In a method for detecting the fuel quantity during the refuelling of a motor vehicle, the volume of the fuel conducted to the motor vehicle during the refuelling operation and the temperature of the fuel are measured. The method comprises providing a gaseous composition comprising at least one compound of said group provided that if the selected compound is either CF3CHCl2 or CF3CHF2, said gaseous composition comprises CF3CHFCl and the mole fraction of the selected compound relative to the other two compounds in said gaseous composition is less than its equilibrium amount at 50 C. The first encoder produces output symbols therefrom.