where did my CYA go?

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M

march2012

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Jan 21, 2012
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TX
I was getting all smug about my pool as it is my first year and the chemistry has been great. We have been using more chlorine than I would expect so I measured CYA yesterday which I havent done in a couple of months. It turns out that it is right around 20 (the dot disappears in indirect sunlight when the tube is full). I was sure that my CYA was 40 as I last measured it because we dont backwash and really havent had much rain except one time. We originally put 4 pounds of conditioner in and then I added another pound a few weeks ago.

We were on vacation a lot and my neighbor did mention that we got like 10 inches of rain once, but that shouldnt have taken my CYA down by half.

What I cant understand is where all the CYA went.

What I am worried about is that we might have a leak, but would a leak really turn over half the pool? It seems like that kind of leak would become obvious.
 
Could it be that your original test of 40 was off? The first time I tested for CYA (and it was my first time ever), I don't think I was as careful as I could have been. A month or so later, I actually did the test several times with the same batch (per suggestions on this board) and realized that I was closer to 20. This is a test that allows your mind to play tricks on you, I think!
 
Pauls234 said:
Could it be that your original test of 40 was off? The first time I tested for CYA (and it was my first time ever), I don't think I was as careful as I could have been. A month or so later, I actually did the test several times with the same batch (per suggestions on this board) and realized that I was closer to 20. This is a test that allows your mind to play tricks on you, I think!
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When I measured 40 I did the test multiple times. It was also not the first time I had measured it. Finally, based on pool volume etc I added the correct amount of CYA to get me to 40. I am confident that the CYA level was in the 40-50 range.
 
I keep a book with pool notes in it, I don't refer to it very often now but I was just flipping though it looking for some pricing history on something. At some point I wrote

"CYA degradation by oxidation (when conversion to ammonia has not occurred) is approximately 12 ppm/month and higher near shock levels and higher at temps above 85 degrees and at higher pH. Therefore regular testing and believe your results."

I know that this was from reading a thread here on TFP, but I am sorry that my notes have no footnotes so I cannot say where this came from actually. Sounds like something ChemGeek would have said, just guessing, as no one else is quite as authoritative as he is when it comes to the chemistry. Based on that, if you started with 40 and lost 12 ppm or more per month that sounds about right.

I suggest that you be more regular about testing CYA, at least once a month. My test record page holds 30 tests, so it may last 2 months, maybe even 3. Within that time span I try to be sure to do a full test at least once or twice, sometimes more if it has rained much or if I've had much evaporation.

[edit] I noticed in another thread ChemGeek said 2 - 3 ppm / month so let's go with that. My notes do say 12 ppm but I'll guess that was my error or a typo in the earlier info.
 
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