ASTM D6378 Standard Test Method for Determination of Vapor Pressure (VPX) of Petroleum Products, Hydrocarbons, and Hydrocarbon-Oxygenate Mixtures (Triple Expansion Method)
13. Calculation
13.1 Calculate the partial pressure of the air, PPAi, after the sample has been introduced into the measuring chamber and expanded to the final volume, using the following equation:
where:
TPX,1 = total pressure after first expansion,
TPX,2 = total pressure after expansion,
TPX,3 = total pressure after third and final expansion,
V1 = volume after the first expansion,
V2 = volume after the second expansion, and
V3 = final volume after the third expansion.
13.2 Calculate the partial pressure of the air at test temperature t, PPAt, using the following equation:
PPAt = PPAi (273.2 + t) / (273.2 + ti)
where:
ti = temperature of the measuring chamber when the test specimen is introduced and expanded, in degrees Celsius, and
t = temperature, in degrees Celsius.
13.3 Calculate the vapor pressure VPX,t at test temperature t, using the following equation:
VPX,t = TPX,t - PPAt
where:
TPX,t = total pressure at temperature t, and
PPAt = partial pressure of the air at temperature t.
13.4 See Appendix X1 for the derivation of the equations.