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ORP and Cynuric Acid (CYA)

This is not meant to be a scientific report but observations from the field regarding cyanuric acid's effect on the ability of chlorine to effectively oxidize swimming pool and spa water in a commercial environment.

There is much debate over the effects of cyanuric acid (CYA) on the ability of chlorine to oxidize and sanitize swimming pool and spa water. There are charts and scientific data that can be confusing and applicable in a controlled environment but in the field can be a whole different scenario so I will attempt to keep it simple and share what I have learned over 17 years.

Does cyanuric acid cause the ORP of swimming pool and spa water to drop? Most definitely! Having worked with ORP - pH controllers since the early 90's, I have seen cyanuric acid interfere with chlorine being able to do it's job of oxidizing water.

However, it wasn't until the mid 90's when I started working with Acu-Trol that I was able to witness the effect in more detail. This was due to the state of the art features that Acu-Trol was offering at the time. Previous to that, I was servicing analog Chemtrol, Strantrol and Aquasol controllers. The features included custom programming, modem communication and downloadable data.

When I first started working with Acu-Trol, I was a partner in a commercial swimming pool service company in Southern California. We started installing some of the original Acu-Trol AK-1000's and after watching the data we saw various ORP values that not only were affected by FAC and pH but also cyanuric acid levels.

Another interesting observation was on our outdoor pools we could tell when the sun was rising and setting based on ORP values alone. When the sun was setting, the ORP level would begin to rise and by nightfall, the ORP had risen by 40 – 50 mV. When the sun started rising, the ORP level started dropping again. This would confirm the theory that HOCl attaches itself to cyanuric acid in the presence of sunlight.

It was also noted that swimming pools with high usage and CYA levels above 70 PPM were almost impossible to keep clear and the ORP levels were below 650mV with FAC in the range or 3-5 PPM.

In my experience, using ORP control on a swimming pool works optimally in the 30-50 PPM range with an upper limit of 70 PPM for swimming pools that are not heavily used. I never advocate using CYA on a spa as it will be difficult to maintain an ORP level sufficient for proper sanitation and the heat will dissipate the chlorine faster than UV.

by Lance Fitzsimmons