Food and water treatment industries often use the term ORP (Oxidation-Reduction Potential). So what is it? ORP can be defined as a measure of water purity and its ability to break down contaminants. The range is from 1000 to + 2,000, and the units are in millivolts (mV). As ozone is an oxidizer, we only need to measure positive level of ORPs (above 0 mV) for ozone.

Oxygen dissolved in water is measured with ORP sensors. There is less dissolved oxygen in the water if there are more contaminants, since organics consume oxygen. The lower the ORP, the more contaminants in the water. An increase in ORP levels increases the water’s ability to destroy microbes and carbon-based contaminants. Various levels of ORP can be identified in the following chart.

ORP Level (MV)Application
0-150No Practical Use
250-350Cooling Towers
400-475Swimming Pools
450-600Hot Tubs
600Water Disinfection *
800Water Sterilization **

A direct link exists between ORP level and Coliform count in water, and this can also be interpreted as a measure of bacterial activity in water. An ORP chart and relative counts of Coliforms are mentioned in the following chart.

ORP Level (MV)Coliform count in 100 ml of water

How Does an ORP Meter Work?

A probe placed in the ozonated water generates very small voltages on an ORP meter. Platinum or gold is used as the electrode, which reversibly loses electrons to the oxidizer. In a silver salt solution, a measurement voltage is generated, similar to that derived from a pH probe, which is compared to an electrode of silver. There is a greater voltage difference between solutions when more oxidizer is available.

Disadvantages Of ORP

We cannot use ORP as direct indicator to determine residual dissolved ozone, except in very clean applications of water.

Advantages Of ORP

Ozone’s ability to perform chemical reactions is measured by its ORP. In addition, ORP probes are rugged enough to be integrated into processes for in-line measurements. Process pressures exceeding 30 PSI can be handled by the probes.

Bottom Line

The ORP method works well for measuring dissolved ozone levels in a clean water system. It has been evaluated by many experts that even moderate turbidity systems with strong oxidative power can produce ORP values well below expected levels and even negative (reducing) values.

In general, measuring ORP at the generator source works well for monitoring ozone, but measuring dirty water is less accurate. It is possible to find ozone detection kits that work well within the limits of approved ozone concentrations for a particular application.