Optimisation of ORP readings for pool water

Category: Pools | Tags:

The Oxidation Reduction Potential, commonly referred to as ORP, is an indicator of chlorine’s capacity to eliminate impurities in pool water.

In 1971, the World Health Organization put ORP as a standardized examination for treating drinking water and it has been used for over four decades to improve pool cleanliness.

This parameter is measured in millivolts and ORP readings in the positive millivolts will correlate to an oxidative water state. The higher the millivolt reading, the more powerfully the swimming pool water is able to oxidize and disinfect.

The ORP level should be maintained between 700 mV and 750 mV for pools and spas.

A decrease in ORP signifies an elevation in chlorine demand brought about by the introduction of reducing agents or contaminants into the water. A decrease in ORP signifies the imminent onset of chemical reactions. ORP is regarded as a more precise indicator of disinfection effectiveness when compared to amperometric control.

There are several common water parameters and factors that can impact the ORP. Here are a few examples:

ORP and pH

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. The ideal pH value for the water of swimming pools and spa is between 7,0 and 7,6.

ORP and Temperature

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.

ORP and Cyanuric acid

Cyanuric acid, also known as stabilizer, 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.

Chlorine concentration - Free Chlorine / Chlorine residual

Depending on the pH level, chlorine will be present as hypochlorous acid (HOCl), the most effective form for sanitization, or as the less effective hypochlorite ion (OCl¯). Chlorine will be present in greater concentrations as hypochlorous acid below pH 7.6; above this pH, hypochlorite is favored. Chlorine can also combine with other molecules present in the water; when it combines with ammonia, chloramines are formed, which are virtually ineffective at sanitizing and responsible for producing the familiar and unpleasant “chlorine” odor.

Hanna Instruments offers a range of devices to assist you in maintaining the sanitation of your pool water and measuring its ORP value.
Whether you prefer automated process controllers or simple handheld testers, Hanna Instruments provides the tools you need to ensure worry-free swimming and enjoyable experiences.

Need assistance?

Measure the pH and ORP levels directly in the pool!


Pool Line Multiparameter Waterproof Portable pH/ORP/EC/Temperature meter

The HI981954 specifically designed to assess important water quality factors in swimming pools, hot tubes or spa water. This advanced device utilizes a single probe with multiple sensors to measure various parameters and present the results in the following format:

  • pH
  • pH in mV
  • ORP
  • conductivity (EC)
  • total dissolved solids (TDS)
  • salinity
  • temperature 

The meter has a built-in pressure transducer for barometric pressure readings.

  • One probe with field replaceable pH/ORP, EC and temperature sensors
  • Auto-ranging for EC and TDS readings
  • Display up to 11 measurements simultaneously


Professional Waterproof Portable pH/ORP/ISE Meter

The HI981914 is specifically designed to measure the pH and ORP levels of swimming pools, hot tubs and spa water.
It is advanced device equipped with various functionalities, including the exclusive CAL Check pH electrode designed by Hanna. This feature enables users to detect possible issues such as the need for electrode cleaning, buffer contamination checks and overall pH electrode health assessment. Moreover, this function can also be utilized to identify problems with process probes that utilize a BNC connector.

  • Laboratory grade measurements in a portable meter
  • CAL Check advanced probe diagnostics
  • Troubleshooting tool for the maintenance of process probes


Waterproof Portable pH/pH-mV/ORP/Temperature Meter with Sensor Check™

The HI991003 is a small, waterproof, portable meter for measuring pH, pH-mV, ORP and temperature. It comes with a durable carrying case and a special pH/ORP electrode made of titanium, which includes a built-in temperature sensor. This meter also includes our exclusive Sensor Check™ feature, allowing users to check the electrode status at any time. The meter has a large LCD display with two levels for easy viewing of pH, pH-mV, ORP, temperature, and status indicators. Operating the meter is made simple with just two buttons. Additionally, the HI991003 boasts an exceptional battery life of 1400 hours of continuous use.

  • Automatic Temperature Compensation
  • Automatic Two-Point Calibration
  • Waterproof


pH/ORP/Temperature Pool Line Combo Tester


pH/ORP/Temperature Pool Line Combo Tester

pH/ORP meter is made for measuring pH and ORP of a swimming pool, hot tub or spa water. This lightweight, waterproof tester offers high accuracy pH, ORP and temperature measurements in a single handheld unit. No more switching between meters for your routine measurements.

  • Waterproof
  • Replaceable pH electrode
  • Automatic calibration

The easiest way to achieve clean and clear water in the pool is with automatic dosing of chemicals.

BL12X Series

The BL12X pumps utilize integrated peristaltic pumps and a multiparameter pH/ORP/Temp probe, to accurately regulate chlorine levels and pH balance. This creates a comprehensive and compact system capable of measuring, analyzing and automatically administering chemicals based on the measured parameters (pH/ORP). As a result, water quality is constantly monitored and precise chemical dosing is facilitated.

BL122 • BL123

pH/ORP Controllers with Cloud Connectivity

BL122 and BL123 are specifically engineered for the purpose of keeping the pH and disinfectant levels consistent in swimming pools, hot tubs and spas. They provide an additional advantage by enabling remote connectivity and device access through the Hanna Cloud web app
These controllers are offered in two variations:

  • BL122 – The first option is the inline model, which facilitates direct installation of probe and chemical injection fitting into pre-existing piping. 
  • BL123 – Alternatively, a panel mounted version with bypass flow cell is also provided. The bypass flow cell allows for convenient calibration and maintenance of the probe without the need to shut down the recirculation pump.

BL120 • BL121

pH/ORP Controllers with Built-In Chemical Feed Pumps

These Swimming Pool Controllers are a complete system designed for maintaining swimming pool, hot tub and spa disinfection water quality.
These controllers are available in two configurations: 

  • BL120 – The basic version is the in-line model which allows for direct installation of probe and chemical injection fittings into existing piping. 
  • BL121 – A panel mounted version with a bypass flow cell is also available. The bypass flow cell allows for calibration and maintenance of the probe without having to shut down the recirculation pump.

PCA300 Series

Chlorine, pH, ORP and Temperature Analyzers

Hanna Instruments PCA310, PCA330 and PCA340 series consist of microprocessor-controlled analyzers designed for monitoring chlorine content, pH, ORP and temperature.

This process analyzers use the DPD Colorimetric method to determine chlorine levels accurately and provide continuous measurements of pH, ORP and temperature values.

  • 4 models available for monitoring chlorine, pH and ORP levels
  • Two separate selectable 0-20 or 4-20 mA signal output (PCA340 only)
  • User-friendly interface simplifies setup and maintenance

Pool Safety: Water Parameter Guidelines

Almost every country has hygiene requirements for swimming pools in its legislation that public swimming pools have to follow.

These parameters have a very narrow allowed range and are recommended to be controlled automatically with additional manually performed measurement.

Have questions?

Contact a Hanna Technical Specialist at info@hannaservice.eu or using our contact form.


Do Traditional Measures of Water Quality in Swimming Pools and Spas Correspond with Beneficial Oxidation Reduction Potential? Public Health Rep.2009 Mar-Apr; 124(2): 255-261. doi: 10.1177/003335490912400213

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