Last week, I noticed my CYA was creeping down to 45ppm. I like to keep her, and she likes to be at 50ppm. So I decided to use two 3" HTH Tri-chlor pucks to try and "bump up" my CYA back to 50ppm. Today I tested for CYA and I was stunned to get a 80ppm reading!! I did a retest, and again ... 80ppm!! What happened?!?! I was expecting each tablet (7.1 oz. a piece) to increase CYA by 2ppm according to poolcalculator.

Unrelated, but maybe a culprit, last week I had to use muriatic acid to reduce my pH from 8.2 after a poolcalculator error on my part. I used one gallon Klean-Strip "Safer (Low odor)" Muriatic acid (<25%). I used the "safer (Low odor)" version because that's all my Home Depot carries (might be a NYS thing). When I had a chance to go over to Lowes two days later, I found full 32.5% strength which, if needed, I will be using from now on.

So what does everyone think? Should I trust the test? Should I chlorinate for that CYA? Or should I trust my gut that says there is NO WAY that 2 pucks of tri-chlor would have increased CYA 35ppm!! I have tested this pool five times for CYA this year. Started out at 65ppm and with rain, splashouts, backwashes, a broken hose that reduced my pool to below the skimmer, etc., and ended up with 45ppm last week.

Latest tests
FC - 7
CC - 0
pH - 7.5 and stable for last five days
TA - 170
CH - 100
CYA - as described above
Temp. - 78 degrees
Water has a case of sparklypoolitis and clear as can be, otherwise, no other symptoms. Filter has been steady at 22 psi. The last time we swam was Saturday. Thanks in advance for any and all replies.

according to pool calculator, two 7 oz. pucks should raise your CYA by 4. Possibly you had a testing error when you got 45?

Regardless whether the first test or the second test was incorrect, two pucks simply cannot add that much CYA.

If you feel that your second test of 80ppm is correct, then you need chlorinate appropriately and can accurately assume your first test was in error.....the pucks didn't do it.

I know my tests results prior to attaining 80ppm were correct because they have been in the same pattern since the beginning of season. That's why I explained that in my initial post ... "I have tested this pool five times for CYA this year. Started out at 65ppm and with rain, splashouts, backwashes, a broken hose that reduced my pool to below the skimmer, etc., and ended up with 45ppm last week." I can't see how I had 5 inaccurate tests with believable and patterned results. I also just looked up last years tests and I ended season with 70ppm ... makes sense.
But this last test(s) is way out of whack. And I remembered that I had bought some fresh R-0013 reagent near start of season and did a third test, and yet again, 80ppm. But I can't see how we got here???

Can the CYA test be skewed by something else added when I used the "Low Odor / Safer" muriatic acid??? It's the first time EVER using any muriatic acid in this pool.

Thanks for the advice so far, I knew that the pucks are not the culprit, but testing error is not doing it for me either.
Fair to say that, for now, I will be dosing for FC in relation to 80ppm CYA until a chemist can figure this out ...

Couple questions.....how did you broadcast the CYA into your pool at the beginning of the summer? And then how did you broadcast the M/A before you saw the stabilizer level jump? Thinking you may have had some undissolved crystals of CYA somewhere in your system (likely the sand filter), and the M/A treatment dissolved it for you.

Didn't broadcast ANY CYA into the pool this past spring. I used two pucks of tri-chlor to try and bump up my CYA by 4ppm. I diluted the M/A in a five gallon bucket (using only about 3 gallons of water so I could handle the weight), lowered bucket almost into pool water fully, and SLOWLY poured it in front of return.

This is a longshot but...

Is there a chance someone else has been feeding the pool chemicals? Dichlor?

Perplexing for sure.

I know this isn't the scientific answer you're looking for but... I'd give it a week or two, then retest the CYA and a good sunny day.

At the beginning of the season, I had something very similar happen to me. My CYA was at 20. I added just enough dry CYA to bring it up to 30. I tested 2 weeks later, the test said I was somewhere between 50-60. I was stunned, how is this possible. I did like you did and just dosed at the new rate. At the next monthly test, I tested it again, I said it was 30. I tested a few more times over the next week and every time is was about 30.

One thing I do know is the test isn't not a pin point accurate, especially at lower level. Additionally there are a lot opportunities for testing errors (differing lighting, subjectivity, water temperature, etc...) Like you said, you know your pool's trends and understand the math, and something is off. Let the pool settle down, and try again, if its back to 50, you know you the 80 was an out of range test, and discount it.

Nope, not a chance ... I am only one maintaining this pool.

Thanks for that affirmative that this has happened to you. I'll test again on a hot sunny day (not many left around here ...LOL) next week and see what I get. Was just my head around here wondering what I did. I'll post back either way, my next CYA results. And you are correct, the test does have many variables to have a "perfect" test. But I do try do do the same thing, same area, same lighting, all things same ... in order to minimize these variables. Thanks again

Well Puffin, thanks again for your help. I retested just now and got the answer I was looking for ... CYA - 50

Thanks for everyone's thoughts, don't know what happened, but it's back where I thought it should be.

Glad to see you discrepancy is resolved, though it would be fun to see what you measure tomorrow. CYA is one value that I trust knowing how much I put in before any single test result. There is a significant potential for error in testing in addition to the +/- 15% ( up to 90 ppm CYA) inherent in the test.

Most definitely agree with that statement, there are a TON of variables with this test. I try my best to weed out the "tester (that's me)" variables ... same place, same lighting, same test height for vial, same eyeball eyeing that dot, only two shots of rum prior to test ... etc.

We'll there's the problem, ONLY two shots?

The same thing happened to me earlier this season using 1" tablets. Being it was late in the season and this was the last season for this pool, I didn't worry much about it. It made me wonder if they're not getting the tablets mixed evenly during manufacture or something, or perhaps it was the dust in the bottom of the bottle of tabs?

Glad to see yours worked out, mines yet to be replaced.

CYA is not added separately to the chlorine in Trichlor tabs. It is a single chemical so the ratio of chlorine and CYA is fixed. There is no way that bad mixing or separation could result in lower or higher CYA levels getting introduced from nearly pure Trichlor tabs. For every 10 ppm FC added by Trichlor, it increases CYA by 6 ppm, period.

Something to keep in mind for sure. Thanks!

I learn something new every day, whether I like it or not! I knew I liked this forum for a reason, thanks for the enlightenment.