Reid vapor pressure (RVP) is a measurement used to determine the volatility of gasoline. Volatility is defined as the absolute vapor pressure exerted by a liquid at 100 °F (37.8 °C). It is calculated by the testing method called ASTM-D-323.This method is valid for volatile crude oil and volatile non-viscous petroleum liquids but excludes liquefied petroleum gases.
The effect of small sample vaporization, and the presence of water vapor and air in the confined space of testing equipment, causes the RVP to vary slightly from the true vapor pressure (TVP) of a liquid. The RVP is the absolute vapor pressure, and the TVP is the partial vapor pressure.
Vapor pressure is critical to the function and operation of gas powered vehicles, especially those with carburetors. To ensure gas engines operate reliably, oil refineries must adjust RVP seasonally. During the winter, higher levels of vaporization are necessary for proper starting and operation. Conversely, lower levels are required during the summer months to avoid vapor lock.
To curb ozone pollution and reduce ozone-related health issues, the EPA strictly regulates the vapor pressure of gasoline sold at retail stations during the summer ozone season (June 1 to September 15) to reduce evaporative emissions.