CYA still high after water change

[BarryY]

100 is the highest on the scale, but it takes the smallest amount of volume. In other words, adding more solution would result in a lower CYA reading, so halfway to the first graduation results in a reading way over 100.

[lagtime]

Barry, I had the same problem. Years of Dichlor left me with an initial CYA that sounds like yours, maybe 1/2" of water in test tube obscured the black dot. I bet it was close to 400 ppm. After the first drain it was still >100 ppm.

I ended up doing 2 drain & fills, removing ~75% of the water each time. My CYA is now between 50~60.

I'm fortunate in that I live in an area where the water table is very far underground so the pool popping out or the sides caving in ( it's 12000 gallon FB) isn't a big concern.

You have to drain & fill again.