A device repeatedly measures a characteristic, such as electrical conductivity, of water in a water line. A plurality of the measurements are averaged to arrive at a water quality value. This value is displayed and another plurality of measurements are averaged to arrive at another value, which is then displayed in place of the previous value. The process continues at selected intervals in order to provide a continuous display of the condition of water in the line. Each new value is compared to a reference value representing a maximum acceptable level of contaminants. If the water quality value exceeds the reference value, an overvalue signal is produced, indicating an unacceptable level of contaminants in the water. The repeated measurements are obtained at selected intervals, and a new water quality value may be derived at the same intervals by discarding the oldest of the measurements from the previous plurality and including the most recent measurement to create a new plurality of measurements.