The pressure of a system is defined as the force exerted by the system on unit area of its boundaries. This is the definition of the absolute pressure. Often in measurements a gauge is used to record the pressure difference between the system and the atmospheric pressure. This is called gauge pressure and can be stated by the following equation:
P=absolute pressure,
Pg=gauge pressure,
Po=atmospheric pressure.

If the pressure of a system is below atmospheric, it is called vacuum pressure.The SI unit of pressure is pascal (Pa): "bar" is used in many industrial applications: This unit is approximately equal to atmospheric pressure: In hydraulic "m of water" is common: Many gauges use Mercury (Hg) as the measurement medium. Other units are:
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