Abstract: A compact, battery-powered water resistivity sensor measures the resistivity of water, sequentially compares the measured resistivity to a plurality of predetermined resistivities, and numerically displays that one of the plurality of predetermined resistivities approximating the measured resistivity. Preferably, the pair of electrodes consists of a pair of printed circuit electrodes disposed conveniently on the end face of an insulating nut extending, in fluid-tight fashion, into the tank of water. The sensor is powered by a battery, and includes circuitry for interrupting the supply of battery power to various circuit elements, the functions of which elements are no longer needed once the water resistivity is determined by the sensor. The sensor also includes temperature-sensitive compensation means responsive to changes in the water temperature for varying an input signal provided to the electrodes.