CYA decreasing over time?

RGS

Bronze Supporter
Jul 17, 2016
36
Carlsbad, CA
I normally maintain my pool with liquid chlorine and muriatic acid. I keep my FC level at the desired percentage of CYA, using the TFP guidelines. Have had no algae problems since I began using this method.

I have an in-line chlorinator that uses 3" tri-chlor tabs, but I only use it for a few weeks at a time, if I am away and cannot tend to the pool. Last September, after using tabs for a few weeks during my absence, I measured the CYA at 65. Since that time, I have only used liquid chlorine. Today, I measured the CYA at 45. I do not backwash the pool, since I have a cartridge filter.

Is it possible for CYA to go down that much in 7 months if the only way that water leaves the pool is by evaporation? Allowing for measurement errors, it could have been 60 last September and 50 today, but that is still a noticeable drop. I'm not complaining, because I may have to use tabs again for 3 weeks when I go on vacation soon. Just wondering what can cause CYA to drop, since I see people on this website saying that it can only go down when some of the water is removed by backwashing or draining, not by evaporation.
 

tim5055

Mod Squad
TFP Expert
LifeTime Supporter
May 11, 2014
10,122
Franklin, NC
Yes, entirely possible. As you point out, there is a testing variance to consider and CYA will naturally degrade a few ppm per month, maybe a little more.
 

RGS

Bronze Supporter
Jul 17, 2016
36
Carlsbad, CA
CYA does reduce over time and a deep end discussion about it is in this thread, Degradation of Cyanuric Acid (CYA).
Thanks. Based on my measurements I lost 20 ppm in 7 months, so about 3 ppm per month, which seems reasonable based on the thread. The pool is never covered and FC is always maintained so that there is no algae or bacteria to affect the CYA. I will probably be using the tri-chlor pucks for 3 weeks during an upcoming vacation, so its good to know that my CYA has dropped since the last time i had to do this.
 

Paul & Crystal

Silver Supporter
May 25, 2014
539
Oklahoma City, OK
I am also experiencing a noticeable drop in CYA. I did back wash twice for a couple minutes about 6 weeks ago when I first got the pool. After the backwash I was rounding up to 50. Now 6 weeks later I'm at 40. I've not decided yet how I will bump it up.

Crystal
 

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