The PVT (pressure, volume, temperature) method of liquid quantity gauging in low-gravity is based on gas law calculations
assuming conservation of pressurant gas within the propellant tank and the pressurant supply bottle. There is interest in applying
this method to cryogenic propellant tanks since the method requires minimal additional hardware or instrumentation. To use PVT
with cryogenic fluids, a non-condensable pressurant gas (helium) is required. With cryogens, there will be a significant amount of
propellant vapor mixed with the pressurant gas in the tank ullage. This condition, along with the high sensitivity of propellant vapor
pressure to temperature, makes the PVT method susceptible to substantially greater measurement uncertainty than is the case with
less volatile propellants. A conventional uncertainty analysis is applied to example cases of liquid hydrogen and liquid oxygen tanks.
It appears that the PVT method may be feasible for liquid oxygen. Acceptable accuracy will be more difficult to obtain with liquid
hydrogen.