The most common reference pressure is a vented gauge or gauge reference which means that the reverse side of the sensing diaphragm is open to atmospheric pressure. If a gauge reference pressure sensor is held open to atmospheric air pressure on the positive side it will indicate zero pressure since the total pressure on both sides of the diaphragm are the same.
For high pressure ranges some manufacturers incorporate secondary pressure containment and it is not possible to provide a true vented gauge reference. In sealed gauge reference pressure sensors either the atmospheric pressure of the day is sealed in on the reverse side of the sensing diaphragm or a vacuum is sealed in and the electronics are offset by 1 bar. Therefore a sealed gauge pressure sensor will read close to zero when the positive pressure port is held open to atmospheric pressure.
For lower sealed gauge pressure ranges, atmospheric pressure will generate a noticeable offset which will affect high accuracy readings. For example a 10 mbar change in atmospheric pressure would generate a 0.1% full scale error for a 10 bar range. Typically sealed gauge pressure references are only used for ranges 70 bar and higher where atmospheric pressure changes will have very little impact on the overall reading accuracy.
An absolute pressure reference is a hard vacuum sealed on the reverse side of the pressure sensor diaphragm so that pressures can be measured independently of atmospheric air pressure changes. When an absolute reference pressure sensor is held open to ambient air it will read the atmospheric pressure.
A differential pressure sensor has two pressure connections and no fixed reference pressure. Both the positive and negative side of the sensing diaphragm can be connected to different parts of the system being measured in order for the difference in pressure to be determined between two points.
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