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                What is ORP?

ORP is a term used frequently in the water treatment & food processing industry.  ORP stands for Oxidation-Reduction Potential.  So what is that?  The best definition I can give is that “ORP is a measure of the cleanliness of the water & its ability to break down contaminants”.  It has a range of –2,000 to + 2,000 and units are in “mV” (millivolts).  Since ozone is an oxidizer, we are only concerned with positive ORP levels (above 0 mV).

ORP Level (mV)



No practical use




Cooling Towers


Swimming pools


Hot Tubs


Water Disinfection *


Water Sterilization **

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, the lower 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 on the left identifies ORP levels for various applications.


ORP level can also be viewed as the level of bacterial activity of the water because a direct link occurs between ORP level and Coliform count in water.  The chart to the right lists ORP levels and relative Coliform counts.

ORP Level

Coliform count in 100 ml of water










How does an ORP meter work?

An ORP meter measures very small voltages generated with a probe placed in ozonated water.  The electrode is made of platinum or gold, which reversibly looses its electrons to the oxidizer.  A voltage is generated which is compared to a silver (reference) electrode in a silver salt solution, similar to a pH probe.  The more oxidizer available, the greater the voltage difference between the solutions.

ORP cannot be used as a direct indicator of dissolved ozone residual, except in very clean water applications.

ORP is a convenient measure of ozone’s ability to perform a chemical task.  ORP is valid over a wide pH range, and ORP probes are rugged enough to be place into processes for in-line measurement. The probes can withstand process pressures in excess of 30 PSI.

ORP and Ozone
In a clean water system, using ORP to measure the dissolved ozone status works well.  In our experience, however, even the strong oxidizing power of ozone in moderate turbidity (cloudiness) systems can result in ORP values far below expected and even negative (reducing) values.  In general, monitoring ozone with ORP at the generator source works well but measuring dirty water becomes unreliable.  Ozone detection kits are available and work well at the limit of approved concentrations for your application.

*  Disinfection is destruction of specific pathogenic microorganisms
 Sterilization is the destruction of all microbial life

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