Atmospheric, gauge and absolute pressures

ATMOSPHERIC, GAUGE AND ABSOLUTE PRESSURES

1. Atmospheric Pressure

• The atmosphere air exerts a normal pressure upon the surface with which it is in contact and it is known as atmospheric pressure.

• The atmospheric pressure is also known as barometric pressure.

• The atmospheric pressure arises at sea level is called standard atmospheric pressure.

2. Gauge Pressure

• If the pressure is measured with reference to the atmospheric pressure, then it is called gauge pressure.

• It is the pressure, measured with the help of pressure measuring instrument, in which the atmospheric pressure is taken as datum. The atmospheric pressure on the scale is marked as zero. Refer Fig.3.3(a).

• Gauge pressure measure is universally used in most of the hydraulic and pneumatic systems.

• The pressure of the liquid may be above or below the atmospheric pressure. If it is below the atmospheric pressure, then the gauge is designated as ‘vacuum gauge' and the pressure is known as ‘vacuum pressure'. The vacuum (i.e., negative) pressure indicates the amount by which the pressure of the liquid is below the atmospheric pressure.

3. Absolute Pressure

• Any pressure measured above the absolute zero pressure is termed as an 'absolute pressure'. In other words, the pressure measured with respect to vacuum is called an 'absolute pressure'. Refer Fig.3.3(b).

• Absolute pressure is important in the comparison of the compression of gases.

4. Differential Pressure

• Some pressure transducers or transmitters measure the pressure difference between the two parts and it is known as differential pressure. Refer Fig.3.3(c).

• The different forms of pressure measurement are depicted in Fig.3.3.

5. Relationship Between Pressures

• Fig.3.4 illustrates the relationship between absolute, gauge and vacuum pressures.

• From Fig.3.4, it can be stated that,

Note

Standard atmospheric pressure has the following equivalent values: 101.3 kPa; 10.3 m of water; and 760 mm of mercury.