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Underwater diving is the human practice of voluntarily descending below the surface of the water to interact with the surroundings, for various recreational or occupational reasons, but the concept of diving also legally extends to immersion in other liquids, and exposure to other pressurised environments. [1]
The human physiology of underwater diving is the physiological influences of the underwater environment on human divers, and adaptations to operating underwater, both during breath-hold dives and while breathing at ambient pressure from a suitable breathing gas supply.
The ambient atmospheric pressure at sea level is not constant: it varies with the weather, but averages around 100 kPa. In fields such as meteorology and underwater diving, it is common to see ambient pressure expressed in bar or millibar. One bar is 100 kPa or approximately ambient pressure at sea level.
Lung air pressure difference moving the normal breaths of a person (only 0.3% of standard atmospheric pressure) [35] [36] 400–900 Pa 0.06–0.13 psi Atmospheric pressure on Mars, < 1% of atmospheric sea-level pressure on Earth [37] 610 Pa 0.089 psi Partial vapor pressure at the triple point of water (611.657 Pa) [38] [39] 10 3 Pa
The pressure of seawater at a depth of 33 feet equals one atmosphere. The absolute pressure at 33 feet depth in sea water is the sum of atmospheric and hydrostatic pressure for that depth, and is 66 fsw, or two atmospheres absolute. For every additional 33 feet of depth, another atmosphere of pressure accumulates. [6]
The absolute pressure on an ambient pressure diver is the sum of the local atmospheric pressure and hydrostatic pressure. [5] [6] Hydrostatic pressure is the component of ambient pressure due to the weight of the water column above the depth, and is commonly described in terms of metres or feet of sea water. [4]
The atmospheric pressure at the surface is 14.7 pounds per square inch or around 100 kPa. A comparable hydrostatic pressure occurs at a depth of only 10 metres (33 ft) (9.8 metres (32 ft) for sea water). Thus, at about 10 m below the surface, the water exerts twice the pressure (2 atmospheres or 200 kPa) as air at surface level.
Atmospheric pressure diving suits are mainly constrained by the technology of the articulation seals, and a US Navy diver has dived to 610 metres (2,000 ft) in one. [ 24 ] [ 25 ] Greater depths can be achieved with submersibles , some of which are capable of " full ocean depth ", the greatest underwater depth known on Earth.