What type of gauge is used to measure pressure both above and below normal atmospheric pressure?

The correct choice, compound gauge, is designed to measure both positive pressure (above atmospheric pressure) and negative pressure (vacuum or below atmospheric pressure) in a single instrument. This dual capability is particularly useful in various applications, such as HVAC systems, refrigeration, and other mechanical processes where monitoring the pressure on both sides of atmospheric conditions is crucial for efficient operation and safety.

The compound gauge typically displays pressure readings on a dial that uses distinct markings or a colored range to differentiate between positive pressure and vacuum. This allows users to easily interpret the pressure conditions without needing multiple gauges.

Other types of gauges do not offer this specific functionality. For instance, a standard gauge generally measures only absolute pressure or may have a restricted range. An absolute gauge measures pressure relative to a perfect vacuum and does not account for pressure below atmospheric levels. A pressure gauge typically refers to gauges measuring only pressure above atmospheric levels. Thus, the compound gauge uniquely serves the need for comprehensive pressure assessment across a wider scope of applications.