I just got off the phone with a guy tasked with purchasing a compound gauge, but he didn’t know what it was or how it was different from a standard pressure gauge. I get this call all the time, often from someone with an application that requires a compound gauge, but unsure how to select the right one. And no wonder, compound pressure gauges can be confusing and masquerade under a variety of names.
Simply put, 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 (see the inset image). 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.
If you have a process that requires a compound vacuum gauge, I recommend going with a liquid filled gauge as the dry ones tend to wear out more quickly under the stress.