Vapor pressure is the property of a substance to exert pressure on it’s surroundings, it is the pressure of the vapor in a closed system in equilibrium with the condensed phase. Vapor pressure for petroleum fuels is generally expressed as kPa – kilo Pascals. All materials have a tendency to evaporate and all gases have a tendency to condense. Materials with high vapor pressures are said to be volatile. Vapor pressure is important in gasoline and other liquid petroleum fuels. Fuel systems are engineered with the vapor pressure of the intended fuel in mind. Vapor pressure of any substance in a closed system is only dependent on temperature not on the amount of the material or the size of the vessel. Fuel systems must be designed allowing for the temperature at which they will operate. Vapor pressure curves (Vapor Pressure vs. Temperature) are determined experimentally for most petroleum fuels (except liquefied petroleum gas) by the Reid vapor pressure method. A fuel is placed in a vapor pressure bomb and a pressure gauge is attached. The bomb is placed in a bath which controls the temperature to ±0.1°C. A pressure reading is taken after 5 minutes at the desired temperature and then pressure readings are taken at intervals until they no longer change. The the temperature is then increased and the process starts again. The relationship between vapor pressure and temperature is exponential. Engineers of fuels and fuel systems will use the Reid vapor pressure test to determine the vapor pressure curve of new fuel formulations.