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Oxidation Reduction Potential, often shortened to the acronym ORP, is a measurement of chlorine’s ability to oxidize contaminants in the pool water. ORP was adopted in 1971 by the World Health Organization as a standard test for drinking water treatment, and has been used to improve pool sanitation for over 40 years.
Public pools and spas are often required to control and monitor the chemistry by electronic means, for health reasons mainly, for the safety of the bathers, but also from a facility perspective, accurate chemical control means less chloramine production, less chemical related damage to pool surfaces, and a reduction in overall chemical costs.
An oxidizer, such as chlorine or ozone is a substance that takes electrons from other molecules, and a in the process is reduced, or rendered useless. In the process of oxidation, chlorine gains an electron and reducers (contaminants) lose an electron.
In a swimming pool, the oxidizers become reduced and the reducers become oxidized. If there were exactly the same amount of oxidizers (chlorine) as reducers (contaminants), the ORP would be zero, because the potential of the oxidizer to reduce exactly matches the level of reducers.
ORP is not a direct measurement of chlorine level or residual, but only a measure of the disinfectant activity or potential. However, with bather load, temperature and pH being constant, one could correlate the levels of chlorine level and ORP.
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.
Pool controllers will display the measurement of ORP, in millivolts, and some controllers also monitor and display pH and temperature, using separate sensors. Many controllers can also dispense chlorine and pH control chemicals in correct proportions to optimize the ORP levels.
The cost of pool controllers has come down in recent years, and many systems are available for less than $1000. Hayward, Pentair, Polaris and RolaChem are manufacturers of ORP controller systems.
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. A controller would call for more chlorine to be delivered to raise the level of oxidizer. It may also call for the pH levels to be reduced.
At higher levels of pH, ORP is reduced, and it becomes difficult to maintain enough oxidation potential, even with high levels of chlorine in the water. For normal chlorine levels of 1-3 ppm, ORP is at its highest level in the 7.0-7.5 range. As pH level rises, chlorine loses much of its efficacy, hence much of its oxidation reduction potential.
At higher levels of water temperature, the activity increases and this reduces ORP by a small factor, usually 10-15 mv per 10° rise in water temperature. Higher temperature water can also cause pH levels to rise slightly. Bright sunlight also reduces chlorine levels, and thus also ORP levels, even on stabilized pools.
Cyanuric acid, also known as stabilizer or conditioner, is a popular chlorine extender that protects chlorine from the sun by limiting its activity. It also suppresses its ability to sanitize, making chlorine somewhat sluggish. This also can be measured by a reduction in ORP, in the presence of stabilizer. For pools without higher requirements for cyanuric acid levels, a level of less than 20 ppm is recommended.
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