Pressure is defined as a force per unit area. The unit of pressure is N/m² or Pa or bar.

• Note: 1 bar = 10⁵ Pa = 10⁵ N/m²

Absolute, Gauge and Vacuum Pressures

Figure: Schematic Diagram Showing Absolute, Gauge and Vacuum Pressure

It is necessary to establish an absolute pressure scale which is independent of the changes in atmospheric pressure. A pressure of absolute zero can exist only in a complete vacuum. Any pressure measured above the absolute zero pressure is termed an ‘absolute pressure’. If the intensity of pressure at a point is measured with reference to atmospheric pressure (p_atm), then it is called gauge pressure at that point. Gauge pressure at a point may be more than the atmospheric pressure or less than the atmospheric pressure.  Accordingly, gauge pressure at the point may be positive or negative. Negative gauge pressure is also called vacuum pressure.
From the figure; It is evident that, 
Absolute pressure at a point = Atmospheric pressure ± Gauge pressure. 
Atmospheric pressure
Air above the surface of liquids exerts pressure on the exposed surface of the liquid and normal to the surface. This pressure exerted by the atmosphere is called atmospheric pressure. Atmospheric pressure at a place depends on the elevation of the place and the temperature. Atmospheric pressure is measured using an instrument called ‘Barometer’ and hence atmospheric pressure is also called Barometric pressure.

Measurement of Pressure
Various devices used to measure pressure can be classified into,

• Manometers

Are the pressure measuring devices which are based on the principle of balancing the column of the liquids whose pressure is to be measured by the same liquid or another liquid.

• Mechanical gauges

It consists of an elastic element which deflects under the action of applied pressure and this movement will operate a pointer on a graduated scale.