I have read through many of the CYA posts on this topic. I understand generally poor results have been obtained. I am going to try this with FC = 0.
Quick background. I am a recovering dichlor abuser of 7 years. Thanks to this site, I, for the first time, feel like I understand how to maintain my pool properly.
I expected my CYA to be astronomically high. Using a 2:1 dilution (multiplying the result by 3) I came up with a CYA reading of 180 - 240. The discrepancy between the readings depended on whether I held the tube with the black dot in my shadow or in the sun. I know the shadow is preferred, but I had a much easier time judging when the dot disappeared with the sun illuminating the tube. That is, the results were more consistently repeatable. For this test, I will take CYA measurements the same way each time with the sun illuminating the tube.
Current water conditions:
FC = 4
CC = 0
pH = 8
TA = 100
CH = 450
CYA = 180
Temp = 83 F
Clear to sparkling
I added 2 cups of 200 Baume acid to bring the pH down to 7.5. Which I find I need to do about every week.
I had a slight brush with algae last weekend. Brushing, vacuuming, bleaching, cleaning the filter, etc. cleaned it up. There has been no CC for the last 4 days.
So, as I said, I want to try this Bioactive CYA reduction test with the FC close to 0. Should take about 3 days for my chlorine level to get there.
Let's see what happens...