I'm new here so it's quite possible I've missed something that's been discussed many times. Anyway, I'm incrementally draining my pool for various dumb reasons and it's darkish when I leave for work and darkish by the time I get home and have dinner all of which adds up to: there are lots of requirements for a good CYA test. Outside, bright sun at your back, waist level, good eyesight and judgment. I haven't been able to perform a decent CYA test for the week that I've had the test kit. My initial, good-as-I-could CYA test was 160 (50% dilution).
However, I know my CH was 820, I've done a few mini-drain/refills on the pool, now the CH is 625ish. That's a 24% drop so it stands to reason that my CYA has dropped by 24% as well, right?
But that doesn't account for the fact the the new water has CH of 80 and CYA of 0 so it's not exactly apples to apples. I can plug the numbers into the PoolCalulator, including the 80, and it says if I want to take my CH from 820 to 625 I should replace 26% of the water. So if I fiddle with the CYA numbers a 26% drain gets me from 160 to 118.
Anything wrong with my logic here?