VAPOR PRESSURE OF PETROLEUM PRODUCTS: ASTM D6378
EXPLANATION
Vapor pressure is a very important physical property of volatile liquids for shipping and storage. The vapor pressure of gasoline and gasoline-oxygenate blends is regulated by various government agencies. Specifications for volatile petroleum products generally include vapor pressure limits to ensure products of suitable volatility performance.
This test method covers the use of automated instruments to determine the vapor pressure exerted in vacuum by volatile, liquid petroleum products, hydrocarbons, and hydrocarbon-oxygenate mixtures. This test method is suitable for samples with boiling points above 0 ° C that exert a vapor pressure between 7 and 150 kPa at 37.8° C at a vapor-to-liquid ratio of 4:1. This test method may be suitable for measurement of volatile petroleum liquids under conditions different than these but the precision statement may not be applicable. This test method can be applied in on-line applications in which an air saturation procedure prior to the measurement cannot be performed. The TPx determined by this test method can be correlated to the dry vapor pressure equivalent value determined by Test Method ASTM D5191.
TEST SUMMARY
Employing a measuring chamber with a built-in piston, a sample of known volume is drawn into the temperature controlled chamber at 20° C or higher. After sealing the chamber, an expansion is performed in three steps to a final volume of (X + 1) times that of the test specimen. After each expansion the TPx is measured. The PPA and the solubility of air in the specimen are calculated from three resulting pressures. The temperature of the chamber is then increased to a specified value, and the TPx is determined. The vapor pressure is calculated by subtracting the PPA in the liquid, which has been gas corrected for the temperature, from TPx.
TEST PRECISION
Repeatability: 0.50 kPa (0.07 psi)
Reproducibility: 1.63 kPa (0.22 psi)
Relative bias is significant relative to Test Method ASTM D5191.
