Abstract: A temperature compensated differential between electrical conductivity measurements of a filter medium during passage therethrough of liquid being treated and measurement of the liquid alone after treatment, is utilized to calculate a conductivity ratio based on a reference value reflecting a minimum electrical conductivity measurement under ion saturation conditions in an ion-exchange filter. The filter is switched from an operational mode to a regenerating mode in response to detection of a predetermined conductivity ratio corresponding to approach of the filter to a clogged condition.