ORP stands for Oxidation Reduction Potential and takes a measurement of how effective the sanitizer is in the water. The North American standard for ORP is in the range of 650-750mv. ORP is a better way gauge how sanitary a sample of water is as opposed to just measuring free chlorine. There are a number of scenarios where you will test your water and have good levels of free chlorine but still have a cloudy or green pool. This can be caused phosphates, improper pH levels or even too much stabilizer in the water. Testing by ORP would show in all the above scenarios a low ORP, indicating something in the water is preventing your sanitizer from doing its intended job, allowing you to correct the issue before it turns into a bigger expensive problem.
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