In my travels around the country, I find most people couldn’t care less about ORP as a measurement of pool-water quality. Instead, they rely solely on free available chlorine, measured in parts per million (ppm). This likely happens because they don’t understand ORP, or because health departments do not recognize it and specify free chlorine residual standards.
ORP is a qualitative measurement, while ppm is quantitative. I have often seen pools and spas with measurements that meet health department standards, but whose ORP levels show an inability to ineffectively sanitize and oxidize. Conversely, I have seen pools and spas with free chlorine residuals that would not pass health department inspection, but that have excellent water quality.
I believe calculated ppm alone cannot be relied upon as an accurate gauge of free chlorine. Many controllers use this way to provide a free chlorine measurement, calculating it based on the pool’s ORP and pH levels.
ORP does not indicate sanitizer levels, but measures the oxidizing properties of any sanitizer in the water. It fluctuates with variables such as usage, weather, fertilizers and treatment chemicals — with the biggest variable being the amount of cyanuric acid (CYA) present. Differing CYA levels have different impacts on the relationship between free available chlorine and ORP. Additionally, sunlight intensity or lack thereof all affect ORP. Because of this fluctuation, the calculated ppm figure is not a hard and fast number, but does provide an idea of how much free chlorine is available. It can be calibrated for specific conditions, but as soon as those conditions change, it will be off.
I believe the only practical way to accurately test for free available chlorine is with DPD or selective membrane free-chlorine sensors, but these can be flow dependent. Various state codes list DPD testing as the preferred method. Amperometric sensors are very dependent on constant flow rates.
The whole picture
But ORP should not be overlooked.
Ppm residuals are only part of satisfying health departments and keeping pool water healthy, since pH, CYA, flow and filtration all work together to keep water clean. Typically, flow and filtration issues are easy to identify, but the invisible things going on in the water can be frustrating to deal with.
ORP offers an excellent way to determine water quality, along with finding the effective level of free chlorine residual that is required within local health department guidelines in a specific body of water to keep it safe.
Keeping water within minimum requirements for free chlorine and under maximum levels for cyanuric acid will not always provide the results required to keep water clear and swimmers safe. Using ORP as a diagnostic tool helps the pool professional identify water-quality problems unrelated to filtration and circulation. Sadly, this incredible tool is often never used to solve simple water quality problems.
