The piezoelectrically activated vibration of a vibrating fork is damped when the fork comes into contact with the medium. Using this change, an electronic system determines the switching signal. The function is independent of fluctuations in the physical properties of the medium.
The two paddles of a vibrating fork are actuated using a piezoelectric source. In air the vibrating fork vibrates at its resonance frequency. When it is immersed in rising liquid, the frequency and amplitude of the vibration is reduced. The change is evaluated electronically and produces the switching signal.