Different Types of Pressure in Fluids [PDF]

Absolute pressure

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Let’s know the definition of Pressure,

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

The pressure is an effect which occurs when a force is applied on a surface.

The pressure is the amount of force acting on a unit area. This is compressive stress on the fluid and the mathematical expression is,

Pressure (p) = Force / Area = F / A

Where p is the pressure and the SI unit is Pascal. (1 Pa = 1 N/m2).

F is the normal force and the SI unit is Newton (N).

A is the area. The SI unit is m2.

Do you know the pressure also expressed in the terms of Bar?. Let’s know in brief the unit conversion,

1 Pa = 1 N/m2

1 kPa = 1000 N/m2

1 bar = 100 kPa = 105 N/m2.

Different types of Pressure:

We will understand the relation between the Absolute, Gauge, Atmospheric and Vacuum Pressure by a line diagram.

Absolute pressure

The pressure on a fluid is measured in two different systems. In one system, It is measured above the absolute zero or complete vacuum and it is called the absolute pressure.

And the other system, the pressure is measured above the atmospheric pressure and It is called gauge pressure. Therefore,

The Absolute pressure is defined as the pressure, which is measured with reference to absolute vacuum pressure.

Gauge pressure is defined as the pressure, which is measured with the help of a pressure measuring instrument, in which, the atmospheric pressure is taken as a datum. The atmospheric pressure on the scale is marked as zero.


Absolute Pressure = Atmospheric Pressure + Gauge Pressure

Vacuum Pressure  = Atmospheric Pressure – Gauge Pressure