More CYA questions...

I've followed this forum and others for a while, and am a bit mystified by my test results regarding CYA. It was my understanding that you only reduced CYA levels by draining/refilling, but not by evaporation/refilling.

This last April, my CYA was down to about 0, as it is most Aprils, so I bumped it up to about 40ppm. About a month ago, chlorine seemed to be getting burned off quickly, so I tested and it was almost down to 0 again!

I bumped it back up to 40 at that time. I tested again today and it was 30ppm. I know that the CYA test is one of the least accurate, but I'm still mystified. The way I test is pretty consistent.

I certainly didn't drain off any water. I live in SoCal, so we have a lot of evaporation, although this summer wasn't particularly hot. I do have a lot of aeration in my pool (25k/plaster).

Any ideas why I would have a constant drain on my CYA level?

Thanks!