Specific gravity is a comparison ratio between the density of a particular substance and the density of a reference substance. Since the density of the reference substance will vary depending on its temperature, the specific gravity should also include the temperature of the reference media.
A very common reference media used for the calculation of specific gravity is fresh water at 4 degrees Celsius, which is very close to the temperature at which water is most dense.
The main use of specific gravity in pressure measurement is to calibrate hydrostatic liquid level measurement equipment. The majority of hydrostatic pressure measurement devices are calibrated in depth of water units such as mH2O or ftWG. Therefore if you want to use these devices to measure the level of a liquid which has a lower specific gravity than water, such as oil for example, you will need to re-calibrate the range from metres of water to meters of oil. The easiest way to do this is to divide the range in meters of water by the SG of the oil.
The calculation of specific gravity is very important to the management of liquid storage tanks, where the quantity of liquid contents has to be determined precisely for commercial purposes.
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