A permanent magnet is integrated into a paddle. The paddle is mounted so that it is able to turn on a stainless steel spindle in the flow cross-section and is in vertical position when deenergized. A reed contact is positioned above the paddle. If a specific flow velocity is exceeded, the paddle is deflected in the flow direction and switches the reed contact. The switching point can be set for increasing and dropping flow velocities by means of an adjusting screw.