need final word on shocking/re-entering pool

[powiebaby]

OK, I essentially got the answers I was looking for from a guy at our local mom and pop pool store. He told me a couple of things. 1. when you shock, initially the Cl goes up, but as it interacts with the water and the nasties within, it comes down to acceptable levels after a short period of time. The regular shock that you put in during the evening and run the filter overnight, the pool should be okay for swimming in the next day. The quick shock that you can swim in after 15 mins or so, it's just that. And for comparative measure, he told me that your typical large public pool, like a YMCA pool or hotel pool, has a much greater level of Cl to account for the large number of bathers. He told me it's typically around 7-10ppm, if not more. And we've had our kids in those pools often. So I think that I should be okay with sending the kids into the pool even when the Cl reading beyond 4.0 on the test kit, provided I follow the directions on the particular type of shock that I use. I'll still aim for the ideal reading between 1-3, but won't fret if the Cl registers 4.0 or above at 10am on a hot, sunny Saturday.

[KurtV]

... I'll still aim for the ideal reading between 1-3, but won't fret if the Cl registers 4.0 or above at 10am on a hot, sunny Saturday.

As others have posted, knowing the free chlorine level without knowing the CYA level is essentially useless. CYA has a great deal of bearing on how the chlorine will effect bathers, liners, swimsuits, etc. In your case, unless you have very little CYA (like less than 20 ppm), 1-3 ppm is far from ideal. In fact, it could be dangerous as you will have insufficient chlorine to keep your water free from algae let alone sanitary.

Post your CYA level and people here will be more than happy to give you advice that will serve you well.

[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