Feet and metres sea water are convenient measures which approximate closely to depth and are intuitively simple to grasp for the diver, compared to the options of more conventional units of pressure which give no direct indication of depth. In US customary units ambient pressure is normally measured in feet of sea water (fsw), and converted to atmospheres absolute or pounds per square inch absolute (psia) for decompression computation. In metric units the ambient pressure is usually measured in metres sea water (msw), and converted to bar for calculations. In diving the absolute pressure is used in most computations, particularly for decompression and breathing gas consumption but depth is measured by way of hydrostatic pressure. (Pressure in ata = Depth in feet/33 + 1) Usage Surface supply air panel for 4 divers with pneumofathometer gauges calibrated in metres and feet seawater Therefore at the surface the gauge pressure of 0 fsw is equivalent to an absolute pressure of 1 standard atmosphere (14.7 psi), and the gauge pressure in fsw at any depth is incremented by 1 ata to provide absolute pressure. For every additional 33 feet of depth, another atmosphere of pressure accumulates. 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. The pressure of seawater at a depth of 33 feet equals one atmosphere. Pressures measured in fsw and msw are gauge pressure, relative to the surface pressure of 1 atm absolute, except when a pressure difference is measured between the locks of a hyperbaric chamber, which is also generally measured in fsw and msw. Atmospheric pressure may be considered constant at sea level, and minor fluctuations caused by the weather are usually ignored. One atmosphere is approximately equal to 33 feet of sea water or 14.7 psi, which gives 4.9/11 or about 0.445 psi per foot. The msw and fsw are the conventional units for measurement of diver pressure exposure used in decompression tables and the unit of calibration for pneumofathometers and hyperbaric chamber pressure gauges. According to the US Navy Diving Manual, one fsw equals 0.30643 msw, 0.030 643 bar, or 0.444 44 psi, though elsewhere it states that 33 fsw is 14.7 psi (one atmosphere), which gives one fsw equal to about 0.445 psi. The unit used in the US is the foot sea water ( fsw), based on standard gravity and a sea-water density of 64 lb/ft 3. The metre (or meter) sea water ( msw) is a metric unit of pressure used in underwater diving. Pressure gauge on Siebe Gorman manual diver's pump calibrated in fsw and psi
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