This has probably been asked, but does CYA ever lower on it's own, or once I reach the level it will maintain? Does anything effect CYA like evaporation causing it to concentrate or adding water/rain diluting it?
CYA naturally degrades very slowly, maybe a few ppm per month. Splash out of the pool is the biggest loss, so,if there are lots of kids, maybe a swimming dog or two - you will see loss there.
You have it correct, if water evaporates the CYA goes up but right back down when you add water back.
CYA testing is a little tricky. Try to recreate the same environment for testing, i.e. same sunlight, back to sun, vial at waist level, vial held with two fingers at the top, etc. If it was not bright out or you held the vial in direct sunlight it will give you a lower reading. This may not be the case but it could be skewing the numbers. Without water replacement your CYA should not change that much.
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