A compound pressure gauge for the low side of the system measures pressure in what?

A compound pressure gauge measures two scales, one in positive pressure and one in negative (below atmospheric) pressure or vacuum

positive pressure is measured in pounds per square inch (psi)

or kilopascals or in some cases bars (1 bar = 14.35 psi) [ the average atmospheric

pressure at 1000 feet above sea level]

vacuum is measured in millimeters of Mercury mmHg or inches of mercury inHg