vapor pressure - pressure of a confined vapor in equilibrium with its liquid at a specified temperature; thus, a measure of a liquid's volatility. Vapor pressure of gasoline and other volatile petroleum products is commonly measured in accordance with test method ASTM D 323 (Reid vapor pressure). The apparatus is essentially a double chambered bomb. One chamber, fitted with a pressure gauge, contains air at atmospheric pressure; the other chamber is filled with the liquid sample. The bomb is immersed in a 37.8°C (100°F) bath, and the resulting vapor pressure of the sample is recorded in pounds per square inch (psi). Reid vapor pressure is useful in predicting seasonal gasoline performance (e.g., higher volatility is needed in cold weather, and lower volatility in hot weather), as well as the tendencies of gasolines, solvents, and other volatile petroleum products toward evaporative loss and fire hazard.