I'm a new pool/spa owner in Novato, CA. The pool/spa are separately plumbed and do not share water. I have clear water in both and have no trouble maintaining it that way thanks to the wonderful advice here.
At the end of June a drained and refilled the spa. It's 600 gallons (I measured during the filling). I treated the water in accordance with the advice here. 34 ppm dichlor (to give 30 ppm CYA), 50 ppm borates (Gentle Spa). After the 34 ppm dichlor I switched to bleach. Since then I've been using only bleach, liquid acid, and 1oz of MPS every 10 days or so. This spa has a low bather load and is not heated when we're not using it. Unheated water is 80F, heated water is 104F. The spa stays covered when not in use and gets about 7 hours of direct sunlight per day. I periodically add make-up water for evaporation.
Anyway... Two months later it seems that the chlorine usage is a little high even when we're NOT using the spa (about 2ppm per day). So I finally broke down and bought a good test kit (up until now I just haven't needed one because I don't have any water problems). The CYA test showed less than 30 ppm (that's as low as my Taylor test kit goes). Hmm. How can this be? I've read here that evaporation doesn't affect CYA. I haven't removed any water...
So for the next two chlorine applications I used dichlor instead of bleach. Another 10 ppm dichlor should have resulted in an increase of 9 ppm CYA. Test again. CYA still less than 30.
If evaporation doesn't affect CYA, how can a total of 44 ppm dichlor result in less than 30 ppm CYA?
I have no problems with water quality, but I think I could lower the chlorine consumption by getting the CYA up to 30. I just can't figure out why it isn't there already...