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:
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: