Vacuum Pressure is defined as the pressure measured with local atmospheric pressure as its reference less the amount of vacuum being drawn.
Selecting Instruments for Vacuum Service - David W. Spitzer - I recently read an Internet posting by a well-respected instrumentation engineer about selecting instruments for vacuum service. His comments are informative in that he mentions that instruments can be damaged when operated under vacuum. Vessels can also be damaged. The plant in which I worked in a previous professional life a tank that held somewhat less volume after it encountered a vacuum. In this case, pumping liquid out of the tank while the inlet and vent valves were closed caused a vacuum to be created in the tank. The tank partially collapsed, but fortunately no one was hurt and no liquid escaped - from www.flowcontrolnetwork.com