# Difference between gauge pressure and absolute pressure measurement

Again and again, we have been asked the question concerning the difference between an absolute pressure measurement and a gauge pressure measurement. To answer this question it helps to look at the definition of gauge pressure and absolute pressure. The difference between your two measurements is then explained relatively simply and thus also the decision 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 is denoted with the subscript ?abs?: Pabs.
Definition of gauge pressure
The gauge pressure means 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.
Unsurpassed between your two measurements is relatively easily clarified: in a gauge pressure measurement, it is always the difference from the existing ambient pressure that’s measured. However, this pressure changes with the elements and the height above sea level. An absolute pressure measurement measures the difference from the ideal or absolute vacuum. This is why this measurement is independent of environmental influences such as for example weather or altitude. Which measurement is currently the right one?
In practice, both measurements can be differentiated as follows: typically, the measuring task would be to determine the gauge pressure. For this reason this type of sensor is most widely used. However, in case a gauge pressure sensor can be used in an application where the actual measuring task would be to gauge the absolute pressure, the next additional errors must be expected:
+/- 30 mbar due to changes in weather
around 200 mbar when changing the positioning (e.g. from sea level to 2,000 m)
According to 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).
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