The “low side” Gauge
The low side gauge, which is most easily identified by the BLUE housing and hand valve, is used to measure the low side pressure at the service ports provided on the low side of the system by the manufacturer. The low side gauge pressure scale reads from 0 to between 130 and 150 pounds per square inch (psi) in a clockwise direction. The vacuum scale reads from 0 to 30 inches of mercury in a counterclockwise direction. The low side gauge is sometimes called a compound gauge because it has a dual purpose – to indicate either pressure or vacuum.