Q: What is the difference between “gauge pressure” (PSIG) and “absolute pressure” (PSIA)?
A: The atmospheric pressure at sea level is 14.67 pounds per square inch, this is essentially the pressure that gravity exerts on us and we don’t feel that pressure because it is what we normally experience in the closed system we call Earth. When you look at a pressure gauge you will see the applied pressure above the atmospheric pressure which is called gauge pressure and indicates the pressure inside a closed system such as a tank or line. If you want the total pressure on the system then you have to add the 14.67 psi to the gauge pressure and this is called “absolute pressure.” Absolute pressure is often used on steam tables. Another commonly used pressure measurement is “bar” which equals one atmosphere of pressure or 14.67 psig.