Where'd my CYA go?

[Lobrien]

tl;dr: My CYA went from 50 6 months ago to under 20. How?

17000 gallon pool, 4.5' average depth.

I noticed that my chlorine consumption has been creeping up. I tested my CYA in July and it was 50 (which made sense in terms of previous readings). I don't regularly test it because I only use bleach and my pool chemistry has seemed stable enough (other than the creeping chlorine). When I tested CYA a few days ago the reagent hardly clouded at all! I sanity-tested my reagent by dropping in a granule of stabilizer and it clouded. I added 16oz of stabilizer and my CYA looks like a shade less than 20 (i.e., I can still barely see the spot at the bottom of the tube). I'm planning on taking my water in to the pool store and getting all my numbers checked by them, but I think my reagent is good and it's a real drop in chemistry.

I was surprised because I thought of CYA as something that only goes up until you dump the water. All I can think of It's been a rainy six months and I imagine that I have lost water through the overflow drain, but I can't really believe that "most of" the pool water I had six months ago overflowed out. I live in Hawaii so very strong sun, if that matters.

What else might cause my CYA to go down like this?

 

[mknauss]

Welcome to the forum!
CYA degrades slowly. In your high UV area I would not be surprised to see ~10 ppm per month. We see that in the summer months here in Nevada.
You should test your CYA monthly.
I suggest you read ABC's of Pool Water Chemistry.

 

[ajw22]

Welcome to TFP.

What test kit are you using? Don’t bother taking your water to the pool store. It will just confuse you. Trust your testing. CYA reagent is very stable and reliable as long as you properly mix it 50/50 with test water.

As Marty said, CYA does degrade. Just add stabilizer and get it back to your target and then check it at least monthly.

CYA - Further Reading

www.troublefreepool.com

 

[Etchase]

I live in Kihei and have experienced a drop in CYA from 50 to 40 during the summer in a month, but 40 to 30 in two months indicating it might be more of a percentage drop than an absolute amount. Since the tests accuracy is +-10 it might not indicate anything. I lived downwind of the wild fires last July on Maui and ended up with a lot of ash in my pool, and my CYA level dropped from 60 to around 20 in a month. Not sure the ash was the reason, but it has never happened before or since. When I say it was level of 20, that is based on how much CYA I added, and back calculating. When I tested it with the Taylor kit, the dot looked completely clear at 30. I was surprised it wasn’t 0.