ORP - What is it and why do you monitor it?
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ORP is not a direct measurement of chlorine level or residual, but only a measure of the disinfectant activity or potential. Oxidizers create a positive millivolt charge when measured by ORP sensors, while reducers, which are negatively charged, will display a negative ORP value. Pool control systems use electronic probes or sensors placed in the plumbing line, to measure millivolts created by oxidizers and reducers in the water.
A minimum level of 650mv has been recommended by the Association of Pool & Spa Professionals (APSP). As pool water becomes contaminated, or chlorine levels are reduced, ORP is also reduced. The pHin monitor would call for more sanitizer to raise the level of oxidizer in most of these situations. However, you may also be called to reduce your pH level, since pH also plays a part in your ORP level. As your pH level increases, your sanitizer will lose much of its efficacy or Oxidation Reduction Potential.
If you are using a stabilizer or conditioner (Cyanuric Acid) to protect your chlorine from the sun by limiting its activity, you will also suppress its ability to sanitize the water. This would be measure as a reduction in your ORP level and would require an increase in free chlorine in order to maintain a proper ORP reading.