A method for operating a reprocessing apparatus for cleaning and/or disinfecting a medical instrument which has at least one channel, wherein a fluid line is provided for connection to the at least one channel, the method including: introducing gas bubbles into a liquid flowing in the fluid line, determining a speed of the gas bubbles flowing in the fluid line； and extrapolating a volume flow of the liquid flowing in the fluid line on the basis of the determined speed of the gas bubbles.
