Method of determining the throughflow of a gas mixture

Method for monitoring the gas return in a filling system, the filling system being set up to feed liquid fuel using a fuel pump from a storage tank into a tank to be filled and to conduct the gas mixture standing above the fuel in the tank to be filled into the storage tank using a gas pump, wherein throughflow of the gas mixture on the way into the storage tank is measured by performing a measuring method comprising:

• flowing the gas mixture past a flow sensor which has a heating device and a first temperature probe reacting to the temperature of the heating device,
• measuring a temperature, which is characteristic of the temperature of a liquid fuel in the tank to be filled above which the gas mixture stands as vapour, with a second temperature probe,
• producing a first signal characterizing the composition of the gas mixture and thus the heat dissipation capacity of the gas mixture using the temperature of the liquid fuel measured by application of the second temperature probe and a vapour-pressure curve defining a relationship of the liquid fuel's vapour pressure as a function of the liquid fuel's temperature to determine the liquid fuel's vapour pressure based on the liquid fuel's temperature measured by application of the second temperature probe, and
• producing a second signal characterizing the throughflow of the gas mixture using the heating power supplied to the heating device, the temperature measured by the first temperature probe and the first signal.