Well I'm not a chemist by trade (I'm a color and imaging algorithm and S/W programming guy at work), but have an interest in pool water chemistry and remember enough chemistry (which was my major along with physics) to know some things, but not others. And it's through real-world observation that we really learn what goes on, regardless of "theory". So that's why I asked.

I found one post where Ben says that PolyQuat and high chlorine levels are incompatible, but nothing else explaining it. The fact that users see chlorine levels drop rather precipitously while the CC only goes up a relatively small amount probably means one of the following:

1) PolyQuat consumes (combines with) chlorine to perform some sort of chlorinated PolyQuat, but does not show up completely in the CC (TC) test.

2) High levels of chlorine oxidize PolyQuat (i.e. break it down) which would explain the large drop in FC while CC doesn't climb very much. If this is the case, then it doesn't make much sense to close the pool with PolyQuat. Perhaps it's OK to use PolyQuat first, at lower chlorine levels, and then later (perhaps a few days or a week later) shock the pool with chlorine. I did read that it is recommended not to shock the pool within 48 hours of using PolyQuat.

3) High levels of PolyQuat interfere with the FC test so that the chlorine level is still high but one does not know it from the test.

If I were a betting man, I'd bet that #2 is what is going on. And yes, I'd sure like to hear from Ben on this, but understand if he's too busy.

As for the black algae problem that keam (Kristi) has in her pool, after she hopefully gets rid of it (at least all visible signs of it), she should probably use a PolyQuat small maintenance dose on a regular basis or perhaps could use 50 ppm Borates to prevent further outbreaks without requiring the higher chlorine levels. It may be that with black algae that nothing will completely get rid of it (especially in plaster pools, though hers is vinyl) so the best one can do is keep it from growing.

The level of chlorine that she reported was the minimum needed to prevent regrowth of the black algae corresponded to a disinfecting chlorine level of 0.07 ppm which roughly corresponds to the Max. column in Ben's best guess CYA chart.

Richard