ORP is a term used frequently in the recreation water industry. ORP stands for "Oxidation-Reduction Potential." So what does that mean? A good working definition is that "ORP is a measure of the cleanliness of the water and its ability to break down contaminants." ORP has a range of -2,000 to +2,000, and units are in "mV" (millivolts). Since Chlorine+ is an oxidizer, we are only concerned with positive ORP levels (above 0 mV).
ORP sensors work by measuring the dissolved oxygen. More contaminants in the water result in less dissolved oxygen because the organics are consuming the oxygen and therefore lowering the ORP level. The higher the ORP level, the more ability the water has to destroy foreign contaminants such as microbes or carbon-based contaminants. The chart below identifies ORP values for various applications.
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The ORP chart on the right show the safe levels for swimming pools and hot tubs. To additionally treat algae and other pathogens, maintaining an ORP level of 700+ mV is recommended.
Bather load affects ORP therefore it is recommended to check your levels depending on amount of activity. In contrast, a commercial pool will have many bathers/swimmers on a daily basis compared to a private pool. Maintaining a safe ORP in a commercial pool requires more attention than a private pool typically does.
In 1971, World Health Organization (WHO) developed this chart for safe ORP levels.