Modes of underwater diving
A mode of diving, or diving mode, is a particular way to dive underwater that requires specific equipment, procedures and techniques, and may expose the diver to a particular range of hazards.
There are several modes of diving; these are distinguished by the type of breathing apparatus, diving equipment, procedures and techniques involved, and whether the diver is exposed to ambient pressure. Ambient pressure diving includes freediving and compressed-gas diving, which may also be classed as air diving, oxygen diving, and mixed gas diving by the breathing gas used, and as open-circuit, semi-closed, or closed-circuit depending on the type of breathing apparatus used. There is also atmospheric pressure diving, which involves encapsulation in an atmospheric pressure diving suit or submersible, and unmanned diving, where there are no human divers involved. The diving equipment, support equipment and procedures used largely depend on the mode of diving.
In certain circumstances, some modes of diving may be impracticable, unsafe, not permitted by the governing organisation, or illegal. All modes of diving carry a certain amount of risk; this is mitigated with planning, training, and the appropriate equipment.
Ambient pressure diving
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, 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. 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 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 modeAir-line diving
, 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.