For gas flows containing condensible vapors, the partial pressures of the vapors must be subtracted
from the absolute pressures (PA and PS) in order that the correction factor accurately reflects
gas behavior alone. A common application of standardized gas flow involving partial pressure
correction is found in compressed air systems, where water vapor (relative humidity) is a factor.
Here too, standards differ as to the humidity conditions of “standard” cubic feet. The API and
PNEUROP/CAGI standards call for 0% humidity (perfectly dry air) as the “standard,” while the
ASME defines 35% relative humidity as “standard” for compressed air calculations