Reid vapor pressure (RVP) is a way to measure how quickly fuels evaporate; it's often used in determining gasoline and other petroleum product blends. The higher a fuel's RVP, the more quickly it evaporates. The more quickly a fuel evaporates, the more it contributes to the ozone layer. RVP represents the fuel's evaporation at 100 degrees Fahrenheit (37.8 degrees Celsius), and is measured in pounds per square inch, or PSIs. The property that RVP measures often is referred to as the gasoline's volatility. To obtain the RVP, a chilled sample is placed in a special vapor chamber that is heated to 100 degrees Fahrenheit (37.8 degrees Celsius). The reading records the observed constant pressure in the chamber. There are national industry standards for the bath, the thermometer and the pressure monitoring device so that consistency can be maintained among RVPs [source: Standard Test Methods].
The RVP for gasoline should always be below 14.7 PSI, which is normal atmospheric pressure. If the RVP is higher than 14.7 PSI, excess pressure will build up, and the fuel might boil and evaporate while in the car's gas tank. The Environmental Protection Agency (EPA) regulates RVP for gasoline that you buy at retail gas stations during the summer months to help reduce emissions [source: Environmental Protection Agency]. The summer generally includes the dates June 1 through September 15, except in certain areas with extended summer seasons. The summer fuel volatility regulations exclude Alaska, Hawaii and U.S. territories, but otherwise are outlined by state and county. During summer months, RVPs typically can go no higher than 9.0 PSI and usually are between 7.0 and 7.8 PSI in metropolitan areas. The precise regulations and amounts on county borders are managed by regional EPA offices [source: Federal Guide].
Measuring vapor pressure of gasoline also reveals the fuel's tendency to vapor lock in high operating temperatures or at high altitudes. Vapor pressure affects starting and warm-up of car and plane engines. For crude oils, understanding vapor pressure can help oil companies more safely gather, refine, store and transport their products [source: Standard Test Methods].
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