PPM sensors for direct measurement of Free Chlorine in water in parts per million (ppm) or milligrams per liter (mg/l). Unlike other chlorine sensors on the market, these sensors are very precise and so reliable that they are covered by a 2-year warranty. For best water management, it is recommended to use ORP control of chlorine oxidizing activity and PPM monitoring of free Chlorine. Alternatively, PPM control can be used for control of the sanitizer and ORP control for a non-chlorine oxidizer.