This doesn't happen every time, but often enough that it makes me wonder what's going on.
Yesterday, FC = 9, pH=7.5+ a little. Added 2 cups MA, pH should have gone down to 7.2
Today I scooped up my sample, and checked pH. Perfect match to 7.2. I would have expected it to be just a little above 7.2, knowing the way my pool runs. FC= 8. Go to empty the two tests out and pH is now 7.5. Not even 2 minutes has elapsed, all I did was the FAS-DPD test.
I could see things changing if it sat in the sun for an hour or something, but not that fast. And as I said, it has happened before a few times. When it does happen, I treat it as the later reading, and so far, it has reacted the way it should. The reagent is fresh, no more than a couple months old.
Does this happen to anyone else? Does anyone (chemgeek??) have a clue why it is so?