Absolute pressure is a particular type of pressure measurement which is always referred to a perfect vacuum.
A very common example of measuring absolute pressure is using an Aneroid Barometer to measure atmospheric pressure. Inside an Aneroid Barometer there is a hollow flexible capsule which has a high vacuum sealed inside. As the atmospheric pressure rises and falls the differential pressure between the inside and outside of the capsule causes it to expand and contract. The expansion and contraction of the capsule is transferred into a mechanical movement by coupling it to a mechanism which moves a needle on a calibrated dial, scaled to read barometric pressure.
Another practicle example of how an absolute reference is the absolute pressure sensor. The manufacturer will seal a high vacuum behind the sensing diaphragm so that the total pressure can be measured on the positive side of the diaphragm independently of the outside atmospheric air pressure. If the pressure connection of an absolute pressure sensor is vented to ambient air pressure it will then be measuring the barometric pressure in a similar way to the Aneroid Barometer.
Absolute pressure is measured in many applications where the changes in atmospheric pressure have no influence on the measurements. For example when leak testing a solid walled vessel over a long period of time the total pressure inside should remain constant independent of the changing atmospheric pressure outside the vessel. Therefore a pressure sensor or instrument with an absolute pressure reference would be the most appropriate choice for leak testing purposes.
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