Add a post lite indicator kit to your deionizer station for continuous monitoring of the quality of the deionized water being produced. The monitoring is simple; the green lite indicates the water conductivity is below the threshold value; the red light warns that it is above. You can use your digital pocket TDS tester [range 0-999 ppm] for the exact level if required from time to time. The patented square wave indicator is a economical method of monitoring water quality via conductivity. The solid state circuitry utilizes a symmetrical square wave sampling voltage to eliminate plating and extend electrode life.