Pressure

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=Pg+Po

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):

1 Pa=1 N/m

"bar" is used in many industrial applications:

1 bar=0.1 MPa

This unit is approximately equal to atmospheric pressure:

Standard atmospheric pressure=1.01325 bar= 101325 Pa

In hydraulic "m of water" is common:

1 m of water=9806.65 Pa

Many gauges use Mercury (Hg) as the measurement medium.

1 mmHg=133.4 Pa

Other units are:

dyn/cm=0.1 Pa
kgf/cm=kp/cm=at=98067 Pa
lbf/ft=47.879 Pa
in (H2O)=249.08 Pa
in (Hg)=3386.4 Pa
psi=lbf/in=6894.7 Pa
ksi=1000 psi=68.947 MPa