A water treatment system has a water treatment tank that stores a resin bed, a conductivity monitor, a valve assembly, a flow meter, and a controller. The control valve assembly is switchable to selectively permit untreated water flow into the tank, treated water flow out of the tank, and regenerant flow into and out of the tank. In one configuration, the conductivity monitor includes at least an uppermost probe located within 6″ of the nominal mid-point of the resin bed and a second probe located half-way between the first probe and the bottom of the resin bed. Signals from these probes are used in combination with flow data as measured by the flow meter to provide precise information regarding volumetric capacity as depletion is registered at each probe. Also disclosed are an improved technique for sampling data from the conductivity monitor, and a wireless conductivity monitor.