ASTM D6378 Determination of Vapor Pressure of Petroleum Products
ASTM D6378 Standard Test Method for Determination of Vapor Pressure (VPX) of Petroleum Products, Hydrocarbons, and Hydrocarbon-Oxygenate Mixtures (Triple Expansion Method)
4. Summary of Test Method
4.1 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 determined. The PPA and the solubility of air in the specimen are calculated from the three resulting pressures. The temperature of the chamber is then increased to a specified value, and the TPX is determined. The (VPX) is calculated by subtracting the PPA in the liquid, which has been gas corrected for the temperature, from TPX. For DUPE results, in accordance with Test Method D 5191, the volume of the measuring chamber shall be five times that of the total test specimen and the measuring temperature shall be 37.8°C.

NOTE 2 - Caution: For liquids containing very low levels of high vapor pressure contaminants, which behave like a gas, this test method of determination of the PPA and gases may lead to wrong results since the partial pressure of the contaminants will be included in the PPA. This effect is shown when the value of the PPA and gases exceeds the average maximum limit of 7 kPa (1 psi).

5. Significance and Use
5.1 Vapor pressure is a very important physical property of volatile liquids for shipping and storage.

5.2 The vapor pressure of gasoline and gasoline-oxygenate blends is regulated by various government agencies.

5.3 Specifications for volatile petroleum products generally include vapor pressure limits to ensure products of suitable volatility performance.

5.4 In this test method, an air saturation procedure prior to the measurement is not required, thus eliminating losses of high volatile compounds during this step. This test method is faster and minimizes potential errors from improper air saturation. This test method permits VP X determinations in the field.

5.5 This test method can be applied in on-line applications in which an air saturation procedure prior to the measurement cannot be performed.