Web Exclusive - February 2016
Find a printable version here

Understanding ORP, Sanitization & How Controllers Work

By Lance Fitzsimmons


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.