On maintaining FC...

[chem geek]

would it be fair to say that adding a solar cover to the mix significantly decreases chlorine breakdown due to sunlight? any numbers on how significantly?

I can tell you that an opaque pool cover certainly eliminates the breakdown of chlorine from the UV in sunlight. My own pool is a case for that since it has only 30 ppm FC and with around 3 ppm FC average it loses around 0.8 ppm FC per day being open around 1-2 hours each day, though in practice even having the pool closed still has a loss of around 0.7 ppm FC or so per day. My daily loss appears to be mostly regular chlorine oxidation of stuff in the pool (including a slow oxidation of CYA by chlorine).

I don't know how much the "clear blue" solar covers block UV. If they did, but let visible and infrared light through, then that would be ideal as it would heat the pool without a loss of chlorine from sunlight.

Richard

[chem geek]

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.

Cyanuric Acid (CYA) itself is able to absorb the UV from sunlight without breaking down. Absoprtion of photons does not necessarily mean destruction of a molecule. There is a number called the "quantum yield" that for CYA is near zero, but for hypochlorous acid and hypochlorite ion are near 1 meaning that virtually every UV photon absorbed by hypochlorous acid and hypochlorite ion results in its breakdown whereas for CYA such absorption just gets translated mostly to heat (i.e. the molecule gets "excited" and then relaxes from this state to mostly vibrational energy).

As for chlorine bound to CYA, the quantum yield must be close to zero or else we wouldn't see that much protection of chlorine from sunlight given that most of the chlorine is bound to CYA. I can't find any definitive information on whether and how much the chlorine bound to CYA breaks down, but it could be by some smaller amount.

As for the effectiveness of chlorine, there are many, many studies that show it is mostly related to the hypochlorous acid concentration and not to the chlorine bound to CYA which has little if any effectiveness (other than as a chlorine buffer or reserve). Also, any breakdown from UV photons getting absorbed by chlorine bound to CYA would have nothing to do with its effectiveness (i.e. rate of killing pathogens or oxidizing organics).

Basically, the bulk of the protection of chlorine is from most of it being bound to CYA and that molecule being much more stable when hit by UV. However, there appears to be a secondary protection that is more of a "shielding" effect and that can be done by CYA itself as well as by chlorine bound to CYA. The best way to determine this would be to measure chlorine loss in a thin layer of water (at various FC/CYA ratios) compared to water with some depth and see if there is a difference (accounting for the small absorption of UV by water itself).

Richard