I have a 20,000 gallon pool with SWG that was installed in the spring of 2010. The pool company put 4 lbs of CYA in at that time. A couple months later I tested the CYA myself (Taylor kit) and it was low, so I added another 4 lbs.
Fast forward to this year, I was having trouble with CL dropping faster than it should, so I tested CYA again, and it was too low to make the black dot disappear (my Taylor tube stops at 30 and the dot was still faintly visible). So I added 8 lbs of CYA last weekend. Today I checked the CYA and it was at 40ppm. According to the online Pool Calculator the 8 lbs I added last weekend should have gotten my CYA up to around 48ppm even if I had zero to start with.
So, my question is: Should I go ahead and add another 4 lbs of CYA (which should get me up to around 65ppm)? Or should I stop and check something before adding more CYA. The reason I am unsure what to do is that I have read that CYA is only lowered by replacing water, and my pool has never been drained or had substantial splash out, so I am wondering if somehow the Taylor black dot test is reading way too low and I really have a ton of CYA in the pool already?
By the way, I took a sample to the local pool store last weekend just after I had added 4 lbs CYA and watched the clerk do the CYA test. She got 40ppm using a black dot test, but when I asked her to show me the black dot it was still visible (but blurry) at 40ppm. I moved the dot to where it truly vanished and it read 30ppm.
By the way, the troublefreepool forums have provided me with an excellent education, so I will take the occasion of my first post to say thanks to all the people who have contributed here!