Any explanation for sudden drop in CYA?

[Guest]

I have slowly beein working on getting my CYA into range. I'ts taken a lot of partial drainings and refills, and with the benefit of lots of rain this summer, it was slowing getting where I wanted it to be. Up until a week ago it was about 55-58..
Measured it yesterday,and was all of a sudden just 45! I'm not complaining, that's about right where I want it to be and my free chlorine is holding fine. I measured again just to be sure I tested right the first time. My numbers matched exactly. I'm wondering if there is a certain threshold with the CYA testing where the black dot becomes more or less visible to the human eye?

The only thing different that I've done is gotten my borates up to about 50PPM a few weeks ago.

Pool looks perfect and all the other numbers look great.

FC: 4.2
CC: 0
PH: 7.5
TA: 90
CH: 250
Borates: 40-50
CYA: 45 (down from 58 two weeks ago)
Stains are staying away with the regular sequestrant treatments and increase in CH.

This is the first time forever that my pool water has been balanced. The only reason for me to keep adding fresh water is to continue to get the metals out.

 

[dattia]

Every time you drain water, you are removing some CYA. Have you replaced water since you had the 55-58 readings for your CYA?

 

[JohnT]

+/- 10ppm is about the accuracy of a CYA test, and the test is not linear, so there may be areas where it's read better.

 

[Guest]

"linear" That's the word I was looking for. In answer to dattia's question, yes, I have been doing partial drainings since last summer, but not enough to explain a sudden 10-12 PPM Drop.

I'm content with the way things are, I was just curious about the drastic results.

The one thing that helped lower my CYA drastically last summer was an ammonia problem. Not saying, I was glad to have that, but it did help get my CYA in a more reasonable range.

+/- 10ppm is about the accuracy of a CYA test, and the test is not linear, so there may be areas where it's read better.