Both PDR1 and Mustafagander have it right. The system that I know best has a tubular capacitor mounted vertically and a flat-plate capacitor at the bottom of the tank where it is always covered by fuel. The flat-plate capacitor provides a reference point so that no matter what the temperature or the type of fuel is in the tank, the system has a point from which to calculate and display the weight of the fuel. Fill a plane with warm fuel in the daytime and the next morning the weight reading will remain the same even though the fuel temperature has dropped overnight.