What is the difference between a PSIg sensor and a PSIa sensor?
At sea level, the atmosphere exerts a pressure of approximately 14.696 pounds per square inch (PSI), usually rounded up to 14.7 psi. You need to know if a pressure reading includes or excludes this pressure. To distinguish this, two specific pressure units are used: pounds per square inch gauge (PSIg), and pounds per square inch absolute (PSIa). PSIg is a reference to atmospheric pressure and the measurement instrument will indicate zero when not connected to the process pressure, with the sensing element exposed to atmospheric pressure. PSIg sensors cannot measure vacuum. A PSIg sensor will read 0.0 PSI when the sensing element is exposed to atmospheric pressure at sea level and the output will not decrease if a vacuum is applied. PSIa is referenced to absolute zero. Absolute zero is the pressure measurement when all the pressure exerted by the atmosphere has been removed. PSIa sensors are required of you wish to measure vacuum. A PSIa sensor will read about 14.7 PSI when the sensing
