What does zero psig (pounds per square inch gauge) indicate?

Zero psig indicates a pressure that is equivalent to atmospheric pressure. In other words, when a gauge reads zero psig, it doesn't mean that there is no pressure; rather, it signifies that the pressure inside the system is the same as the surrounding atmospheric pressure. This is a crucial concept in understanding how pressure gauges operate, as they measure the pressure of a medium relative to atmospheric pressure.

Atmospheric pressure is approximately 14.7 psi at sea level, and any gauge that shows zero psig does so relative to that standard. Thus, any positive reading on that gauge means the pressure is above atmospheric pressure, while a negative reading would indicate a pressure below it, which would be the case with vacuum gauges.

The other options describe conditions that do not apply to a gauge reading of zero psig. For example, a perfect vacuum is represented by a pressure significantly below atmospheric pressure, not at it. A slight positive pressure would show some gauge reading above zero. Similarly, operational pressure would typically indicate a working state that exceeds atmospheric pressure, emphasizing that zero psig is distinct from operational conditions. Understanding this relationship is essential for correctly interpreting pressures in HVAC applications.