mickey4paws said:
Thanks, that makes sense. On the other hand, with CYA at a higher level, the corresponding FC level has to be higher, and wouldn't that make the cell work harder too? or not?
You might think so but it doesn't work out that way for reasons we don't fully understand. CYA seems to shield chlorine from breakdown from sunlight with a mechanism beyond just being bound to it, perhaps shielding lower depths from UV (though some sources say that shouldn't be the case). So even though the FC is proportionately higher so you've got the same active chlorine level while having more chlorine bound to CYA, the overall FC drop is less.
This post shows the effect where the FC/CYA ratio is kept constant so the active chlorine level is the same, but the % Loss is lower at higher CYA and enough to make even the absolute FC loss lower.