I started the season with a CYA of 30. On 3/21 while trying to get to 50/60 CYA I put in 4lbs. and it moved to 40 after a week then dropped back to 30 a few weeks later. On 4/19 I purchased 4 more lbs. and put in 2lbs. on 4/13 and 2lbs. on 4/20. Today my CYA is between 30 and 40. My TA has shot from 60 to 100 and I would guess that is due to the CYA addition. While I did have to drain around 6 to 7 inches off due to excessive rain fall in late March I would not have thought that that would have made that big a difference in my CYA readings. I can also tell my CYA is low due to the increased chlorine usage. I have never had this problem with CYA from Pinch A Penny. These last two batches came from their competitor.
My question is what has caused this problem? Bad CYA? Bad Math (me)? Water drain off? Test Accuracy?