Atmospheric pressure:

The pressure due to air surrounding the earth’s surface is called as atmospheric pressure.

Absolute pressure:

It is known that pressure is force per unit area when the interaction of fluid particles among themselves is zero, a zero pressure intensity will occur. This is possible only when the population of molecules is negligibly small which is nothing but perfect vacuum. Hence the pressure intensity measured from a state of perfect vacuum is called as absolute pressure.

Gauge Pressure:

A pressure measuring instrument generally measures the difference between the unknown pressure (p) and the atmospheric pressure (pa). When the unknown pressure (P) is greater than the atmospheric pressure (Pa), the pressure measured by the instrument is called as the gauge pressure.

Vacuum pressure:

A Pressure measuring instrument generally measures the difference between the unknown pressure (P) and the atmospheric pressure (Pa). When the atmospheric pressure (Pa) is greater than the unknown pressure(P), the pressure measured by the instrument is called as the vacuum pressure.

Static Pressure:

the pressure caused on the walls of the pipe due to a fluid at rest inside the pipe or due to the flow of a fluid parallel to the walls of the pipe is called as static pressure. This static pressure is measured by inserting a pressure measuring tube into the pipe carrying the fluid, so that the tube is at right angle to the fluid flow path.

Total or Stagnation pressure:

the pressure which is obtained by bringing the flowing fluid to rest isentropically is called as total or stagnation pressure. Hence the pressure will be a sum of static pressure and impact pressure.

Dynamic – or – Impact – or – Velocity pressure.

The pressure due to fluid velocity (flow speed) is called as impact pressure.

Impact pressure = Total pressure – static pressure.