Difference between gauge and absolute pressure

The most common pressure reference is gauge pressure signified by a (g) after the pressure unit e.g. 30 psi g, which means 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 the pressure referred to ambient barometric pressure. Thus a vented gauge reference pressure sensor should always read zero pressure when the process pressure connection is held open to the 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 ranges such as hydraulics where atmospheric pressure changes will have a negligible effect on the accuracy of the sensor so venting is not necessary. This also allows some manufacturers to provide secondary pressure containment as an extra precaution for pressure equipment safety if the burst pressure of the primary pressure 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 because atmospheric pressure is always changing and the reference in this case is fixed at 1 bar.

An absolute pressure measurement is one that is referred to absolute vacuum and this is absolutely the lowest pressure that the laws of physics will allow. The best example of an absolute referenced pressure is atmospheric or 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 an absolute pressure or a gauge pressure?

This is not always straightforward but generally if you want to measure or control a pressure that is dependent on changes in atmospheric pressure, e.g. the level of liquid in an open tank or the output pressure of an air compressor, you will need to use a vented gauge pressure instrument.

If you want to measure pressures that are not influenced by changes in atmospheric pressure, e.g. leak testing a gas mains pipe, you would typically use an absolute pressure indicator.