Probe rod with length corresponding to required switching level are immersed into the liquid which changes conductivity between probe rod and tank wall.
A low alternative voltage is applied between the probe rod and the conductive tank wall. When the filling liquid contacts the probe rod, a low alternating current flows through the liquid. The current is amplifiered by a TR and process chip by which the liquid level is detected and converted into an output signal. The signal is displayed by relay of barrier in the control panel.