Finder level control monitoring relay 72 series finder 720180240000 the finder 720180240000 monitoring relay is from the 72 series this level control relay is for measuring conductive liquids the level control relay works by measuring the resistance through the liquid between the common b3 electrode and min and max electrodes b2 and b1 if the tank is metallic then this can be substituted as the b3 electrodethe 24v ac 1 co spdt 10a relay has a sensitivity range between 5