Pressure - Thermodynamics:

While working in the system, thermodynamic medium exerts a force on boundaries of vessel in which it is kept. The vessel may be a container, or an engine cylinder having a piston etc. The exerted force F per unit area (A) on a surface that is normal to the force is called as intensity of pressure or simply pressure p. Therefore

P = F/A=   .g.h

It is expressed in Pascal (1 Pa = 1 Nm²), bar (1 bar = 10⁵ Pa),

standard atmosphere (1 atm =1.0132 bar),or technical atmosphere (1 kg/cm² or 1 atm).

1 atm means 1 atmospheric absolute.

The pressure is usually represented in following terms.

1.   Atmospheric pressure

2.   Gauge pressure

3.   Vacuum pressure

4.   Absolute pressure