Atmospheric pressure:
The pressure due to air surrounding the earths surface is called as atmosperic 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 prefect vacuum is called as absoulte pressure.
Gauge Pressure:
A pressure measuring instrument generally measures the difference between the uknown pressure § and the atmospheric pressure (pa). When the unknown pressure § 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 § and the atmospheric pressure (Pa). When the atmospheric pressure (Pa) is greater than the unknown pressure§, the pressure mesured 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.
Pressure relation
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.