1. Absolute Pressure:
It is the zero reference to a perfect vacuum, which exists in the air free space of the universe. It is denoted by Abs. Derived from Latin “absolutus”, meaning independent.
Absolute pressure is the sum of gauge pressure and atmospheric pressure.
P(abs) = P(gauge) + P(atm)
2. Gauge Pressure:
It is the pressure, zero referenced against ambient pressure. It is also called overpressure. Gauge pressure is positive for pressures above the atmosphere and negative for pressures below it.
P(gauge) = P(abs) – P(atm)
3. Atmospheric Pressure:
It is the pressure exerted by the weight of the atmosphere. It is basically pressured around you i.e.ambient pressure.
The standard value of air pressure at sea level is 14.696 psi at 59 Deg F heat. In imperial units and metric units – it is 1013.25 millibar at 15 Deg C.
4. Vacuum Pressure:
It is the pressure below atmospheric pressure is measured by vacuum gauges. It indicates the difference between atmospheric pressure and absolute pressure.
P(vac) = P(atm) – P(abs)
OR P(vac) = P(–gauge)