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In underwater diving, ambient pressure diving is diving exposed to the water depth pressure, rather than in a pressure-excluding hard suit or submarine.
The SI unit of pressure is the pascal (Pa), which is a very small unit relative to atmospheric pressure on Earth, so kilopascals (kPa) are more commonly used in this context. 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 scuba diving, it is common to see ambient pressure expressed in bar or millibar. One bar is 100kPa or approximately ambient pressure at sea level. Ambient pressure may in other circumstances be measured in pounds per square inch (psi) or in atmospheres (atm). One atmosphere is also approximately the ambient pressure at sea level and is equal to 14.7 psi or 1.01325 bar, which is close enough for bar and atm to be used interchangeably in many applications.
In scuba diving, the ambient pressure on the diver increases linearly with depth. Since water is much denser than air, a diver will experience much greater changes in ambient pressure than can be experienced above water. Each 10 metres (33 ft) of depth adds another bar to the ambient pressure.
Within the atmosphere, the ambient pressure decreases with height above ground and by measuring ambient pressure, a pilot may determine height. Near the ground, a change of ambient pressure of 1 millibar is taken to represent a change of height of 9 metres (30 ft).
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