ASTM D2878 Test Method for Estimating Apparent Vapor Pressures and Molecular Weights of Lubricating Oils
4. Summary of Test Method
4.1 The test is run at the selected temperature for a sufficient time to give the selected amount of evaporation, which is 5 +/- 1 % unless otherwise specified. This evaporation rate is compared with a standard value for pure m-terphenyl to yield the apparent vapor pressure and molecular weight of the lubricating oil as defined in Section 3.
5. Significance and Use
5.1 The vapor pressure of a substance as determined by measurement of evaporation reflects a property of the bulk sample. Little weight is given by the procedure to the presence of low concentrations of volatile impurities.
5.2 Vapor pressure, per se, is a thermodynamic property that is dependent only upon composition and temperature for stable systems. In the present method, composition changes occur during the course of the test so that the contribution of minor amounts of volatile impurities is minimized.
