Reid vapor pressure (RVP) is a common measure of the volatility of gasoline. It is defined as the absolute vapor pressure exerted by a liquid at 37.8 °C (100 °F) as determined by the test method ASTM-D-323. The test method measures the vapor pressure of gasoline, volatile crude oil, and other volatile petroleum products, except for liquefied petroleum gases. RVP is stated in kilopascals and represents a relative pressure to the atmospheric pressure because ASTM-D-323 measures the gauge pressure of the sample in a non-evacuated chamber. All values are in SI units and are regarded as standards. Imperial units are for information only.
The matter of vapor pressure is important relating to the function and operation of gasoline-powered, especially carbureted, vehicles. High levels of vaporization are desirable for winter starting and operation and lower levels are desirable in avoiding vapor lock during summer heat. Fuel cannot be pumped when there is vapor in the fuel line (summer) and winter starting will be more difficult when liquid gasoline in the combustion chambers has not vaporized. Thus, oil refineries manipulate the Reid Vapor Pressure seasonally specifically to maintain gasoline engine reliability.
The Reid vapor pressure (RVP) differs slightly from the true vapor pressure (TVP) of a liquid due to small sample vaporization and the presence of water vapor and air in the confined space of the test equipment. That is, the RVP is the absolute vapor pressure and the TVP is the partial vapor pressure.