HVAC - EPA Certification Practice Exam

The type of gauges that can measure both above and below 0 psig are compound gauges. This is because compound gauges are designed to measure pressure in both the vacuum (below atmospheric pressure) and positive pressure ranges. They typically have a dual scale, often displaying pressure in psi alongside vacuum measurements in inches of mercury or similar units.

The ability to measure negative pressures is particularly useful in HVAC applications where systems may operate below atmospheric pressures, such as those involving refrigerants in a vacuum state during certain maintenance or service conditions. This versatility allows technicians to effectively monitor and diagnose system performance over a wider range of conditions critical for maintaining system efficiency and safety.

Other types of gauges mentioned, such as barometric gauges, primarily measure atmospheric pressure and do not typically measure vacuum. Digital gauges can measure various pressure types, but their ability to read below 0 psig depends on their specific design and calibration, making them less consistent in this regard compared to compound gauges. Thermocouples, on the other hand, are used for temperature measurement and are not applicable for pressure measurement at all.