Understanding ORP, Sanitization & How Controllers Work

Correct water balance in commercial pools and spas is the difference in having a safe and attractive pool and a disaster. Advances in chemical automation, development of new methods of disinfection and better education on the problems associated with water chemistry have made the aquatics facility manager's job easier while providing safer water to the bather.

Swimming pool ORP/pH controllers have been around since the mid-1970s, but were large and expensive so typically were found only in municipalities. The greatest innovations came in the early '90s when controllers became powered by microprocessors instead of analog technology. The introduction of the two-pool controller with modem communications became an extremely powerful tool, which allowed an operator to monitor and control pool water remotely as well as send information to a pager.

Remote communication has made it possible to become proactive in swimming pool and spa management. Whenever water chemistry falls below the threshold established, a message is sent to a pager or cell phone so pool operators can log in to the controller to diagnose and correct the problem before it becomes a health issue. Chemical automation using direct connections from microprocessor-based programmable controllers to a wireless Internet database management system provides the aquatic facility manager "real time" information regarding water chemistry.

Programmable controllers have now become the reporting media transferring information from the aquatic facility to a "readily accessible" record of the pool's current and past status. This offers the aquatics facility manager better monitoring and control of the chemical balance in the aquatic facility with more accuracy than at any time in the past. A new age of management and control of commercial pools is at hand.

But despite the fact that controllers are now commonplace on commercial swimming pools, users do not often fully understand how ORP works. ORP is a qualitative measurement, not a quantitative measurement, and is affected by many factors, including the biggest variables in swimming pool water chemistry, pH, cyanuric acid and now salt.

Health departments require a specific range of chemical residuals, which leads people to look at ORP and PPM charts to establish a chlorine residual that is acceptable to their local health authority. This situation confuses pool operators. Oftentimes, pool water is in compliance with PPM, pH and cyanuric acid, yet has unacceptably low ORP in the water to effectively oxidize and sanitize the water. Conversely, there are often pools and spas that are well below the required PPM levels but have exceptional water quality.

ORP & PPM (Oxidation-Reduction Potential & Parts Per Million)

The misunderstanding about ORP and PPM has been going on since ORP controllers have been used on swimming pools and spas. If properly understood, pools can be run very efficiently with an ORP controller. For many operators, water chemistry is a small part of their job, and the health department wants water with a specific PPM range. Controller manufacturing companies have ventured into utilizing sensors that measure PPM or quantity of chlorine to help solve the problem.

Most controllers will measure the levels of sanitizer and pH in a body of water. The three most popular measures of sanitizer available through chemical automation are ORP, Amperometric PPM, and Colormetric PPM. Understanding the basis of each measure can assist pool operators in managing their pools.

ORP (Oxidation-Reduction Potential): ORP is a measure of the oxidizing properties of any sanitizer present in the body of water. Most controllers will use a bypass system, taking a small sample of water out of the plumbing and directing it to a small container that houses the sensors. An ORP sensor contains two electrodes, generally a platinum electrode in contact with the water and a reference electrode inside an electrolyte chamber. The electrodes are separated from the electrolyte solution by a membrane. This membrane prevents the loss of the electrolyte solution, and prevents contaminants from entering the sensor. The sensor acts like a small battery. The output from the sensor is a voltage, typically expressed as milli-volts. ORP measures the water's ability to oxidize and is therefore dependent on pH when the primary sanitizer is chlorine. Cyanuric acid levels can affect the readings taken by an ORP sensor. Salt systems can also affect ORP sensors; often, special metals must be used for the electrodes in order to prevent buildup on the sensor. ORP is probably the most common method used by chemical controllers to measure sanitizer level, and the most inexpensive type of system to buy. However, understanding that the measurement is the water's ability to oxidize rather than the actual amount of sanitizer can be misleading for the novice. Some controllers calculate the actual chlorine concentration based on the ORP, pH and temperature measurements. The important thing to remember is that the ORP sensor doesn't measure anything specific; it will not differentiate between ozone and bromine.

Amperometric PPM: PPM measures the total amount of sanitizer in the system. Like ORP, an Amperometric PPM measurement is taken by a sensor, consisting of copper and a platinum or gold element. These sensors are housed in a small chamber, usually located on a bypass from the main plumbing of the equipment set. A liquid sample is delivered to the annular space between the two fixed electrodes in the sensing cell. A small amount of potential is applied to the working electrode, made of gold wire and the counter electrode, made of silver. An electrical current is generated by the reduction of HOCL and/or OCL at the gold electrode. Using a gold and silver electrode, the process consumes the electrons from the oxidation of the silver electrode, creating a small amount of electrical current. This charge is in direct linear proportion to the amount of residual chlorine present in the sample. This charge is sent from the sensors to the controller. The electrical voltage is measured, and converted to parts per million. The residual value is displayed on the digital indicator in mg/l (PPM).

PPM: Probably the easiest measurement for end users to understand. The system is easy to set up and maintain. As long as the pH and the flow are relatively stable, the Amperometric PPM measurement is accurate. The Amperometric sensor is also affected by pH, cyanuric acid and salt systems, but the effect is less than on the ORP measurement because of the linear relationship of the sensor. Amperometric measurements also require flow to operate; when there is no flow the oxidizer is soon consumed by the measurement leaving none left to measure.

Selective Membrane PPM: Provides direct readings of free chlorine in water. These are true readings of free chlorine and not indirect values derived from ORP. An advantage of the PPM sensor is that it's not affected by cyanuric acid (CYA) and MPS, thereby assuring constant readings of free chlorine at all times.

