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.