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}, and converted to bar for calculations.
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.
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.
The distinction between gauge and absolute pressure is important for calculation of gas properties and pressure must be identified as either gauge or absolute.
Gauge pressure in msw or fsw is converted to absolute pressure in bar or atm for decompression and gas consumption calculation, but decompression tables are usually provided ready for use directly with the gauge pressure in msw and fsw.
Depth gauges and dive computers with readouts calibrated in feet and metres are actually displaying a pressure measurement, usually in feet or metres sea water, as most diving is done in the sea.
If ambient pressure in fresh water and hyperbaric chambers is measured in feet and metres sea water, the same decompression algorithms and tables can be used, which eliminates the need to use calibration factors when diving in these environments.
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}, and converted to bar for calculations. 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. 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. The distinction between gauge and absolute pressure is important for calculation of gas properties and pressure must be identified as either gauge or absolute. Gauge pressure in msw or fsw is converted to absolute pressure in bar or atm for decompression and gas consumption calculation, but decompression tables are usually provided ready for use directly with the gauge pressure in msw and fsw. Depth gauges and dive computers with readouts calibrated in feet and metres are actually displaying a pressure measurement, usually in feet or metres sea water, as most diving is done in the sea. If ambient pressure in fresh water and hyperbaric chambers is measured in feet and metres sea water, the same decompression algorithms and tables can be used, which eliminates the need to use calibration factors when diving in these environments.
Metre sea water