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.

Pressure can be expressed in units such as pascal, bar, atmosphere, kilogram per square centimeter, and pounds per square inch (PSI). In the international system it is normalized as the unit of pressure the pascal, which is 1 newton per square meter (1N/m2).

Pascal is a small unit. The pascals (1kPa = 0.01 bar), I megapascal (1 MPa = 10 bar), and 1 gigapascal ( 1 GPa = 10,000 bar).

In industry, the bar is also used (1 bar = 105, Pa = 1.02 kg/cm2). The unit kg/cm2, despite its wide spread use, increasingly used less frequently.

**Types of Pressures **

**Atmosheric 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.

The unit bar, despite its wide spread use, increasingly used less frequently.

Absolute pressure

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

Absolute pressure = Gauge pressure + 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 (ATM) 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.