DIFFERENCE BETWEEN GAUGE PRESSURE AND ABSOLUTE PRESSURE EXPLAINED !!!

Introduction:

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Mathematically, it is symbolized with a ‘P’. To put it briefly, it is the amount of force acting on a unit area. The simple formula for pressure is:

P = F / A; where P = pressure

F = force
A = area

The SI unit for pressure is in Pascals (Pa). Other non-SI units are PSI and bar.

There are two kinds of references to measure pressure ‘“ the gauge pressure and the absolute pressure. 

Absolute Pressure:

The actual pressure at a given position is called the absolute pressure and it is measured relative to absolute vacuum. One concept should be taken into consideration is that to measure any quantity we require a base line with respect we are going to measure it.

To learn this concept let us take an example, suppose we need to measure distance of Chennai.Distance can be measured in meter. Can we measure distance of Chennai by this input? Obviously your answer is no because we need a reference from which we want to measure distance. Now suppose we need to measure distance of Chennai from Delhi. Now we are able to measure this distance in some meters or kilometers.

Similarly pressure cannot be measured without a reference. When we take vacuum or no pressure condition as reference, the measured pressure is called absolute pressure.

Gauge Pressure:

When we take atmospheric pressure as reference to measure pressure of any system, the measured pressure is known as gauge pressure. Most of pressure devices work in atmospheric condition always measure gauge pressure. We can convert this gauge pressure in absolute pressure by adding atmospheric pressure in gauge pressure. 

It should be noted that atmospheric pressure may vary, depending on many factors, such as locality. Altitude and temperature are essential factors. The standard atmospheric pressure (1 ATM) is about 14.7 PSI.

P (absolute) =  P (Gauge) +  P (Atmospheric)

Most of gauge read zero in atmosphere but there is some atmospheric pressure. They read atmospheric pressure as absolute zero pressure. Pressure below atmospheric pressure is called vacuum pressure and is measured by vacuum gauges that indicate the difference between the atmospheric pressure and absolute pressure.

P (vacuum) = P (Atmospheric) – P (Absolute)

Machines like air compressors, well pumps, and tire gauges will all use gauge pressure. 

Summary:

1. Absolute pressure is measured in relation to the vacuum, while gauge pressure is the difference between the absolute pressure and the atmospheric pressure.

2. Absolute pressure uses absolute zero as it’s zero point, while gauge pressure uses atmospheric pressure as it’s zero point.

3. Gauge pressure is commonly used, while absolute pressure is used for scientific experimentations and calculations.

4. To indicate gauge pressure, a ‘g’ is placed after the unit. Absolute pressure, on the other hand, uses the term ‘abs’.

5. Due to varying atmospheric pressure, gauge pressure measurement is not precise, while absolute pressure is always definite.

6. Absolute pressure is sometimes referred to as ‘total systems pressure’, while gauge pressure is sometimes called ‘overpressure’.