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Pressure is directly proportional to depth. The equation used is for hydrostatic pressure:
P = Po + dh
Where P = pressure, Po = initial pressure at surface, d = density of the fluid, and h = height to the free surface (or depth). In diving, Po is usually the atmospheric pressure. This is the only thing that elevation affects, as sea level air pressure is higher than the air pressure at higher elevations.
The density of sea water is generally accepted as 64 lbs/cu ft. There is variation, but usually not enough to be meaningful, unless you are diving in a very high salinity location, like the Dead Sea, in which the salt content is so high that the density is considerably higher.
For most purposes, atmospheric pressure at sea level is accepted to be 14.7 lbs/sq in (14.7 psi), or 2116.8 lbs/sq ft (2116.8 psf). Diving in sea water then, the equation becomes:
P = 2116.8 lbs/sq ft + 64 lbs/cu ft x h.
Then, at the surface, h = 0, and P = 2116.8 psf = 14.7 psi. At h = 10 ft, P = 2116.8 + 64 x 10 = 2756.8 psf = 19.1 psi. At h = 33 ft, P = 4228.8 psf = 29.4 psi. At h = 100 ft, P = 8516.8 PS = 59.1 psi. And so on. The general rule-of-thumb for sport/recreational diving is that the pressure increases by 14.7 psi (1 atmosphere) for every 33 ft of depth. So that at 100 ft, the pressure is 58.8 psi (4 atmospheres). (Remember that you start at the surface at 1 atmosphere.)
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LaoYes, changes in elevation and depth can affect pressure. In general, as elevation or depth increase, pressure decreases, and as elevation or depth decrease, pressure increases. This is due to the weight of the overlying air or water column exerting pressure on the lower layers.
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