When the Reid Vapour Pressure is measured there is air in the confined space which was previously dissolved in the test sample. Corrections for this can be made by means of charts and the vapour pressure so determined is called the True Vapour Pressure (TVP). Moreover, if the RVP, which is of course always determined at 38°C (100°F), is known for a particular hydrocarbon stock its TVP at other temperatures can be determined from the chart. The chart itself will not be reproduced as an interested reader can very easily download it from web sites including.
Hatzioannidis et al. found that for a limited number of gasolines experimentally examined the following holds:
RVP/kPa = 1.0091(TVP/kPa) + 5.53
from which, for example, a gasoline having a Reid Vapour Pressure of 50 kPa will have a TVP of 44 kPa. This The present author has checked this against the plot in and obtained a value of 46 kPa to the nearest whole number.
This is reasonable agreement, but it is emphasised that the plot in is intended to be used across a wide range of distillate RVP values whereas the correlation above appertains only to those particular gasolines (some of which contained additives) examined in so if it is used more widely it must be with caution.
Of course, the so-called true vapour pressure is not the vapour pressure any more than the RVP is. It has already been explained that no such vapour pressure exists for a distillate because of the dependence of the vapour pressure on the space which the vapour occupies. The TVP is merely the RVP corrected for air (and water vapour) in the measurement space.