What does zero psig represent?

Zero psig indicates a pressure level that is equal to atmospheric pressure. Pressure measurements in pounds per square inch gauge (psig) measure pressure relative to atmospheric pressure, where zero psig represents no pressure difference between the measured pressure and the surrounding atmospheric pressure. At this level, the system is neither under any additional pressure nor in a vacuum condition; it simply reflects the ambient pressure conditions.

This understanding is crucial because it clarifies how various pressures are referenced and what zero psig really signifies in practical applications, such as HVAC systems, fluid dynamics, and process engineering, where accurate pressure measurements are essential for safety and efficiency. The other options describe conditions outside the state of zero psig, either indicating pressure lower than atmospheric (vacuum or below atmospheric pressure) or a high-pressure context, which is not applicable.