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 can help to look at this is of gauge pressure and absolute pressure. The difference between your two measurements is then explained relatively simply and thus also the choice of the appropriate measurement.
Definition of absolute pressure
Absolute pressure is a pressure that is relative to the zero pressure in the empty, air-free space of the universe. This reference pressure may be the ideal or absolute vacuum. It really is denoted with the subscript ?abs?: Pabs.
Crush of gauge pressure
The gauge pressure is defined as the difference between a complete pressure (Pabs) and the prevailing atmospheric pressure (Pamb). It 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 will always be 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. This is the reason this measurement is independent of environmental influences such as weather or altitude. Which measurement is now the right one?
In practice, the two measurements can be differentiated the following: typically, the measuring task is to determine the gauge pressure. For this reason this kind of sensor is most widely used. However, if a gauge pressure sensor is used in an application in which the actual measuring task would be to measure the absolute pressure, the following additional errors should be expected:
+/- 30 mbar caused by changes in weather
around 200 mbar when changing the positioning (e.g. from sea level to 2,000 m)
Depending on 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
If you are uncertain whether your measuring task needs an absolute pressure or gauge pressure measurement, simply contact us ? we?ll be glad to assist you.