True vapor pressure (TVP) is a common measure of the volatility of petroleum distillate fuels. It is defined as the 
equilibrium partial pressure exerted by a volatile organic liquid as a function of temperature as determined by the test method ASTM D 2879.
The true vapor pressure (TVP) at 100 °F differs slightly from the Reid vapor pressure (RVP) (per definition also at 100 °F), as it excludes dissolved fixed gases such as air. Conversions between the two can be found in [https://www.epa.gov/sites/default/files/2020-10/documents/ch07s01.pdf AP 42, Fifth Edition, Volume I Chapter 7: Liquid Storage Tanks] (p 7.1-54 and onwards)