Relation between Gauge, Vacuum & Absolute Pressure

Pressure:

Pressure is defined as a force per unit area. Pressure are exerts by gases, vapors and liquid.

                                    Pressure = Force/Area
                                               P = F/A

Atmospheric Pressure:

The atmospheric air exerts a normal pressure upon all the surfaces with which it is in contact and it is known as atmospheric pressure.

Standard atmospheric pressure:

The atmospheric pressure at sea level (above absolute zero) is called standard atmospheric pressure and its value is given as follows:

Standard atmospheric pressure = 101.3 KN/m² 

                                                 = 10.3 m of water

                                                 = 760 mm of Hg

Gauge Pressure:

The pressure measured with the help of a pressure gauge is known as gauge pressure, in which atmospheric pressure is taken as datum. All pressure gauge record the difference between the actual pressure  and the atmospheric pressure.

Absolute pressure: 

The actual pressure is known as absolute pressure.

Mathematically, 

Absolute Pressure = Atmospheric pressure + Gauge pressure (Positive)

This relation is used for pressures above atmospheric, i.e for positive gauge pressure. 

Vacuum pressure:

For pressure below atmospheric, the gauge pressure will be negative. This negative gauge pressure is known as vacuum pressure.

Absolute Pressure = Atmospheric pressure - Negative gauge pressure or vacuum pressure.