What does zero PSI gauge (P S I G) indicate?

A zero PSI gauge, or PSI G, indicates pressure relative to atmospheric pressure. This means that when the gauge reads zero, it is measuring the pressure at the same level as the current atmospheric pressure, which is typically around 14.7 PSI at sea level. Thus, any positive reading on a PSI G gauge indicates pressure above this baseline atmospheric pressure.

For instance, if the gauge reads 10 PSI, it implies that the actual pressure is 10 PSI higher than the atmospheric pressure, leading to a total pressure of approximately 24.7 PSI absolute (14.7 PSI atmospheric + 10 PSI gauge pressure).

Understanding this concept is crucial in HVAC applications, as it helps technicians ensure that the system operates within the correct pressure range, vital for efficiency and safety. The other options discuss pressures that don't accurately describe the function of a PSI G gauge. Absolute pressure isn't directly read on a gauge marked in PSI G, nor is maximum allowable pressure a measure displayed on this type of gauge. Standard atmospheric pressure is a benchmark for these measurements but does not describe what the zero PSI gauge itself indicates.