A compound gauge is a device that can display both positive and negative (vacuum) pressures. You need to use a compound gauge when you are measuring a system that is exerting both positive and negative pressure on the gauge. Gauges are designed to measure specific types of pressure, so if you pick a simple pressure or vacuum gauge for a system that requires a compound gauge, the gauge will most likely be damaged. The exertion of negative pressure on a pressure gauge will cause damage, and likewise the exertion of positive pressure on a vacuum gauge will also damage the gauge.
Pressure gauges use pounds per square inch (psi) as the unit of measure. Vacuum gauges, on the other hand, measure force in units of inches of mercury (in.Hg). Both of these measurements will be displayed on the face of a compound gauge. The needle in a compound gauge will move clockwise when measuring positive pressure, and counterclockwise when measuring negative pressure.
To check if your gauge is operating properly remove the gauge from the line. Turning off your process is not sufficient because pressure may still be present. Once pressure has been removed from the line, a properly working gauge will read zero. If the gauge does not return to zero, then the gauge is likely damaged.
