On maintaining FC...

[chem geek]

In theory, the rate of oxidation of contaminants and the breakdown of chlorine from sunlight are both proportional to the amount of "active" chlorine. With twice the CYA and no change in FC, there is about half the amount of "active" chlorine and as expected there would be a slowdown in chlorine usage.

The problem is that what we are comparing what happens when you raise BOTH FC and CYA to keep the FC/CYA ratio constant which is roughly what Ben's table does in order to provide the same amount of algae prevention. In theory, this has the rate of loss of "active" chlorine from sunlight and the oxidation of stuff be about the same as before if the only effect of CYA were lowering the active chlorine concentration. We know that CYA does more than that and shields lower depths as well, so that's the extra effect that depends only on the CYA level and is not diminished by higher FC.

In addition to "active" chlorine, the chlorine bound to CYA may also breakdown, though not as quickly. The amount of chlorine bound to CYA roughly doubles if both the FC and CYA are doubled. The amount of shielding is roughly proportional to the amount of CYA. So these two factors should roughly cancel each other out unless there was less chlorine at the surface.

Thanks for having me think it through a little more.

Richard