Pressure is force per unit area on a surface. There are some relative terms such as gauge pressure, atmospheric pressure, and absolute pressure those can be understood easily with Figure 18.2.
Figure 18.2 Relative Terms of Pressure
Gauge Pressure: It is the pressure relative to atmospheric pressure. Gauge pressure is positive for pressures above atmospheric pressure and negative for pressures below it.
Absolute Pressure: It is the sum of gauge pressure and atmospheric pressure.
Pabs = Pgauge + Patmospheric
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