Definition
An instrument that measures pressure both above and below atmospheric pressure on a single dial. The scale typically reads positive pressure (in psi) on one side of zero and vacuum (in inches of mercury) on the other side, allowing a single gauge to display either condition.
Plain English
A pressure gauge that can read both pressure (pushing) and vacuum (sucking) on the same dial, with zero in the middle.
Context Anchor
Seen during aircraft maintenance and system checks where a technician needs to measure both pressure and suction with one instrument.
Derivation
Compound comes from the Latin componere, meaning 'to put together.' The gauge is called compound because it combines two measurements — pressure and vacuum — into one instrument.
Why Pilots Care
Allows a mechanic to verify correct pressure or suction in critical aircraft systems without switching instruments, reducing the chance of missed faults.
Intuition Check
Do not read compound here as simply “made of chemicals” or “complex.” In this term, compound means the gauge combines two kinds of pressure reading on one instrument.
Example Sentence 1
The technician connected the compound gauge to the system to verify it pulled down to the required vacuum before charging.
Example Sentence 2
During fuel system inspection the compound gauge read both positive pressure from the pump and vacuum on the suction side.