4.1 The true vapor pressure of a distillate fuel is a relative measurement, both of the tendency of the most volatile portion of the fuel to gasify, and of the restraining pressure required to prevent gasification of the most volatile portion. Thus the measurement is of importance when a fuel is to be utilized in applications where no gasification may be tolerated, and temperature-pressure conditions are expected to be near the true vapor pressure of the fuel.

1.1 This test method describes the calculation of true vapor pressures of petroleum distillate fuels for which distillation data may be obtained in accordance with Test Method D86 without reaching a decomposition point prior to obtaining 90 volume % distilled.

1.2 The test method may be used to calculate vapor pressures at temperatures between the 0 % equilibrium flash temperature and the critical temperature of the fuel. Provision is included for obtaining a calculated critical temperature for fuels for which it is not known.

1.3 Critical pressure-temperature data are usually not available for petroleum fuels. However, if both the critical pressure and critical temperature are known, the values shall be used as the coordinates in Fig. 1 to establish a critical point to be used instead of the focal point established as described in 6.5.4; and the calculations described in 6.5 through 6.5.4 are not required. If either a determined true boiling point or determined equilibrium flash vaporization temperature at 0 % distilled at atmospheric pressure is known, the determined value shall be used to establish the lower limit of the bubble-point line referred to in 6.4.

1.4 The method is not reliable for distillate fuels having a boiling range of less than 100 °F (38 °C) between the Test Method D86 10 volume  % and 90 volume % distilled temperatures.

1.5 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.