Metre sea water


The metre sea water is a metric unit of pressure used in underwater diving. It is defined as one tenth of a bar.
The unit used in the US is the foot sea water, based on standard gravity and a sea-water density of 64 lb/ft3. According to the US Navy Diving Manual, one fsw equals 0.30643 msw,, or, though elsewhere it states that 33 fsw is , which gives one fsw equal to about 0.445 psi.
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.

Feet of sea water

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. Atmospheric pressure may be considered constant at sea level, and minor fluctuations caused by the weather are usually ignored. 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.
The pressure of seawater at a depth of 33 feet equals one atmosphere. The absolute pressure at 33 feet depth in sea water is a combination 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. Therefore at the surface the gauge pressure of 0 fsw is equivalent to an absolute pressure of, and the gauge pressure in fsw at any depth is incremented by 1 ata to provide absolute pressure.

Usage

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. In metric units the ambient pressure is usually measured in metres sea water (msw

Conversions

In the metric system, a pressure of 10 msw is defined as 1 bar. Pressure conversion between msw and fsw is slightly different from length conversion between metres and feet; 10 msw = 32.6336 fsw and 10 m = 32.8083 ft.
The US Navy Diving Manual gives conversion factors for "fw" based on a fresh water density of 62.4 lb/ft3 and for fsw based on a sea water density of 64.0 lb/ft3.
One standard metre sea water equals:
One standard metre sea water is also approximately equal to:
One standard foot sea water is approximately equal to:
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