The conductivity of a liquid medium may vary widely. Once the liquid reaches the fill limit determined from the installation height of the electrode, the liquid medium closes the DC free alternating current circuit between the two electrodes (or between the container wall and an electrode). A switching signal is produced from the sudden increase in current consumption.
Acids, lyes and solutions containing water are conductive and are detected very well. Aggressive liquids can be detected without problems using probes made from highly-resistant materials. Combustible liquids such as fuels, oils and solvents are nonconductive and cannot be measured by this measurement principle.