Chemistry tip: Understanding ORP

By David Lloyd

Oxidation Reduction Potential (ORP) measures the true efficacy of the oxidisers present in the water in millivolts. An ORP reading of 700mV to 720mV is considered adequate for fast and effective disinfection in pools and most Australian health departments recommend it for commercial pools.

ORP will rise and fall according to the level of cyanuric acid, pH and contaminants in the water. For example, a pool showing a Free Chlorine level of 2ppm may show an ORP reading of 720 at a pH level of 7.4. As the pH level changes so too will the ORP. At a pH of 7.8 the ORP may read below 700 or at a pH of 7 may read above 750.

Similarly a stabiliser reading of 20 will show a much higher ORP level than the same pool with a stabiliser level of 50+.

As swimmers introduce contaminants such as sweat, body fats or urine into the pool water, the ORP will reduce. As the ORP level changes it enables the sanitiser controller to react to the increased chlorine demand and commence feeding as required.

Chlorine controllers will only monitor and control the Free Chlorine set point but do not allow for changes in pH, contaminants etc.