The term "psig" refers specifically to what kind of pressure measurement?

The term "psig" stands for pounds per square inch gauge, and it specifically refers to pressure measured relative to the ambient atmospheric pressure. This means that when measuring in psig, the reading accounts for local atmospheric pressure, effectively subtracting it from the absolute pressure value. In practical terms, if a pressure gauge reads 10 psig, it indicates that the pressure is 10 pounds per square inch above the surrounding atmospheric pressure.

This measurement is particularly useful in HVAC and other engineering applications where it's essential to know the pressure relative to the atmospheric conditions to evaluate system performance or safety.