Absolute Pressure Gauges
One big difference in how pressure gauges are rated is gauge pressure vs.
absolute pressure. Both measure pressure, but a gauge pressure device is zeroed against atmospheric conditions, where an absolute pressure gauge is not.
The zero point for an absolute pressure gauge is absolute zero or absolute vacuum. Therefore the displayed value on an
absolute pressure gauge will always be positive. This differs from a gauge pressure device, which will show a negative value for a vacuum measurement, as it will be less than atmospheric pressure.
We have
pressure measurement devices that measure both gauge pressure and absolute pressure. We even have devices that measure both absolute pressure and gauge pressure.