Lower CYA for no apparent reason

[a_t]

I use TF-100 test kit. Two months ago my CYA was 60. Today it is down to less than 20. I filled the CYA test tube to the top and the water is cloudy, but I can still see the black dot. I measured a second time with the same result. This is confusing because as far as I know CYA doesn't decrease. I have not used the pool at all this season, so no splash out. I barely even add any water because the solar cover really helps with evaporation. So the only thing I can think of is that the CYA reagent is bad. I got the kit in April. What else can make CYA decrease?

It's a fresh water 14,500 gal in-ground pool.

Here are my test results from 2 months ago:
FC 7
TC 7
CH 225
TA 100
CYA 60

Here are my results from today:
FC 2.5
TC 2.5
CH 175
TA 90
CYA < 20

I also used a test strip for comparison and stabilizer seems to be in the ideal range - 30-50ppm. Chlorine in this picture is higher because I did the test after adding chlorine.

 

[kimkats]

Do you have a overflow tube and auto refill system? Have you had a lot of rain in your area? I am having a hard time keeping my CYA up due to the amount of water turn over with the rain.

Kim

 

[mknauss]

Be aware that CYA does degrade by 3-5 ppm per month. You have a greater drop than that, but degradation does occur.

There is also a +/- 10 ppm testing error no matter what.

I am not aware of that reagent ever being reported as bad. But you never know. Not sure I would believe a test strip other than it shows there is some CYA in the water, maybe.

I would add enough CYA to get back to the level you want.

There have been reports on this Forum that the Clorox brand of CYA has not raised the CYA to the level expected by the pool owner whom used it.

Take care.

 

[Philo]

"Be aware that CYA does degrade by 3-5 ppm per month"

That's new to me. Is that referenced in Pool School somewhere? I thought I read everything.

Thanks

 

[mknauss]

"Be aware that CYA does degrade by 3-5 ppm per month"

That's new to me. Is that referenced in Pool School somewhere? I thought I read everything.

Thanks

Read this -- Degradation of Cyanuric Acid (CYA))

 

[Kiss4aFrog]

You don't have the pools equipment outlined in your signature so I'm not sure if this applies but it might also be due to how much you may have needed to backflush the filter if it's sand. Each time you backflush you are kicking out some CYA.

 

[Philo]

Read this -- Degradation of Cyanuric Acid (CYA))

I read that a couple of times but can't find the where Richard/Chemgeek mentions that?

 

[mknauss]

I read that a couple of times but can't find the where Richard/Chemgeek mentions that?

"Some report rates of around 10 ppm CYA per month which is higher than the roughly 2-3 ppm CYA per month seen in pools not exposed to sunlight (or the 5 ppm CYA per month seen in hot spas) which would be the degradation rate from chlorine oxidation of CYA."

 

[JoyfulNoise]

Richard talks about it in the context of spas and hot tubs due to the higher water temps. 3-5ppm/month is just a guesstimate made from some of the scientific literature on chlorine and CYA. There's no hard & fast rule. My pool has lost as much 10-12ppm per month during the hot, high UV season of May to July. I absolutely have NO water leaks and no algae but saw a 20ppm drop from late May to the middle of July.

It can happen. Just carefully add back the CYA 10ppm at a time so you don't overshoot.

 

[Philo]

"Some report rates of around 10 ppm CYA per month which is higher than the roughly 2-3 ppm CYA per month seen in pools not exposed to sunlight (or the 5 ppm CYA per month seen in hot spas) which would be the degradation rate from chlorine oxidation of CYA."

Still a little fuzzy on this issue but thank you for the reply.

 

[a_t]

Thanks for your responses. Since I have no water turnover, CYA must be degrading over time. I'll just throw in a floater with a bunch of chlorine pucks to raise CYA for me. I still have half a bucket of those. I only use them when I need to increase CYA.