Why is the RVP calculated in the ProMax Vapor Pressure Analysis listed with units of psig when my specification states psia?
Often the RVP is reported as “psia” although the way the test is performed the pressure is actually a gauge pressure. The D 323 – 99a standard says “Note 1–Because the external atmospheric pressure is counteracted by the atmospheric pressure initially present in the vapor chamber, the Reid vapor pressure is an absolute pressure at 37.8 C (100 F) in kilopascals (pounds-force per square inch).” However, a bourdon tube pressure gauge is used to measure the pressure so the reported pressure is actually in psig. That is confirmed by procedure 5B1.4 in the API Technical Data Book.
