The typical altimeter (and/or depth gauge) is a pressure measuring instrument that calculates its altitude (or depth) according to the local air or water pressure.
Below we explain how is pressure related to altitude and depth and show you how to calculate both air pressure and water pressure.
1 litre of air has a weight of approximately 1 gram.
A column of air 10m high and 1cm² area (1 litre) creates a pressure of 1g/cm² = 1mbar, so the pressure at sea level is approximately 1000mbar.
Pressure at 100m
At 100m above sea level, the air pressure is approximately 990mbar, so the air pressure has decreased by approximately 10mbar.
Altimeters are based on these changes of the atmospheric pressure and are used in aircraft and by mountain climbers. They always need correction for the variation of that pressure. Pilots will ask for the barometric pressure at runway level prior to landing, and mountain climbers will correct their altimeters at a location of known altitude.
Water is about 1000 times heavier than air, and so 1 litre of water has a mass of 1kg and a volume of 1000cm³.
Again, if we produce a column 10m high and 1cm² area, we will create a water pressure of 1kg/cm² at the bottom (1kg/cm² = 1bar). The pressure in water increases by 1bar for every 10m depth.
Water Pressure at 100m
At 100m, water pressure is 10bar, relative to the surface. But the absolute pressure is 11bar (1bar is the atmospheric pressure on the water surface, plus 10bar water pressure).
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