascent vertically to a free breathable air surface. Cave diving, wreck diving, ice diving and diving inside or under other natural or artificial underwater structures or enclosures are examples. The restriction on direct ascent increases the risk of diving under an overhead, and this is usually addressed by adaptations of procedures and use of equipment such as redundant breathing gas sources and guide lines to indicate the route to open water.^[20]^[21]^[22]

Surface-supplied diving

Surface-supplied diving in surface-oriented and saturation modes

Surface-oriented diver entering the water

Closed diving bell, also known as a diver transfer capsule

An alternative to self-contained breathing systems is to supply breathing gases from the surface through a hose. When combined with a communication cable, a pneumofathometer hose and a safety line it is called the diver's umbilical, which may include a hot water hose for heating, video cable and breathing gas reclaim line. The diver generally wears a full-face mask or helmet, and gas may be supplied on demand or as a continuous free flow. More basic equipment that uses only an air hose is called an airline or hookah system.^[23]^[14]^[24] This allows the diver to breathe using an air supply hose from high pressure cylinders or a diving air compressor at the surface. Breathing gas is supplied through a mouth-held demand valve or light full-face mask. Airline diving is used for work such as hull cleaning and archaeological surveys, for shellfish harvesting, and as snuba, a shallow water activity typically practised by tourists and those who are not scuba-certified.^[24]^[25]^[26]

Bounce diving

Commercial divers refer to diving operations where the diver starts and finishes the diving operation at atmospheric pressure as surface oriented, or bounce diving.^[27] The diver may be deployed from the shore or a diving support vessel and may be transported on a diving stage or in a diving bell. Surface-supplied divers almost always wear diving helmets or full-face diving masks. The bottom gas can be air, nitrox, heliox or trimix; the decompression gases may be similar, or may include pure oxygen.^[28] Decompression procedures include in-water decompression or surface decompression in a deck chamber.^[29] Surface oriented dives may use a transportation platform to move the diver vertically through the water column, may be assisted by an attendant controlling the umbilical, or the diver may control their own descent and ascent.^[1]

Stage diving

A diving stage or diving basket is a simple platform lowered and lifted from the surface by a winch under the control of the diving team. The diver's umbilical leads directly to the diver and is managed at the surface by the diver's attendant.^[1]

Open bell diving

A wet bell with a gas filled dome provides more comfort and control than a stage and allows for longer time in water. Wet bells are used for air and mixed gas, and divers can decompress on oxygen at 12 metres (40 ft).^[28]

Closed bell bounce diving

Small closed bell systems have been designed that can be easily mobilised, and include a two-man bell, a launch and recovery system and a chamber for decompression after transfer under pressure (TUP). Divers can breathe air or mixed gas at the bottom and are usually recovered with the chamber filled with air. They decompress on oxygen supplied through built in breathing systems (BIBS) towards the end of the decompression. Small bell systems support bounce diving down to 120 metres (390 ft) and for bottom times up to 2 hours.^[28]

Scuba replacement

A relatively portable surface gas supply system using high pressure gas cylinders for both primary and reserve gas, but using the full diver's umbilical system with pneumofathometer and voice communication, is known in the industry as "scuba replacement". It is generally used where scuba equipment cannot be used for reasons of safety or when it is not allowed by regulations or code of practice.^[30]

Air-line diving

Hookah, Sasuba and Snuba systems are categorised as "air-line" equipment, as they are supplied through a basic air line, and do not include the communication, lifeline and pneumofathometer hose characteristic of a full diver's umbilical. A bailout system is not an inherent part of an air-line diving system, though it may be required in some applications.^[31]^[32]

Their field of application is very different from full surface-supplied diving. Hookah is generally used for shallow water work in low-hazard applications, and sometimes for open water hunting and gathering of seafood,^[31] shallow water mining of gold and diamonds in rivers and streams, and bottom cleaning and other underwater maintenance of boats, hull cleaning, swimming pool maintenance, and shallow underwater inspections.^[33]^: 29

Sasuba and Snuba are a shallow water recreational application for low-hazard sites, using air supplied through a short hose of about 7 m to a demand valve mouthpiece,^[34]

Compressor diving

Compressor diving is a rudimentary method of surface-supplied diving used in some tropical regions such as the Philippines and the Caribbean. The divers swim with a half mask and fins and are supplied with air from an industrial low-pressure air compressor on the boat through plastic tubes. There is no reduction valve; the diver holds the hose end in his mouth with no demand valve or mouthpiece and allows excess air to spill out between the lips.^[35]

Saturation diving

Saturation diving lets professional divers live and work under pressure for days or weeks at a time. After working in the water, the divers rest and live in a dry pressurised underwater habitat on the bottom or a saturation life support system of pressure chambers on the deck of a diving support vessel, oil platform or other floating platform at a similar pressure to the work depth. They are transferred between the surface accommodation and the underwater workplace in a pressurised closed diving bell. Decompression at the end of the dive may take many days, but since it is done only once for a long period of exposure, rather than after each of many shorter exposures, the overall risk of decompression injury to the diver and the total time spent decompressing are reduced. This type of diving allows greater work efficiency and safety.^[36]

Air and gas diving

The original breathing gas for diving was atmospheric air, and compressed air remains an important breathing gas for ambient pressure diving. Oxygen is limited to shallow water to avoid toxicity problems, and is usually used to accelerate decompression, or by tactical divers to provide a long endurance with a small amount of gas, and to minimise bubbles where detection would be a tactical problem. Some physiological problems of deeper diving, such as inert gas narcosis and high work of breathing, can be mitigated by the use of breathing gases based on helium, and experimental work which includes hydrogen in the mixture for extreme depths, continues. The common terminology refers to air diving and gas diving,^[37]^[38] which includes oxygen diving,^[39] and mixed gas diving, which includes nitrox diving, trimix diving, and heliox diving.^[40]^[39]

Air is available for the cost of operating the compressor, so it is supplied on open circuit, and discharged into the surroundings on exhalation, and may be supplied through a free-flow system where this has advantages. Oxygen and nitrox are also cheap enough that it is usually economical to supply by demand open circuit except for long duration scuba operations, but helium is expensive and sometimes in short supply, so recycling can be viable for moderate usage, and essential for high volume usage. The costs of recycling by rebreathing or reclaiming helium based gases include high capital investment in the equipment, and additional running costs compared to open circuit.^[41] The use of hydrogen as a breathing gas component is still experimental.^[42]