Reid Vapor Pressure and True Vapor Pressure

Gasoline and other fuels have the ability to vaporize, or change from liquid to vapor, when exposed to heat or air at temperatures above the boiling point of the fuel. The rate of vaporization 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 vapor and liquid mixture in a closed container at 100°F (37.8°C).

All liquids have a vapor pressure, which is the pressure of vapor above a liquid in a container. One measurement is the true vapor pressure, which is the pressure without any air present. A measurement used by the petroleum industry is the Reid vapor pressure, which measures the vapor pressure in a closed container at one temperature, without first removing the air. This measurement allows petroleum engineers to compare different fuels, or to show the effects of performance additives.

Reid vapor pressure does not eliminate air or water vapor from the sample, but compares all fuels at 100°F (37.8°C). Typically, the Reid pressure will be lower than the true vapor pressure, because the water and air included in the sample container affect it. Some industry organizations issue Reid vapor pressure data for different motor fuels to provide consistent data for analysis.

The test apparatus uses a closed metal container called a bomb, which holds the sample. A known quantity 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 gauge and recorded. Measurements are repeated at specific intervals until pressures readings are the same.

A type of graph called a nomograph has been developed that allows Reid vapor pressure data to be converted to true vapor pressure, and in reverse. These graphs are important because true vapor pressure is needed for certain petroleum fuel properties. The effects of air and water from the Reid test need to be eliminated from the pressure measurements.

Gasoline and other motor fuels can have different additives and mixtures, for stability and minimizing chemical attack on storage tanks and engines, but also because of seasonal temperature changes. A fuel with a high Reid vapor pressure will vaporize easily, which can be an advantage in colder weather, but can cause vapor lock if used in hot weather areas. Vapor lock is vapor that forms in the fuel line in a hot climate, which can prevent the engine from starting. Refineries will change the additives and fuel mixtures in different seasons to provide the best fuel performance

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