In pools and spas that are exposed to the sun, Cyanuric Acid (CYA) is typically used to stabilize the available chlorine by reducing the rate of decomposition by the sun’s ultraviolet rays. For that reason, CYA is also sometimes referred to simply as “stabilizer”.
Stabilized forms of chlorine, such as dichlor and trichlor, have CYA in their formulas, but non-stabilized chlorines like calcium hypochlorite (a.k.a. cal hypo), lithium hypochlorite, sodium hypochlorite, and chlorine gas do not. Uncovered outdoor pools and spas that use these non-stabilized chlorines require the separate addition of CYA.
However, in either situation, it’s important to monitor the CYA level. With too little stabilizer, the free chlorine will not be adequately protected. With too much stabilizer, the chlorine can become overstabilized or "locked" which decreases its sanitizing and oxidizing effectiveness.
Overstabilization tends to occur most often in water treated with dichlor or trichlor, particularly if dichlor granules are used for shocking. Both dichlor and trichlor are more than 50% CYA by weight. Daily application can easily lead to a concentration of 80 parts per million (ppm) CYA.
For every pound of trichlor* added to 10,000 gallons of water, CYA increases 7 ppm. For every pound of dichlor* added to 10,000 gallons of water, the CYA increases 6 to 7 ppm. At this point, action must be taken to avoid reaching overstabilization. This typically requires partial draining with the addition of fresh water and a temporary switch to unstabilized chlorine for daily sanitization.
When stabilized water is outside of the recommended range for pH or above the ideal range for CYA (7.4 - 7.6 ppm and 3050 ppm, respectively), water balance calculations can be noticeably affected by the total alkalinity concentration unless a CYA correction factor is applied.**
Taylor makes water test kits that include a CYA test based on turbidimetric analysis. To perform the test:
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Rinse and fill the CYA dispensing bottle to the 7mL mark with sample water.
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Add the Cyanuric Acid Reagent (R-0013) to the 14 mL mark. Cap and mix for 30 seconds.
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Slowly transfer the cloudy solution that develops to the small comparator tube while viewing from the top.
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Stop when the black dot on the bottom of the small comparator tube disappears.
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Check the liquid level on the back of the comparator block. The reading indicates a parts per million concentration of CYA.
It’s important to note that, while industry-accepted guidelines for the minimum‒maximum range of CYA is 10–100 ppm, some health departments limit CYA to less than 100 ppm in public pools and less than 40 ppm in public spas.
*Two 3” tablets of trichlor or dichlor equals approximately one pound.
**Refer to page 14 in Pool & Spa Water Chemistry: A Testing and Treatment Guide (#2004B).