the chlorine bound to CYA may also breakdown, though not as quickly.
This may well be the key to the advantage Jan (aylad) has recorded year in and year out. If the bound-up chlorine ALSO breaks down, it's still doing work in addition to the un-bound chlorine. This MUST happen or why would there be any protection at all of the UNbound chlorine? Isn't it logical if the unbound chlorine isn't protected it would STILL break down as if there were no CYA at all to the residual level. Yet we know this doesn't happen. So there MUST be an effect from the bound chlorine.

So I think the next step is to evaluate the effectiveness of chlorine when the residual required base level is reached for increasingly greater CYA levels.

I don't understand chemistry at the scientific level, but I do follow logic and cause and effect. I leave to you, Richard, to evaluate and confirm or refute my hypothesis.