<P> 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 . </P> <P> 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 . </P> <P> 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 . </P>

Difference between reid vapour pressure and true vapour pressure
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