There are three pressure scales used for measuring pressure.
- The Absolute pressure scale
- The Gauge pressure scale
- The Vacuum pressure scale
In the pressure gauges shown in Figure all pressure gauges show a reading of 20, but their values of pressure are all different.
Gauge A Shows a pressure of 20 pounds per square inch absolute
Gauge B Shows a pressure of 20 pounds per square inch gauge
Gauge C Shows a pressure of 20 inches Mercury Vacuum
In order to understand why these pressure gauges represent different values of pressure, you have to understand the concept of Absolute pressure, Gauge pressure and Vacuum pressure.
Pressure gauges with different pressure scales
Absolute Pressure scale
Absolute pressure is when the pressure is measured using absolutely no pressure as a starting point.
Air, which is a mixture of gases, has weight. The force of gravity attracts the air. At sea-level the ‘standard’ weight of the earth’s atmosphere exerts a pressure of 14.7 PSI Atmospheric pressure.
Since we all are subjected to the pressure of the earth’s atmosphere all the time, we may think of it as no pressure at all and consider the pressure of the earth’s atmosphere to be zero. Yet it is a very real pressure. If we move all the air out of a closed chamber, there would be no pressure within the chamber, so the pressure would be absolutely zero.
The absolute pressure scale uses absolute zero pressure as its starting point. Absolute pressure is the complete absence of pressure, even Atmospheric pressure.
Pressure readings taken from an absolute pressure scale are expressed as pounds per square inch absolute (PSIA).
An unconnected pressure gauge that uses absolute pressure scale will read 14.7 PSIA if it is accurate.
Absolute pressure = Gauge pressure + Atmospheric pressure.
An unconnected pressure gauge with absolute pressure scale
Gauge Pressure scale
Gauge pressure is pressure measured using atmospheric pressure as a starting point and calling this pressure ZERO.
Most pressures measured in Gas/ Oil processing are gauge pressures. The gauge pressure uses atmospheric pressure as the starting point for all measurements. This means that the gauge scale starts at zero even though there is still atmospheric pressure on it.
An unconnected pressure gauge using gauge pressure scales will read zero if it is accurate.
Gauge pressure = Absolute pressure – Atmospheric pressure
An unconnected pressure gauge using gauge pressure
Vacuum Pressure scale
A vacuum is a space that contains no air. In hydrocarbon processing plants, vacuum means pressure lower than atmospheric pressure. Vacuum pressure is commonly measured in inches of mercury. A reading of zero on the mercury vacuum gauge is equal to atmospheric pressure. The vacuum scale is used to measure pressures in pipes or vessels which are below atmospheric pressures.
An unconnected pressure gauge with a vacuum scale will read zero.
An unconnected pressure gauge with a vacuum pressure scale