Measure of propellant level in zero-gee tankage is a tricky problem.
Shuttle attempted it in its Orbital Manuevering System.
A capacitance gauging system in each OMS propellant tank measures the
propellant in the tank. The system consists of a forward and aft probe
and a totalizer. The forward and aft fuel probes use fuel (which is a
conductor) as one plate of the capacitor and a glass tube that is
metallized on the inside as the other. The forward and aft oxidizer
probes use two concentric nickel tubes as the capacitor plates and
oxidizer as the dielectric. (Helium is also a dielectric, but has a
different dielectric constant than the oxidizer.) The aft probes in
each tank contain a resistive temperature-sensing element to correct
variations in fluid density. The fluid in the area of the
communication screens cannot be measured.
The system was problematic and Orbiters often flew with it nonfunctional.
The Orbiter's Reaction Control System tanks used algorithmic gauging
The RCS quantity monitor uses the GPC to calculate the usable percent
of fuel and oxidizer in each RCS module. The quantities are computed
based on the pressure, volume, and temperature method, which requires
that pressure and temperature measurements be combined with a unique
set of constants to calculate the percent remaining in each of the six
propellant tanks. Correction factors are included for residual tank
propellant at depletion, gauging inaccuracy, and trapped line
propellant. The computed quantity represents the usable (rather than
total) quantity for each module and makes it possible to determine if
the difference between each pair of tanks exceeds a preset tolerance
(leak detection). The sequence assumes that helium flows to the
propellant tanks to replace propellant leaving. As a result, the
computed quantity remaining in a propellant tank will be decreased by
normal usage, propellant leaks, or helium leaks.
(GPC = General Purpose Computer)
Propellant transferred between the two systems was estimated by a burn-time algorithm.
Just for completeness, the shuttle's External Tank didn't have a sophisticated gauging system for its propellant. Instead, simple on/off level sensors were used to aid in filling the tank, and to shut off the engines if the tank ran dry prematurely.