What measurement unit is indicated for vacuum on a compound gauge?

The indicated measurement unit for vacuum on a compound gauge is typically expressed in inches of mercury. This unit is derived from the way a vacuum is created — as pressure decreases, the height of mercury in the gauge rises, allowing for the measurement of the vacuum in relation to atmospheric pressure. Since the standard atmospheric pressure can support a column of mercury approximately 29.92 inches high, a gauge reading in inches of mercury effectively shows the degree of vacuum created.

The other units, while they measure pressure, do not specifically relate to the vacuum measurement as effectively as inches of mercury. Pounds per square inch (PSI) is a common unit for measuring pressure but does not specifically denote vacuum. Pascals and bars are metric units also used for pressure measurements, but again, they do not provide the same direct interpretation of vacuum in relation to atmospheric standards as inches of mercury do. Thus, inches of mercury are the standard and most recognized measure for vacuum in various applications, particularly in fields like HVAC and engineering.