The Cassini probe, presently still exploring the Saturn system, has two main propellant systems. The bipropellant system uses nitrogen tetroxide as the oxidiser and monomethylhydrazine as the fuel. The monopropellant system just uses hydrazine, but how do they tell how much fuel has been used, or how you even spell them? This guy knows, that’s his job.
The two methods used are either to estimate the amount of fuel coming out and relate that to how much there once was or knowing the volume of the fuel tank and taking pressure and temperature readings, use the ideal gas laws (or some derivative thereof) – Pressure x Volume = number of molecule x Gas Constant x Temperature – to calculate what’s in there. Initially, these gave results that agreed to about 20%. This error was halved through work done on the fuel coming out method and a few further corrections drawn from performance tests have reduced the error further.
The result? Half the monopropellant remains as well as one tenth of the bipropellant, but that’s enough bipropellant to make it to 2017.