Suited for use in a range of water applications, the FCL free chlorine sensor from Sensorex, Garden Grove, CA, is an amperometric sensor that measures free chlorine concentration in real time and is offered in 0–2 ppm, 0–5 ppm, and 0–10 ppm measurement ranges. After hypochlorous acid diffuses through the sensor's membrane, it is electrochemically reduced, releasing electrons and creating a current flow proportional to the free chlorine concentration in the medium outside the sensor. The current output is then conditioned to a 4–20 mA direct output. The sensor operates in a range of 4–11 pH, a temperature range of 0°C–45°C, and at a max. pressure of 14.7 psig.