There are different options for measuring pressure. Absolute pressure is going to be a measurement that is referenced against a perfect vacuum. Gage pressure is measured or referenced against the ambient air pressure. In other words, the gage pressure measurement will be the absolute pressure (referenced against a vacuum), minus the surrounding atmospheric pressure.
There is also a third possibility in measuring pressure. This can be done through a differential pressure. In this method, the difference between two reference points, when one point is not a vacuum or the atmospheric pressure, is measured and transmitted as an electric signal. This is also a common option, but it requires two reference points, so it is not a good option for measuring many types of pressure.
Not all applications and systems are a good match for gage pressure measurement. However, because of the reference point, the atmosphere, these pressure sensors can be used anywhere in the world. It will automatically use the environmental atmospheric pressure where it is used, so the reference is always consistent.
This is essential when there is an ongoing pressure measurement need over time. This also allows measurements to be taken in different areas and compared to the same reference for study, tracking and longitudinal recordings.
These are also relatively simple gauges with only one pressure port. This port is designed to allow the ambient air pressure to access the sensing unit. This is typically accomplished through the use of a vent tube or a vent hole. It will always direct the air to the negative side of the sensing element, or the negative side of the diaphragm.
To determine if gage pressure measurement has been used, look for a “g” behind the unit being measured. For example, BARG or PSIG will be measurements to indicate the measurement was completed through gauge pressure measurement devices.