A compound pressure gauge measures

• A. Pressure and vacuum
• B. Temperature
• C. Humidity
• D. Flow

Answer: (A)

A compound pressure gauge is designed to read both positive pressure and vacuum on a single instrument. It uses a Bourdon tube connected to the system and has two scales on the dial: one for pressures above atmospheric and another for vacuum below atmospheric. This lets you see how the system is performing under charging or evacuating conditions, since refrigerant systems can operate with pressure above atmospheric or go into vacuum during evacuation or after a system is recovered.

In refrigeration work, you monitor pressure to ensure the correct charge and to diagnose operating conditions, and the vacuum side helps verify that there’s no remaining air or moisture when evacuating.

The other measurements aren’t what this tool provides: temperature requires a thermometer, humidity needs a hygrometer, and flow requires a flow meter.