ASTM D6897 Standard Test Method for Vapor Pressure of Liquefied Petroleum Gases (LPG) (Expansion Method)
12. Procedure
12.1 Connect the pressurized sample container to the inlet of the apparatus and position it in such a way that the outlet valve of the container is below the liquid level. Open the outlet valve of the pressurized sample container.
12.2 Rinsing - Open the inlet valve and draw in the sample by moving the piston from zero-volume to the filling position. Close the inlet valve and open the outlet valve, move the piston to zero-volume position. Close the outlet valve. Repeat this procedure two more times.
12.3 Filling - Regulate the measuring chamber to the filling temperature of 5 °C +/- 0.5 °C. When the measuring chamber is at the filling temperature, close the outlet valve and open the inlet valve. Draw in the sample from the pressurized sample container by moving the piston from zero-volume to the filling position. Close the inlet valve.
12.4 Expansion - Move the piston to the final volume to provide the necessary vapor to liquid ratio (the overall volume of the measuring chamber is 1.5 times the fill volume of liquid for a vapor to liquid ratio of 0.5:1).
12.5 Total Pressure Determination - Adjust the temperature regulator of the measuring chamber to the test temperature of interest, such as 37.8 °C. After the temperature equilibrium observe the pressure reading. If two consecutive readings remain constant within +/- 3 kPa after 1 min, record the observed pressure as total pressure of the sample at test temperature.
13. Calculation
13.1 Record the total pressure readings from the instrument.
13.2 Calculate the LPG vapor pressures relative to the standard atmospheric pressure of 101.3 kPa by using Eq 1. Ensure that the instrument reading used in this equation corresponds to the absolute pressures (relative to vacuum) and has not been corrected by an automatically programmed correction factor:
VP(LPG) = Ptot - 101.3 kPa
where:
Ptot = measured total vapor pressure, kPa.
13.3 The calculation described by Eq 1 can be accomplished automatically by the instrument, if so equipped, and in such cases the user shall not apply any further corrections.