is the 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.