Gasoline and other fuels vaporize when exposed to heat or air, and their volatility is measured by the Reid vapor pressure test. This test compares fuels at 100°C and uses a closed container to measure pressure. A nomograph can convert Reid vapor pressure data to true vapor pressure. Fuel additives and blends are used to minimize chemical attack and seasonal temperature variations. High Reid vapor pressure can cause vapor lock in hot climates, so refineries adjust fuel blends accordingly.
Gasoline and other fuels have the ability to vaporize, or turn from liquid to vapor, when exposed to heat or air at temperatures above the fuel’s boiling point. Vaporization rate is often referred to as volatility, and one test to measure this is the Reid vapor pressure test. This test measures the pressure of a mixture of vapor and liquid in a closed container at 100°C (37.8°F).
All liquids have vapor pressure, which is the pressure of vapor above a liquid in a container. One measurement is true vapor pressure, which is the pressure with no air present. One measure used by the petroleum industry is Reid’s vapor pressure, which measures the vapor pressure in a closed container at a temperature, without first removing the air. This measurement allows petroleum engineers to compare different fuels or show the effects of performance additives.
Reid’s vapor pressure does not remove air or water vapor from the sample, but it compares all fuels at 100°C (37.8°F). Typically, the Reid pressure will be lower than the true vapor pressure, as the water and air included in the sample container affect it. Some industry organizations publish Reid vapor pressure data for different motor fuels to provide consistent data for analysis.
The test rig uses a closed metal container called a bomb, which holds the sample. A known amount of sample is placed in the bomb, which is then sealed and placed in a temperature controlled water bath. When the bomb has been in the bath for five minutes, the pressure is measured with a pressure gauge and recorded. Measurements are repeated at specific intervals until the pressure readings are the same.
A type of graph called a nomograph has been developed that allows you to convert Reid vapor pressure data to true vapor pressure and vice versa. These graphs are important because true vapor pressure is required for certain petroleum fuel properties. The effects of air and water from Reid’s test must be eliminated from pressure measurements.
Gasoline and other fuels can have different additives and blends, for stability and to minimize chemical attack on storage tanks and engines, but also due to seasonal temperature variations. A fuel with high Reid vapor pressure will vaporize easily, which can be an advantage in colder climates, but can cause vapor lock when used in hot climate areas. Vapor lock is vapor that forms in the fuel line in hot weather, which can prevent the engine from starting. Refineries will change fuel additives and blends in different seasons to provide the best fuel performance.
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