ABSOLUTE, GAUGE, ATMOSPHERIC AND VACUUM 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 absolute pressure and in other system, pressure is measured above the atmospheric pressure and it is called gauge pressure.

Absolute Pressure 

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

Gauge 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 datum. The atmospheric pressure on the scale is marked as zero.

Vacuum Pressure 

Vacuum pressure is defined as the pressure below the atmospheric pressure.

The relationship between the absolute pressure, gauge pressure and vacuum pressure are shown in fig.

Mathematically,

Absolute pressure = Atmospheric pressure + Gauge pressure

          P_ab = p_atm + p_gauge

Vacuum pressure = Atmospheric pressure – Absolute pressure

Note:

1. The atmospheric pressure at sea level at 15⁰C is 101.3 KN/m³ or 10.13 N/cm² in SI unit. In case of MKS units, it is equal to 1.033 kgf/cm².
2. The atmospheric pressure head is 760mm of mercury or 10.33m of water.

Problem: 

 What are the gauge pressure and absolute pressure at a point 3m below the free surface of a liquid having a density of 1.53 × 10³ kg/m³ if the atmospheric pressure is equivalent to 750 mm mercury?  The specific gravity of mercury is 13.6 and density of water is 1000 kg/m³.