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Difference between gauge and absolute pressure

The absolute pressure pabs is the pressure compared with the zero pressure of empty space (i.e. a vacuum).

The gauge (relative) pressure prel is the pressure compared with atmospheric pressure pamb (surrounding air pressure).

The atmospheric pressure (in hPa) changes continually, according to the altitude or weather.
According to the application, the pressure will need to be measured either as gauge (relative) pressure or as absolute pressure.

Example for absolute pressure: Measuring air pressure


The atmospheric air pressure is an absolute measurement.
This is used, for example, in weather forecasting or altitude measurement.

Example for gauge (relative) pressure: Filling level measurement

. pressure_t1q4_1pressure_t1q4_4

The gauge pressure transmitter measures the hydrostatic pressure (prel) of the liquid and converts it into an electrical signal.
If the density of the liquid is known, then this signal provides a direct indication of the filling level h.

Applications include:
Wells, lakes and ponds, swimming baths, reservoirs and tanks, heating systems.

Using a vacuum pump to evacuate a vessel
Aim: to switch off the pump at a residual pressure of 5 mbarabs



The aim is to switch off the pump at a residual pressure of 5 mbarabs. The diagram shows how this can be achieved through absolute or relative (gauge) pressure.
The switching point will only remain constant if absolute pressure measurement is used.

State of the system
Fine weather = air pressure about 960 mbar
Bad weather = air pressure about 940 mbar
Absolute measurement
Relative measurement
Absolute measurement
Relative measurement
Vessel contains air (atmospheric)
960 mbarabs
0 mbarrel
940 mbarabs
0 mbarrel
Vessel is empty (residual pressure)
5 mbarabs
-955 mbarrel
5 mbarabs
-935 mbarrel