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.
