The liquid is heated by a heating element mounted on the pipe and the temp. in the liquid is detected with the aid of temp. sensors (4,6) mounted on the pipe at distances from the heating element. The pipe is bent into a U-shaped section with two arms (10,11) in one region. A flow sensor (2) is mounted on each arm. The sensors measure the flow speed of the liquid with different methods involving proportional physical measurement parameters which are fed to an evaluation unit in which physical measurement values are formed from each parameter's mean value. The current flow speed is derived from these values.
