What is the relationship between zero psig (pounds per square inch gauge) and atmospheric pressure?

Zero psig indicates a pressure measurement relative to atmospheric pressure. Specifically, it means that the pressure being measured is equal to the pressure of the atmosphere at that particular location.

To elaborate, atmospheric pressure is the pressure exerted by the weight of the air above us and is typically around 14.7 psi at sea level. When we say something is at zero psig, we mean that it is at the same pressure level as the surrounding atmosphere – in other words, it is not elevated above atmospheric pressure nor is it below. Therefore, zero psig is a direct measure of atmospheric pressure, making it accurate to say that it is the same as atmospheric pressure.

This understanding is fundamental in fields like HVAC, where pressure measurements are commonly referenced to atmospheric pressure. In contrast, absolute pressure, which is the total pressure measured relative to a vacuum, always includes atmospheric pressure, meaning that as a result, zero psig will be lower than absolute pressure since absolute pressure includes the atmospheric pressure factor.