by Rob Cox January 31, 2014
Chlorine Levels and Cyanuric Acid
Your Test Kit has been Lying to you. Cyanuric acid, aka pool conditioner or stabilizer, slows down the kill rate, or activity of chlorine in your pool. Experts such as Robert Lowry state that to counter the effect of loss efficacy, target free chlorine (FC) levels in the pool should be as much as 10% of the cyanuric acid level (CYA) in the water. This means that if you have a 50 ppm CYA level in the pool, your target FC level would be 5.0 ppm.
I was always taught that 1.0 ppm of chlorine was sufficient to kill bacteria and such. It turns out that it is in most cases. Most types of bacteria or water borne pathogens can be effectively controlled at levels over 1.0 ppm. Especially in residential pools that are clean and used very little. There are some persistent strains of bacteria such as Cryptosporidium or Giardia which can grow faster than a 1-3 ppm chlorine level can control.
A study performed by the Pinellas County, Fla Health Department found that when pools with 1-5 ppm of chlorine were treated with 100 ppm of cyanuric acid, nearly 80% of the pools would be deemed unfit for swimming by the sanitarian.
Experts on poolforum and TroubleFreePool such as Richard Falk (chemgeek) and Ben Powell have led lengthy discussions on the topic of the depressive effect of cyanuric acid to the efficacy of free chlorine levels. Two charts have been produced to help pool owners find the best chlorine level, based on their level of cyanuric acid, the one shown here is the CYA-Chlorine-Slam-Shock chart.
In most cases, and with the exception of Crypto or Giardia contaminations, these higher chlorine levels will compensate for the effects of cyanuric acid on chlorine activity. For heavily used commercial pools, it has been suggested that double this amount be used. This would roughly equate to a maintaining chlorine levels at 20% of the cyanuric acid level.
Notice also in the chart that it's not only the daily chlorine residual that needs to be boosted to compensate for the CYA, but also when shocking the pool. CYA bonds almost instantly to chlorine in the water, and makes it difficult to reach breakpoint chlorination.
When shocking the pool for sanitation and chloramine removal, these charts suggest shocking to higher FC levels based on your level of CYA in the pool. If using cal hypo pool shock, 1.3 lbs will raise 10,000 gallons to about 10 ppm free chlorine.
If you want your pool to be truly sanitary, you have to understand that your test kit is not really measuring Free Available Chlorine, and what is actually in the pool can be Free, but not Available - or is it the other way around?
Anyway - if you really want a sanitary pool, this new data suggests that stabilized pools begin using a lot more chlorine than in the past.