Absolute and gauge pressure:
Pressure in the fluid is defined as normal force per unit area exerted on the imaginary or real plane surface in a gas or a fluid. The equation for pressure is expressed as folloes:
p = F / A
Absolute Pressure:
The absolute pressure - pa - is the measured relative to absolute zero pressure - the pressure which would occur at absolute vacuum. All the calculation including the gas laws requires pressure and temperature to be in absolute units.
Gauge Pressure:
A gauge is used to measure the pressure difference in between system and surrounding atmosphere.
There are 2 kinds of references to measure pressure -gauge pressure and absolute pressure. The absolute pressure can be measured relative to the absolute zero pressure. Absolute pressure is pressure which would occur at absolute vacuum, or zero pounds per square inch (PSI). All computations involving the gas laws need pressure, and temperature, to be in the absolute units. Absolute pressure is referred to as 'total systems pressure' also. To distinguish in between it from gauge pressure, the term 'abs' is commonly placed after the unit.
On the other hand, gauge pressure is most commonly used reference of pressure. Machines such as well pumps, air compressors, and tire gauges will all use gauge pressure. Gauge pressure uses atmospheric pressure (14.7 PSI), as its zero point. Sometimes, it is referred to as 'overpressure'. A 'g' is often placed after the unit of pressure to signify that a particular measurement is a gauge pressure. Absolute pressure uses absolute zero as its zero point, whereas gauge pressure uses atmospheric pressure as its zero point. The Gauge pressure is commonly used, whereas absolute pressure is used for the scientific experimentations and calculations. To indicate the gauge pressure, a 'g' is placed after unit. Absolute pressure conversely uses the term 'abs'. Because of varying atmospheric pressure, gauge pressure measurement is not precise in nature, while the absolute pressure is always definite.