need final word on shocking/re-entering pool

[chem geek]

The "quick shock" your pool store guy is referring to isn't chlorine, it's non-chlorine shock (potassium monopersulfate) and they recommend waiting 30 minutes before going in the pool after use (it used to be 15 minutes, but they changed that). Anyway, as for chlorine, with an outdoor pool with Cyanuric Acid (CYA) in it, the disinfecting chlorine level that actually does the oxidation of organics, killing of bacteria and algae, etc. is very very low. A pool with 10 ppm Free Chlorine (FC) and 30 ppm CYA has the equivalent disinfecting chlorine level of 0.45 ppm FC with no CYA so is less than found in most indoor pools.

He is wrong about chlorine dropping quickly after shocking. It all depends on whether there is a lot of algae or other substances for chlorine to combine with and you can test the chlorine drop yourself to see. It is true, however, that chlorine will drop quite a bit during the day in direct sunlight, but that happens whether you have something for chlorine to combine with or not.

At any rate, yes, it is safe to swim after a modest shock, but if there are organics to oxidize, then they may smell briefly until sunlight breaks them down so I'd wait a half hour or so right after shocking before going in.

Richard

[AquaBill]

Richard:
How is it that "a pool with 10 ppm Free Chlorine (FC) and 30 ppm CYA has the equivalent disinfecting chlorine level of 0.45 ppm FC with no CYA"? Is there a typo in that? That is a ratio of over 20 times. Looking at Ben's "best guess" chart below I would have thought that 10 ppm FC at 30 CYA would have been equivalent to somewhere around 5 ppm FC at no CYA.

Stabilizer . . . . . . Min. FC . . . . Max FC . . . 'Shock' FC
=> 0 ppm . . . . . . . 1 ppm . . . . . 3 ppm . . . . 10 ppm
=> 10 - 20 ppm . . . . 2 ppm . . . . . 5 ppm . . . . 12 ppm
=> 30 - 50 ppm . . . . 3 ppm . . . . . 6 ppm . . . . 15 ppm

..Bill