Abstract: An apparatus controls the treatment of water that flows through a cooling system. The electrical conductivity of the water is sensed. A valve control opens a system drain valve when the sensed conductivity is greater than a threshold, and closes the drain valve when the conductivity drops below the threshold by a given amount. The water drained form the system is replenished through an inlet and a mechanism measures the volume of water added to the cooling system. Chemicals to treat the water are fed into the system when specified volumes of water have been added. The sensing of conductivity can be inhibited for a certain period following the application of the chemicals. A possible system malfunction is indicated when the conductivity does not drop below the threshold after the drain valve is open for a given period of time.