Oxidation-Reduction Potential (ORP) measures the oxidizing properties of any sanitizer (chlorine) present in the water. When chlorine is free to oxidize, sensors generate a millivolt reading, expressed as ORP. If the chlorine isn’t free and available, a millivolt reading will not be generated.
It is important to remember that the “P” in ORP stands for the “potential” of the water’s ability to oxidize and is therefore dependent on pH when the primary sanitizer is chlorine. CYA levels can also affect the readings taken by an ORP sensor, as can salt systems.
ORP is probably the most common method used by chemical controllers to measure sanitizer level, and the most inexpensive type of system. ORP is a qualitative measurement that is constantly changing, especially outdoors. This is primarily due to varying cyanuric acid (CYA) levels and sunlight intensity, but also from pH, combined chlorine and salt-chlorine generators. As these factors fluctuate, ORP measurements will change throughout the day.
The ORP reading is a constantly moving target, it is best practice to monitor ORP in combination with chlorine residual measured in parts per million (PPM) this will give the assistant a complete picture of the chlorine within the pool(s) to comply with prescribed health regulations.
Fact: Sunlight DOES affect ORP readings. That’s why when taking a ORP reading pool operators always see a distinct pattern at morning and afternoon in outdoor pool levels. ORP will go down as the sun rises because UV rays begin hitting the pool water, causing the chlorine to combine with CYA. At that time, the chlorine is less “free” and, therefore, has a lower oxidation potential at those two times of the day. This doesn’t mean the chlorine can’t do its job, because it will break free as needed to oxidize and sanitize. In the afternoon, ORP will again increase, because of the lower UV levels degrading the chlorine.
It is suggested that readings taken at these times take into consideration the above information, as if the levels are slightly higher, this may not always be a true representation of the available chlorine in the pool before adjusting set points or adding chlorine remover.
It is often difficult to obtain a satisfactory ORP reading in an outdoor pool stabilised with cyanuric acid. It may be necessary to limit the cyanuric concentration to 25 mg/L or even 20 mg/L to obtain a satisfactory ORP reading.