Know the Difference between Gauge Pressure and Absolute Pressure

Two pressure scales are utilized to measure pressure values in fluid power systems: a gauge scale and an absolute scale. The gauge pressure is the pressure indicated by a pressure gauge. It is a measure of the pressure with reference to the atmospheric pressure (1.013 bar or 14.7 psi) and it is not with reference to the absolute vacuum. Therefore, it does not include the pressure exerted by the atmosphere. The zero gauge pressure corresponds to one atmospheric pressure. The absolute pressure scale begins at the point where there is complete absence of pressure (zero absolute pressure).

The gauge scale is represented with a ‘g’ suffix to the unit or with no suffix at all {for example, bar(g) or bar} and the absolute pressure scale is always represented with an ‘a’ suffix {for example, bar(a) or psi(a)}.

Gauge pressures may be converted to absolute pressure by adding the atmospheric pressure to the gauge reading. Absolute pressures are to be used, for example, in calculations using gas laws. The relationship between absolute pressure and gauge pressure is illustrated graphically in the figure.

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