Hydrostatic pressure is what is exerted by a liquid when it is at rest. The height of a liquid column of uniform density is directly proportional to the hydrostatic pressure.
The hydrostatic properties of a liquid are not constant and the main factors influencing it are the density of the liquid and the local gravity. Both of these quantities need to be known in order to determine the hydrostatic pressure of a particular liquid.
The formula for calculating the hydrostatic pressure of a column of liquid in SI units is:
Hydrostatic Pressure (Pa, N/m2) = Height (m) x Density(kg/m3) x Gravity(m/s2)
The density of a liquid will vary with changes in temperature so this is often quoted alongside hydrostatic pressure units e.g. mH2O @ 4 deg C.
The local gravity depends on latitudinal position and height above sea level.
For convenience the most common standard for hydrostatic pressure is metres of water or feet of water at 4 deg C (39.2 degF) with a standard gravity of 9.80665 m/s2. The density of pure water at 4 deg C is very close to 1000 kg/m3 and therefore this has been adopted as the standard density of water. Another reason for the significance of choosing 4 deg C is that it is very close to the temperature that water reaches its maximum density.
In practical terms hydrostatic pressure units are rarely absolutely precise because the temperature of any liquid is not always going to be 4 deg C. You will also come across another temperature standard of 60 deg F (15.56 deg C). This can lead to confusion and inaccuracies when the temperature is not labelled alongside the hydrostatic pressure unit. For most applications these differences are not significant enough to influence the results since the reading accuracy is often much wider than the difference in the pressure unit conversion factor at these 2 temperatures.
In summary hydrostatic pressure units are a very convenient method for relating pressure to a height of fluid but they are not absolute pressure units and it is not always clear what density/temperature has been assumed in their derivation, so be very cautious when using them for high precision level measurements. In fact some institutions are discouraging their use because of the very reasons mentioned above.
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