The correct approach: if you use cyanuric acid to stabilize pools and spas, you need to apply a correction factor. Here's how it works.(Tech Talk)

Most operators know that total alkalinity tests are a critical measure of water's ability to resist changes in pH and stay' balanced. Maintaining proper alkalinity also prevents corrosion. But if you use cyanuric acid or certain types of sanitizers, you may not be getting an accurate alkalinity reading--unless you apply a correction factor.

Stabilized pools and spas use CYA as a chlorine stabilizer (so-called conditioner) to minimize the loss of free chlorine due to the action of the sun's ultraviolet rays. In these pools, cyanurates are frequently present in high concentration, along with carbonates and bicarbonates. The typical TA test kit, however, measures the presence of all dissolved materials more alkaline than approximately pH 4.5 and does not distinguish cyanurates from carbonates/bicarbonates.

That gap can interfere with measurement of the actual carbonate alkalinity necessary for establishing the correct calcium saturation index, or CSI--a pool's first line of defense against corrosion, etching and scaling.

Getting an accurate reading is further complicated when chlorinated isocyanurates (trichloroisocyanuric acid or dichloroisocyanurate) are used to sanitize pool and spa water. Besides adding free available chlorine, these sanitizers also contribute CYA with each application. And the higher the pH, the more cyanurate is produced, further upping the "total alkalinity."

Because the alkalinity component of the CSI is based solely on carbonate alkalinity and TA testing cannot distinguish cyanurates from carbonates/bicarbonates, calculating the index in the presence of high cyanurates will produce erroneous results.

Fortunately, a simple factor can be applied to correct the total alkalinity results fur the effect of cyanuric acid and provide a good indicator of the actual carbonate alkalinity. The factor was first developed by John Wojtowicz who is now a consulting chemist fur Chemcon in Goodyear, Ariz.

It works like this: Simply measure the amount of stabilizer in the water and subtract one-third of the CYA ...