There are four types of pressure measurement required when monitoring industrial processes,
They are: ·
- Atmospheric Pressure
- Gauge Pressure
- Vacuum Pressure
- Absolute pressure
This is the pressure experienced on the earth’s surface due to the surrounding air. It may be considered that each square meter of the earth’s surface supports a column of air, exerting a pressure of 1.013 bars. The pressure reduces progressively towards the top of the column. This is illustrated by the difference between pressure at sea level and that at the top of a high mountain. It also varies due to changes in temperature and composition of air.
Consider a gauge connected to an air supply and reading 6 bar. The reading is called gauge pressure. This gives the pressure over and above that due to the atmosphere in any sealed
This is pressure below atmospheric, that is where a degree of vacuum exists and it may be expressed in one of two ways. Atmospheric pressure may be used as a datum, and the degree of vacuum expressed as a number of pressure units below this datum, or in terms of absolute pressure using absolute zero pressure as a datum.
If a vessel with a gauge attached and open to the atmosphere is then sealed, the pressure
gauge reading remains zero. Existence of atmospheric pressure can be provided by completely exhausting the vessel when the gauge will read a vacuum (or suction) corresponding to “Absolute Zero” of pressure. Pressure measured from this datum is referred to as absolute pressure.
Thus it follows that:Absolute pressure = gauge pressure + atmospheric pressure