Colorimetric PPM: Measures sanitizer level in parts per million, however it takes measurements in a very different way. The term colorimetric is used to describe a system of measurement where reaction-based chemical indicators are used to detect the presence of a specified chemical. The indicator reacts with the specified chemical and produces a visible color change in the solution. The stronger the color of the solution, the more compound is in the solution. DPD test kits, an example of colorimetric measuring devices, are very familiar to those in the pool and spa industry. However, studies have shown that there can be significant differences in readings from these kits due to differences in color perception between individual users. However, when the eye that evaluates the test is automated, a DPD test becomes a very accurate measure. Essentially, a colorimetric system is an automated hand check. The controller is programmed to test the chemical levels in the water at certain times or time intervals each day. When it is time to test, the controller sends a signal to the colorimetric unit, which is separate from the controller. This signal starts the testing process. A test chamber is filled with water from the pool or spa. A beam of light goes through the test chamber and is detected on the other side to establish a base for the reading. Reagents are added and mixed with the sample. The beam of light is again measured and the difference between the amount of light received in the base reading and the reading with reagents is the measure of the amount of sanitizer. Colorimetric systems are some of the most accurate automated sanitizer measuring systems available. The measurement is not affected by the presence of salt or cyanuric acid in the water. As there are no electrodes to corrode or gather buildup, they also work wonderfully on salt systems. Colorimetric sensors also require flushing the sample to waste; this can amount to 2 to 3 gallons of water per day.

Primary Disinfection & pH Control

Primary disinfection using sodium hypochlorite, calcium hypochlorite, bromine, di-chlor, tri-chlor or chlorine generation (salt systems) has always been a part of the automation of commercial pools and spas. However, controllers now go beyond primary disinfection and pH control offering solutions for secondary disinfection, as recommended by the CDC's Model Aquatic Health Code (MAHC).

Secondary Disinfection Support With Controllers

More and more often, the industry is now seeing the need for a secondary disinfection system as a backup to the primary system. Controllers now control, manage and support the use of backup sanitizing systems, which is mandatory for salt water pools.

Enzymes & Shock Treatments With Controllers

Some controllers can also be set up to add enzymes and shock treatments automatically. This treatment can be scheduled at a time and date when no bathers are in the pool and starts automatically. Be aware, though, that non-chlorine shock will cause a temporary increase in ORP and a false DPD reading.

Using a DPD colorimetric chlorine measurement system, a water test is automatically taken in a pre-programmed frequency as often as every 15 minutes. Reagents are automatically mixed with a sample of pool water and optically read to determine total chlorine, free chlorine and combined chlorine levels. A true PPM reading is then transmitted through the controller to the Internet-based data management system. The colorimetric system is connected directly to the controller, providing a true PPM reading not subject to problems associated with PPM sensors.

Choosing the Best Controller for Your Aquatic Facility

The type of measurement system selected for a chemical controller should take all of these factors into account. The cost of any controller is often a hurdle in the decision making process; ORP measurement will require the lowest investment but will not provide chlorine concentrations as accurately as a colorimetric system. The type of measurement system selected for a chemical controller should take all of these factors in to account. The cost of any controller is often a hurdle in the decision-making process; ORP measurement will require the lowest investment, but will not give as accurate chlorine concentrations compared with a colorimetric system. Colorimetric systems will require a higher initial investment, as well as the ongoing operational costs of reagents. Cost often becomes the focus of a decision-making process; however, it is more important to focus on the needs of the facility, and to then balance those needs with any monetary limitations. Most controllers will justify the initial expense with the savings in labor and longevity of equipment and surfaces. The type of measurement chosen should reflect the other equipment on the pool. ORP measurements are the most susceptible to outside effects, and are therefore less accurate; cyanuric acid, salt, temperature and pH levels all affect ORP measurements. Amperometric measurements have small pH, temperature and cyanuric effects, but are flow-dependent. Colorimetric measurements are the least susceptible. ORP will tell you how much of your sanitizer is available for use. Amperometric and selective membrane sensor measurements tell you how much sanitizer is in the water. A controller that measures both can help a pool operator to identify and solve issues when combined chlorine is present in the water. ORP, selective membrane and amperometric PPM use electrochemical detection, which offers several advantages: simple instrumentation, ease of use, low cost and low power requirements. Colorimetric units will use more water in their testing, and require minimal power.


Every commercial swimming pool and spa should have chemical automation for maximum comfort, safety and longevity of the surfaces and equipment. Utilizing ORP controllers is the most cost effective solution, as long as the operator understands how ORP works, the swimming pool is indoors or the outdoor pool is using no more than 30 PPM cyanuric acid and it is understood that actual chlorine PPM levels will fluctuate when there is ORP control due to the variables affecting ORP.

Ideally, a facility will have ORP and PPM control, which will help ensure that the PPM levels required by local health authorities will remain constant, while also measuring the true water quality.


Lance Fitzsimmons is the inside sales manager for Pool Warden controllers manufactured by ControlOMatic. He has been in the pool industry since 1984 and has worked directly in the servicing of chemical automation products since 1991. Fitzsimmons worked with Acu-Trol, Ecolab and Leslie's in everything from technical support to sales roles. He is a CPO instructor and has a degree in sales management. For more information, visit www.poolwarden.com.


Lance Fitzsimmons