Ambient pressure diving modes are those in which the diver is exposed to the ambient pressure in the water due to combined
hydrostatic and local
atmospheric pressure. Ambient pressure diving can also be classified as surface-oriented diving, where the diver is decompressed to surface ambient pressure at the end of each dive, and
saturation diving, where the diver remains under pressure between in-water exposures. There is also a distinction between
freediving, where the diver holds their breath, and
compressed-gas diving, where the diver breathes gas at ambient pressure during the dive. They may be further classified by the type of breathing apparatus used, and by the level of confinement of the diving environment.
Surface-oriented (bounce) diving Professional divers refer to diving where the diver starts and finishes the diving operation at atmospheric pressure as surface-oriented, or bounce diving. In
recreational diving there is no need to make this distinction, as all recreational diving is surface-oriented, usually without decompression stops.The diver may be deployed from the shore or a diving support vessel and may be transported on a diving 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. Decompression procedures include in-water decompression or
surface decompression in a
deck chamber. 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.
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. It is the original diving mode. 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. Freediving excludes the use of
underwater breathing apparatus, 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.
Swimfins and a
diving mask are often used in free diving to provide more efficient propulsion and improve
underwater vision. A short breathing tube called a
snorkel allows the diver to breathe at the surface while the face is immersed.
Scuba diving Scuba diving is a mode of compressed-gas 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), and much greater endurance than freediving.
Open circuit scuba Open circuit scuba systems discharge breathing gas into the environment as it is exhaled, and consist of one or more
diving cylinders containing pressurized breathing gas, supplied to the diver at ambient pressure through a
diving regulator. They may include additional cylinders for decompression gas or emergency breathing gas.
Scuba rebreather Closed-circuit or semi-closed circuit
rebreather scuba systems allow the recycling of exhaled gases; the volume of gas used is reduced compared to that of open circuit, so smaller cylinders may be used for an equivalent dive duration. They allow a diver to spend much more time underwater while consuming the same amount of gas. Rebreathers produce fewer bubbles and less noise than scuba, which makes them attractive to covert military divers to avoid detection, to scientific divers to avoid disturbing marine animals, and to media divers to avoid bubble interference.
Surface-supplied diving 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 also 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 either on demand or at a constant, continuous rate. More basic equipment that uses only an air hose is called an
airline or hookah system. 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.
Stage diving Stage diving may refer to surface-supplied diving from a
diving stage, or
technical scuba diving where
stage cylinders are used for different stages of a long dive, and may contain different gases. A diving stage or
diving basket is a simple platform lowered and lifted from the surface platform 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. In-water decompression is facilitated as the stage can be hoisted at a controlled rate and held at reasonably constant depths for stops.
Open bell diving Open bell diving uses an ambient pressure diving bell to transport the diver through the water column. 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 can be used for air and mixed gas diving, and divers can decompress on oxygen at .
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 and for bottom times up to 2 hours. Larger closed bells can be used the same way and also for saturation diving with up to three divers, including a
bellman.
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, and full surface supplied equipment is inconvenient, impractical, or unsafe. A lightweight helmet or full-face mask and bailout cylinder are standard for this mode
Air-line diving Hookah, 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. 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, 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. 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.
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 divers hold the hose ends in their mouths with no demand valve or
mouthpiece and allow excess air to spill out between the lips.
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 at the surface, usually on the deck of a
diving support vessel,
oil platform or other floating platform, at a similar pressure to the ambient pressure at the work depth. They are transferred between 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.
Modes of breathing gas management There are three basic modes of breathing gas management available for use in both self-contained (scuba) and surface-supplied underwater breathing apparatus for ambient pressure diving.
Open-circuit Open circuit diving uses breathing apparatus which discharges exhaled breathing gas directly into the environment. The simplest case of this system is , when the same
breathing gas mixture is used throughout the dive. However, open circuit can also be used with
gas switching, where the breathing gases used on different stages of the dive are changed to better suit the depth and decompression requirements.
Air diving is the classic example of single gas open circuit diving, but most
recreational diving and shallow surface-supplied diving falls under this mode. Open circuit systems may provide gas by
constant flow or
supply on demand, and some equipment can be switched between these modes by the diver.
Closed-circuit A closed-circuit apparatus recirculates breathing gas indefinitely, with carbon dioxide removed and oxygen added, generally to maintain a
constant partial pressure, regardless of depth. This is also called and is a feature of
electronically controlled closed circuit rebreathers (eCCR)
Gas reclaim systems and
push-pull diving systems are forms of closed circuit gas recycling equipment not carried by the diver, used mainly to recover expensive helium-based diluent gases.
Semi-closed-circuit A semi-closed-circuit apparatus partly recycles the breathing gas mixture. Oxygen partial pressure is maintained at a breathable level by addition of breathing gas mixture at a rate sufficient to make up the losses due to exhaust gas and metabolism. The oxygen concentration is not constant, but varies between predictable limits. The equipment used for this may be called a
semi-closed circuit rebreather or a
gas extender. Such systems have been used in scuba rebreathers, and surface supplied and self-contained free-flow helmet equipment.
Mode by breathing gas 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 in closed circuit rebreathers 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, which includes
oxygen diving, and
mixed gas diving, which includes
nitrox diving,
trimix diving, and
heliox diving. 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. The use of hydrogen as a breathing gas component is still experimental.
Modes of decompression Decompression is a part of every ambient pressure dive. Modes of decompression range from "
no-stop dives" where a limited and controlled
ascent rate is sufficient decompression, to decompression from saturation over several days. Decompression can be continuous, where no stops are required, and the rate of ascent is limited to provide sufficient time to
outgas safely, or staged, where ascent is made up to and between stops a limited rate, but most of the outgassing occurs during periods of constant depth, (pressure) called
decompression stops. Continuous decompression rates depend on the theoretical gas loading of the controlling tissue, and may be fixed or, more often, variable with depth. Decompression can also be done entirely in the water, partly in the water and partly in a surface decompression chamber or entirely in one or more decompression chambers. It can also be classified by the type of breathing gases used while decompressing, whether there are changes in gas composition during decompression, and whether the changes are stepwise or continuous, or a combination of both. Air diving traditionally uses air as breathing gas for the entire dive, including for in-water staged decompression. It is simple, low cost, requires little or no special equipment, but is inefficient and limited to tolerable in-water exposures. Modes of decompression: • • • • • • • • • • • • ==Atmospheric pressure diving==