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The outside water pressure increases with depth and so the stresses on the hull also increase with depth. Each 10 metres (33 ft) of depth puts another atmosphere (1 bar, 14.7 psi, 101 kPa) of pressure on the hull, so at 300 metres (1,000 ft), the hull is withstanding thirty standard atmospheres (30 bar; 440 psi; 3,000 kPa) of water pressure.
Pressure as a function of the height above the sea level. The human body can perform best at sea level, [7] where the atmospheric pressure is 101,325 Pa or 1013.25 millibars (or 1 atm, by definition). The concentration of oxygen (O 2) in sea-level air is 20.9%, so the partial pressure of O 2 (pO 2) is 21.136 kilopascals (158.
The ambient pressure in water with a free surface is a combination of the hydrostatic pressure due to the weight of the water column and the atmospheric pressure on the free surface. This increases approximately linearly with depth. Since water is much denser than air, much greater changes in ambient pressure can be experienced under water.
The lower initial pressure at the surface means that a mass of gas occupying a given volume will be compressed more than the same volume at sea level for the same depth. The formula for Boyle's law applies: P 1 V 1 = P 2 V 2 or: V 2 = P 1 V 1 /P 2 or: V 2 /V 1 = P 1 /P 2. Example: At sea level P 1 = 1 bar (approximately), and at 10 m depth at ...
As of 1990, ascent rates used in tables and dive computers ranged from constant ascent rates of 33, 40, and 60 fpm, from all depths, to variable depth dependent rates of 60 fpm for depths below 100 ft, 40 fpm for depths between 100 and 60 ft, and 20 fpm for depths less than 60 ft in the ORCA dive computers.
For example, a diver ascends from a maximum depth of 60 metres (200 ft), where the ambient pressure is 7 bars (100 psi), to a decompression stop at 20 metres (66 ft), where the pressure is 3 bars (40 psi). The first Pyle stop would take place at the halfway pressure, which is 5 bars (70 psi) corresponding to a depth of 40 metres (130 ft).
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So the 1 atmosphere or bar contributed by the air is subtracted to give the pressure due to the depth of water. The pressure produced by depth in water, is converted to pressure in feet sea water (fsw) or metres sea water (msw) by multiplying with the appropriate conversion factor, 33 fsw per atm, or 10 msw per bar. In feet