Weirdo question time again.
Let me preface this with the disclaimer that it seems I'm missing something. I'm not advocating high levels of anything. Nor am I planning on increasing my CYA to astronomical levels, I just think that I'm not getting it.
I understand that the purpose of CYA is to protect chlorine from UV, et.al. I understand that by protecting the chlorine, a lower percentage of the chlorine is available for disinfection, organics control, etc. so that as CYA rises, you must introduce a greater amount of chlorine to provide the same benefit. This is because the CYA binds with some of the chlorine, thus making it less readily available for use. Right so far, even if it is a simplistic description?
Now, one of the more common statements I've seen here regarding high CYA seems to go something like "when CYA reaches X ppm, the chlorine demand becomes unmanageable, and you won't be able to maintain such high levels of FC." So, accepting that as CYA increases it binds with more and more of the available chlorine, I see how getting to the target level of FC requires higher and higher levels of chlorine, which can become quite expensive to reach.
(The following numbers are from the Pool Calculator and Chem Geek's CYA chart) In a 10,000 gallon pool, from 0 FC, getting to FC 5 (target for CYA 40) requires 104 ounces of bleach. In the same pool, with a CYA of 100, to get to the target FC 12 requires 249 ounces. Making the assumption that the requirement is somewhat linear (perhaps a bad assumption) if CYA were something like 200, the target FC would be about 24 and require 498 ounces to reach. (Even if my numbers are off at this level, the concept should remain basically the same.)
Here's where I'm missing something. Shouldn't the actual chlorine demand remain the same? If the above pool uses 1ppm FC/day with a CYA of 40 and a target FC of 5, shouldn't the demand remain 1ppm FC/day regardless of the CYA level? Shouldn't actual maintenance of the appropriate level remain 21 ounces (1ppm) per day, regardless of the actual target level or amount of chlorine bound to CYA? My thinking (probably flawed) is that once the chlorine is bound to the CYA it is no longer available for immediate use, but the CYA is also already bound, so cannot bind with additional chlorine. Therefore it will not bind with the additional chlorine added to the pool to maintain the target. So it will take a large amount of chlorine to initially reach the target level, but once there, should not take any more to maintain it at that level than it would require to maintain it at lower levels.
What am I missing?
A side question: Based on other comments that indicate that swimming is safe "up to shock levels" this must mean that with a CYA of 40, an FC level of 16 is safe for swimming. At CYA 40, is 24ppm FC unsafe? I assume that although 24 might be too high with a CYA of 40, that it is perfectly safe at a CYA of 200 since the majority of the chlorine is bound. Right?
Another side question: Never mind. I'll wait on that one.