Stubborn algae - Am I getting it yet?

[chem geek]

Given the 75-80 CYA level, an FC of 40 still only generates about 0.6 ppm of HOCl (see this table for some values going from FC to HOCl and this table for some values going from HOCl to FC similar to Ben's chart if you look at the 0.03 as "min", 0.06 as "max" and 0.5 as a possible "shock") so assuming that this disinfecting form of chlorine is what fades vinyl liners, then even an FC of 40 is likely to be safe. Nevertheless, it's probably not worth threatening a vinyl liner just because the chemistry shows disinfecting chlorine levels that are even lower than when if you didn't use CYA. It is possible, though unlikely, that the chlorinated cyanurates (that is, the chlorine bound with CYA) may have some destructive capability to vinyl liners, but I doubt it.

It may turn out that up to 0.5 ppm HOCl is needed to truly kill algae in which case we may be updating Ben's table in the future, increasing "shock" chlorine levels at higher CYA (and possibly lowering the minimums needed at lower CYA, though more chlorine would probably just make the algae kill go faster so isn't a bad thing, up to a point).

Anyway, I'm just throwing in my two cents in. I can't prove my assertion (with regard to vinyl liners) without an experiment.

Richard