Facts about the ASTM D5191 Standard Test Method
The ASTM D5191 test method measures the total vapor pressure of petroleum products. It is based on a known volume of chilled, air-saturated sample that is introduced into a thermostatically controlled, evacuated test chamber. Alternatively, the test chamber can have a moveable piston that expands the volume after the sample introduction – the internal volume of which is five times that of the total test specimen introduced into the chamber. The air saturation process requires complex preperations, for example a consequent shaking of the sample or a refrigerator cooling. Because of that, this test method can produce an operator bias, a fact that marks a considerable disadvantage compared to ASTM D6378. Also EN 13016 can be considered as an equivalent to this method.
Inside the chamber, the test specimen is allowed to reach thermal equilibrium at the test temperature of 37.8°C or 100°F at a vapor-to-liquid ratio of 4:1. A pressure transducer is measuring the resulting rise in pressure. (Andreas Schwarzmann, R&D)
At the end of a measurement, the total vapor pressure is converted to a dry vapor pressure equivalent (DVPE). Therefore the correlation equation DVPE (kPa) = (0.965 X) – 3,78 kPa is applied where X denotes the measured total vapor pressure in kPa. This test method is suitable for calculation of the DVPE of gasoline and gasoline-oxygenate blends by means of a correlation equation.