OK, so this issue came up and I'd appreciate any clarification/thoughts on this from you experts. As most know, Leslie's Pools allows a monthly "free" water test, which includes the CYA value. So they don't do the test the same as I would do per the instructions on this site: Outside, back to the Sun. I've tried this myself - testing outside, then indoors. The results are wildly different. For whatever reason, when indoors, the black dot take longer to disappear - which suggests a much lower CYA. (and PS - the guy(s) at Leslie's are very quick with this test and seem to never get the same results that I get ... I know, it's subjective anyway, but just sayin')
So I guess my question is WHAT is the point of testing CYA in the Sunlight (or bright light source) vs. in some other light?