Again and again, we are asked the question about the difference between a complete pressure measurement and a gauge pressure measurement. To answer this question it helps to look at this is of gauge pressure and absolute pressure. The difference between the two measurements is then explained relatively simply and therefore also the choice of the correct measurement.
Definition of absolute pressure
Absolute pressure is really a pressure that is relative to the zero pressure in the empty, air-free space of the universe. This reference pressure is the ideal or absolute vacuum. It is denoted with the subscript ?abs?: Pabs.
Definition of gauge pressure
The gauge pressure is defined as the difference between a complete pressure (Pabs) and the prevailing atmospheric pressure (Pamb). It really is denoted with the subscript ?e?: Pe and is calculated as follows: Pe = Pabs ? Pamb.
Absolute vs gauge pressure measurement
The difference between your two measurements is relatively easily clarified: in a gauge pressure measurement, it is usually the difference from the existing ambient pressure that is measured. However, this pressure changes with the weather and the height above sea level. A complete pressure measurement measures the difference from the ideal or absolute vacuum. Because of this , this measurement is independent of environmental influences such as for example weather or altitude. Which measurement is currently the right one?
In practice, the two measurements could be differentiated as follows: normally, the measuring task is to determine the gauge pressure. Hate is exactly why this kind of sensor is most widely used. However, in case a gauge pressure sensor is used in an application in which the actual measuring task would be to gauge the absolute pressure, the following additional errors must be expected:
+/- 30 mbar caused by changes in weather
around 200 mbar when changing the location (e.g. from sea level to 2,000 m)
Depending on the measuring range, these errors could be substantial (e.g. in pneumatics at a measuring range of 1 bar) or negligible (in hydraulics at 400 bar).
Note
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