Gauge Pressure
Gauge Pressure: The pressure measured with the help of a pressure gauge is known as gauge pressure, in which atmospheric pressure is taken as datum. All the pressure gauges record the difference between the actual pressure and the atmospheric pressure. The actual pressure is known as absolute pressure. Mathematically
Absolute pressure= Atmospheric pressure + Gauge pressure (Positive)
Atmospheric Pressure + Gauge Pressure
Atmospheric pressure: The atmospheric air exerts a normal pressure upon all surfaces with which it is in contact and it is known as atmospheric pressure. Atmospheric pressure is also known as barometric pressure. The atmospheric pressure at sea level (above absolute zero) is called standard atmospheric pressure and its value is given as follows:
Standard atmospheric pressure = 101.3 kN/m2 or kPa (1 kN/m2 = 1 kPa)
= 10.3 m of water
= 760 mm of Hg
Atmospheric pressure = Absolute pressure - Gauge pressure
Gauge Pressure: The pressure measured with the help of a pressure gauge is known as gauge pressure, in which atmospheric pressure is taken as datum. All the pressure gauges record the difference between the actual pressure and the atmospheric pressure. The actual pressure is known as absolute pressure. Mathematically
Absolute pressure= Atmospheric pressure + Gauge pressure (Positive)
Vacuum pressure: For pressures below atmospheric, the gauge pressure will be negative. This negative gauge pressure is known as vacuum pressure. Therefore
Absolute pressure = Atmospheric pressure - Negative gauge pressure or vacuum pressure
Pascal
When a liquid is contained in a vessel, it exerts force at all points on the sides and bottom of the vessel. The force per unit area is called intensity of pressure. Mathematically, intensity of pressure,
p = P/A
where
P = Force acting on the liquid, and
A = Area on which the force acts.
The intensity of pressure may be expressed either in N/m2, N/mm2 or in metres of liquid or mm of liquid. Intensity of pressure is also expressed in pascal (briefly written as Pa), such that
1 Pa= 1 N/m2; 1 kPa= 1 kN/m2 and 1 MPa = 1 MN/m2= 1 N/mm2
