Puzzled about CYA drop

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patf11

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May 21, 2008
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Fair Oaks Calif
About two weeks ago my CYA reading was 70 to 80 just like it has been since April. Last weekend it suddenly was 40 to 50. We have the Taylor K-2006 kit. No other parameters are affected.

This seems like a very sudden change even taking into account the test is somewhat subjective. I did try the bucket test to ensure there are no new leaks (we do have auto fill).

Searching TFP I found a few references to unexplained CYA loss such as this one.

We have spent quite a bit of time recently cleaning the tile using pool tile soap purchased at Home Depot. Ingredients are not listed. We also used Mr. Clean Magic Eraser.

Is it possible the tile soap/Magic Eraser is involved?
 
If you have autofill and a leak the water gets replaced as soon as it leaks out and your CYA level falls. If you suspect this may be an issue, turn off the autofill system and check for leaks.

Something similar can happen if you have an automatic overflow drain and get a lot of rain or leave the hose filling the pool overnight by accident.
 
Appreciate the ideas.

Understand about the auto fill possibility. That is why I did the bucket test to ensure there are no leaks (auto fill was off). Pool and bucket drop matched.

The drain is part of the auto fill well and its input is below water line and protected from floating debris. I did check for any blockages, did not see anything to account for large water change. Never had to use other fill methods such as hose.

TA and CH have remained steady, would they not also reflect drop if cause is water replacement?

I am aware of Waterbear’s advice to wait 2-5 minutes for the CYA test.

I can certainly add stabilizer, but I really have no idea why the sudden change. :scratch:
 
I see Chem Geek has started a thread to explore CYA loss that some folks seem to be experiencing similar to mine. In an attempt to add data to the great CYA mystery, here is my experience for the past two week with CYA levels:
1. Ran bucket test to ensure no leaks.
2. According to Pool Calculator over 7,000 gallons of water would have to be replaced to drop CYA from 70 to 40. Water bill does not show any indication that this happened
3. CH stayed at 280 since early June. Pool Calculator shows a 7,000 gallon water replacement would have dropped CH to about 160.
4. On July 31 I added enough stabilizer to bring CYA back to 70 where it has stayed ever since.

So what was different during the period that CYA dropped? Here is what I can think of, I am certainly open to other ideas:
1. We did have hot spell, water temp went up to 87-89 degrees with minimal cool down at night. Water temp is normally 82-84 with cooling to about 80 at night.
2. Had unusual winds that blew extra leaves and some dust/dirt into pool.
3. Used tile soap to clean tiles.
4. Normally I test CYA about once a month however I noticed FC dropped to 2 from normal 5. This is what led me to test CYA early. Water always stayed clear. I suspect if we had not been home this would have changed. I did shock pool with liquid chlorine when I added stabilizer.

Possibly the water temp is involved?

Maybe wind brought something that thinks cyanuric acid is snack food?
 
Thanks for the info. Your CYA drop is quite rapid -- much faster than what would occur with the "chlorine oxidizes CYA" theory even under the best of circumstances. Also, it takes 2.5 times as much FC to oxidize CYA so if the CYA dropped 30 ppm it would use up 75 ppm FC in doing so and I don't think you added that much chlorine to the pool over 2 weeks. The only time we've seen such CYA drops occur that quickly other than from dilution are from bacteria in the pool converting the CYA into ammonia but that usually leads to a very high chlorine demand. Also, in your situation, you never had zero or near-zero FC in the pool so bacteria should not be growing anyway.

You said you used tile soap or Mr. Clean Magic Eraser. The MSDS for this product is shown here and interestingly the main ingredient is "Formaldehyde-Melamine-Sodium Bisulfite Copolymer". Note in particular the "melamine" which is what is used in the CYA test to precipitate CYA. Did your water turn cloudy or did you find any white precipitate (maybe it got filtered out)? The sodium bisulfite is a reducing agent (among other things) and will lower the chlorine levels, so that may explain the drop in FC that you saw. I believe this is a plastic resin anyway so the individual components won't necessarily come apart as they are bound tightly in a polymer -- but then again there's chlorine around so it's possible. I suppose it's also possible that CYA could bind to that polymer directly (sounds like an experiment someone can do). This link gives info on the product from the manufacturer.

In any event, I doubt you used enough of the product to drop the CYA that much. 30 ppm CYA in 17,000 gallons is 4-1/4 pounds. Interesting info, though.

Richard
 
I used both tile soap and Mr. Clean Magic Eraser. Probably not a factor but I will see if I can find MDS for tile soap tonight.

Water never turned cloud. I run the filter for about 10 hours split into three periods. I never noticed any white precipitate but between the Dolphin (Tuesday & Friday nights) and filter run times I could have easily missed chance since it never dawned on me to look.

I used 1.5 gallons of 12% chlorine to shock one time on August 1. That is only time I used liquid chlorine since installing SWG.

Agree the volume of sodium bisulfate required seems out of the question for delivery by Mr. Clean Magic Eraser. Interesting thought about experiment, not sure how to get proper ratios but seems like putting a Mr. Clean Magic Eraser into a bucket would at least give some indication if CYA is affected.
 
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