Pressure (P) is defined as the physical force per unit area applied in a direction perpendicular to the plane surface of an object.

Mathematically it is **P = F/A,**

where F = Force and A = Area.

The air pressure varies with location and weather.

Types of Pressures:

- Atmospheric pressure.
- Absolute pressure.
- Gauge pressure.
- Differential pressure.

Atmospheric pressure:

Atmospheric pressure is also known as barometric pressure, surrounded by atmospheric gases on the earth. It varies with altitude and weather. The standard device used to measure is a barometer.

Absolute pressure:

Absolute pressure is measured relative to a full vacuum. Absolute pressure is the sum of the gauge pressure and atmospheric pressure.

Gauge pressure:

Gauge pressure is relative to atmospheric pressure. Gauge pressure is positive for pressures above the atmosphere and negative for pressures below it.

Differential pressure:

Differential pressure is simply the difference between two applied pressures. Differential pressure can be measured as gauge pressure or absolute pressure.

The basic atmospheric pressure unit – 101300 Pa (Pascal).

= 101.3 k Pa (Kilopascals)

= 0.1013 MPa (Megapascal).

= 1.013 bar = 1.033 kg/cm² absolute.

= 760 mm of Hg (torr).

= 14.7 psia.

= 29.92 inches of mercury (Hg).

There are three standards available:

1. API Standard: 14.7 psia, 60°F, 0% relative humidity.

2. ASME Standard: 14.7 psia, 68°F, 36% relative humidity.

3. CAGI Standard: 14.7 psia, 60°F, 36% relative humidity.