Also if you have taken delivery of a pressure transducer with the wrong pressure reference it will normally mean that the manufacturer will have to build a replacement because the reference pressure often determines the base construction of the sensing element.
The most common pressure reference is gauge pressure which is signified by a ‘g’ after the pressure unit e.g. 30 psi g, this indicates that the pressure measured is the total pressure minus atmospheric pressure. There are two types of gauge reference pressure: vented gauge (vg) and sealed gauge (sg),
A vented gauge pressure transmitter for example allows the outside air pressure to be exposed to the negative side of the pressure sensing diaphragm via a vented cable or a hole on the side of the device, so that it always measures with reference to the ambient barometric pressure. Thus a vented gauge reference pressure sensor reads zero pressure when the process pressure connection is held open to atmospheric air.
A sealed gauge reference is very similar except that atmospheric pressure is sealed on the negative side of the diaphragm. This is usually adopted on high pressure applications such as measuring hydraulic pressures where atmospheric pressure changes will have negligible effect on the accuracy of the sensor so venting is not necessary. This also allows manufacturers to provide secondary pressure containment for extra protection if the burst pressure of the sensing diaphragm is exceeded.
There is another way of creating a sealed gauge reference and this is to seal a high vacuum on the reverse side of the sensing diaphragm. Then by adjusting the electronics, the output signal is offset by 1 bar so the pressure sensor reads close to zero when measuring atmospheric pressure.
A sealed gauge reference pressure transducer will never read exactly zero when left open to atmospheric air, because atmospheric pressure is always changing and the reference in this case is fixed at a nominal 1 bar.
An absolute pressure measurement is one that is referred to absolute vacuum. The best example of an absolute referenced pressure is the measurement of barometric pressure.
In order to produce an absolute pressure sensor the manufacturer will seal a high vacuum behind the sensing diaphragm. Therefore if you hold open the process pressure connection of an absolute pressure transmitter to the air it will read the actual barometric pressure.
So how do you know when to measure absolute pressure or when to measure gauge pressure?
This is not always straightforward but generally if you want to measure or control a pressure that is influenced by changes in atmospheric pressure, e.g. the level of liquid in an open tank or the output pressure of an air compressor; you would use a vented gauge pressure instrument since you are interested in the pressure reading minus the atmospheric pressure component.
If you want to measure pressures that are not influenced by changes in atmospheric pressure, e.g. leak testing a completely sealed non-flexible container, you would use an absolute pressure sensor. If a gauge pressure sensor was used instead to measure the container pressure, and the barometric pressure changed, then the sensor’s reading would change, despite the fact that the pressure in the container remains the same.
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