ASTM D4953-20 Standard Test Method for Vapor Pressure of Gasoline and Gasoline-Oxygenate Mixtures (Dry Method)

The standard “ASTM D4953-20 Standard test method for vapor pressure of gasoline and gasoline-oxygenate mixtures (dry method)” published by the American Society for Testing and Materials (ASTM) describes a test method for determining gasolines and gasoline-oxygenate mixtures having a vapor pressure range of 35 kPa to 100 kPa (5 psi to 15 psi).

The standard in question is a modification of the method described in the standard “ASTM D323-20 Standard test method for vapor pressure of petroleum products (Reid method)” and includes two procedures for determining the vapor pressure of gasoline and gasoline-oxygenate mixtures.

Since the external atmospheric pressure is initially balanced by the atmospheric pressure in the air chamber, this vapor pressure is the absolute pressure in kilopascals at 37,8 degrees. This vapor pressure differs from the actual vapor pressure of the sample due to some vaporization of the sample and the air in the confined space.

The vapor pressure of gasoline or gasoline-oxygenate mixtures below 35 kPa (5 psi) or above 100 kPa (15 psi) can be determined by this test method, but sensitivity and bias are not applied. For materials having a vapor pressure greater than 100 kPa (15 psi), the 323 kPa to 20 kPa (0 psi to 200 psi) gauge is used, as specified in the appendix to ASTM D0-30.

Some gasoline-oxygenate mixtures may appear hazy when cooled to 0°C to 1°C. If haze is observed, it should be noted in the reporting of results. Sensitivity and bias statements for hazy samples have not been determined.

For the purposes of this standard, the following definitions apply:

• A Bourdon spring gauge is a pressure measuring device that uses a Bourdon tube attached to a gauge.
• A Bourdon tube is a flat metal tube bent into a curve that flattens under internal pressure.
• The dry method is a special empirical test method for measuring the vapor pressure of gasoline and other volatile products in vapor pressure methods, in which the test sample is not allowed to come into contact with water.
• The dry vapour pressure equivalent is a value calculated from a defined correlation equation that is expected to be comparable to the vapour pressure value obtained by Procedure A defined in this standard.
• Oxygenate is an ashless organic compound containing oxygen, such as alcohol or ether, that can be used as a fuel or fuel supplement.
• Vapor pressure is the pressure value exerted by the vapor of a liquid when in equilibrium with the liquid.

In performing the test, the liquid chamber of the vapor pressure device is filled with the cooled sample and connected to the vapor chamber at 37,8°C. The device is immersed in a bath at 37,8°C until a constant pressure is observed. The pressure reading, suitably corrected, is reported as the vapor pressure.

Procedure A uses the same apparatus and substantially the same procedure as described in ASTM D323-20 except that the inside surfaces of the liquid and vapor chambers are completely free of water. Procedure B uses a semiautomatic apparatus in which the liquid and vapor chambers are the same in volume as those in Procedure A. The apparatus is suspended in a horizontal bath and rotated while reaching equilibrium. A Bourdon gauge or pressure transducer may be used in this procedure. The inside surfaces of the liquid and vapor chambers are kept completely free of water.

Vapor pressure is an important physical property of volatile liquids. This test method is used to determine the vapor pressure of petroleum products and crude oil at 0 degrees with an initial boiling point above 37,8 degrees.

Vapor pressure is critical for both automotive and aviation gasolines and affects starting, warming up, and vapor lock tendency at high operating temperatures or high altitudes. Maximum vapor pressure limits for gasoline are legally required in some areas as a measure of air pollution control.

The vapor pressure of crude oil is important to the crude oil producer and refiner for overall handling and initial refinery processing. Vapor pressure is also used as an indirect measure of the evaporation rate of volatile petroleum solvents.

Vapor pressure is an important physical property of liquid spark ignition engine fuels. It provides an indication of how a fuel will perform under different operating conditions. For example, vapor pressure is a factor in determining whether a fuel will vapor lock at high ambient temperature or high altitude, or provide easy starting at low ambient temperature. Petroleum product specifications often include vapor pressure limits to ensure products have adequate volatility performance.

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