Water quality monitor

An apparatus for monitoring the performance of a reverse osmosis water purification system. The resistivity of the water, and hence its dissolved ionic material concentration, is measured at both the inlet and product ports of the system by resistance sensors forming two legs of a wheatstone bridge. The bridge is powered by two oppositely phased AC signals. The output of the bridge drives a phase sensitive null detector which generates a null error signal. After thresholding, the null error signal actuates a bidirectional meter to indicate the degree of bridge unbalance, as well as to actuate an alarm circuit if the outlet resistance falls below a predetermined value, thereby indicating an unacceptable degree of purification. The bridge is balanced by a potentiometer forming a pair of bridge legs on each side of the potentiometer wiper. Thus, when the bridge is balanced, the setting of the potentiometer wiper corresponds to a given ratio of inlet resistance to outlet resistance. This ratio corresponds to the percentage of ionic material in the product water which has been rejected from the inlet water. The potentiometer dial is calibrated in percent rejection, thereby giving an indication of the performance of the reverse osmosis water purification system. Means are also provided for actuating an alarm when the percent rejection falls below a predetermined value and for reading directly the resistivity, and hence ion concentration, of both inlet and product water.