Not all gasoline is exactly alike and in some formulations, a sizable mixture of “light ends” is blended into the gasoline at therefinery. This makes the gasoline vaporize more easily, a characteristic desirable in cold weather. In hot weather, however, gasoline with an excess of volatility tends to emit more vapors during transfer operations. This, in turn, can contribute to air pollution and can also cause vapor lock in engines during the summer months.
Environmental regulators have adopted rules which limit the degree of volatility present in gasoline in certain regions at certain times of the year. The accepted method for determining the volatility of a batch of gasoline is a laboratory procedure that measures the pressure of the gasoline’s vapors. The procedure, named for its originator, produces a numerical value called Reid vapor pressure (RVP).
One batch of gasoline might show an RVP rating of 8.37; another might show a rating of 7.49, etc. The higher the number, the higher the volatility. Environmental regulations in some states prohibit the sale of gasoline with an RVP in excess of a specified number, particularly during the summer months.