Reid Vapor Pressure (RVP): Is it Expressed in Units of Absolute Pressure?
September 26, 2013
The Reid Vapor Pressure (RVP) is determined according to ASTM Standard D323. Many have expressed confusion related to the pressure units reported for RVP. Since most vapor pressures are reported in absolute pressures, most observers expect the RVP to follow this norm. To make matters worse, the published standard uses the term "absolute pressure" in its description. Further, many publications and reports carelessly provide values with units such as "psia" when providing RVP values.
In reality, the value is a gauge pressure. It is measured using a pressure gauge that reports a value of zero pressure when open to the atmosphere. The gauge also indicates zero pressure when attached to the ambient air filled chamber of the apparatus. The gauge is not of the type that has vacuum measurements.
The RVP is measured directly from the pressure gauge, and reported without addition of atmospheric pressure. No place in the ASTM standard are units such as "psia" stated. Instead, pressure units such as "psi" or "kPa" are given. Earlier versions of our software incorrectly reported values in psia. However, this is no longer the case as we had similar internal uncertainty until we carefully considered the experimental procedure, equipment used, and the method to report the results. Following the convention of D323, we now report the RVP as psi or kPa, which would represent the value taken from the gauge if the process was done experimentally. It would be just as correct to report the same value with units of psig or kPa(g), but that would not have followed D323.
Due to this confusion, I contacted the chairman of the ASTM D323 committee on July 20, 2009, to attempt to obtain better presentation of this information in future editions of the standard, especially with regard to the term "absolute pressure". You can read my comments and the response from the committee using the links below.
Review the comments to the ASTM D323 Committee
Review the D323 Comments from Rey Montemayor on Michael Hlavinka's e-mail request
Authored by Michael Hlavinka (BR&E Technical Director)