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Thread: Please help me understand FC/CYA

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    Whatrymes's Avatar
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    Please help me understand FC/CYA

    I've been reading up on CYA/chlorine relationships. Some of ChemGeeks works and some of the papers he recommends. I understand the basic ideas of what's going on but I'm still confused why high levels of FC are less harmful in the presence of CYA. I'm really struggling to accept the premise of this forum that such high levels of FC are fine. It's a constant pounding, the cry of conventional pool experts, that FC should be between 1 - 3 ppm and swimming should not be allowed when it is above 5 ppm. I keep getting ready to raise my levels but then I get squeamish again and adjust my water to be no higher than 4 ppm. At my level of CYA, I should keep FC levels between 3 - 7 ppm and a target of 5, but I'm afraid.

    Please explain (in layman's terms) why the belief is so different here. Does CYA actually make chlorine less potent? Less of an oxidizer? Why has conventional wisdom not caught up? I don't see how their views help profits any? There is much experience here of pools having FC levels above 5 and there is no harmful effects?

    Thanks for all your time,
    Ed
    15 X 30 X 3.6 Ester williams AGP
    10,800 gallons
    Haywood EC40C90 DE filter 1 HP pump
    TF-100 Test Kit

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    Re: Please help me understand FC/CYA

    Mind if I send you to a recent link? Respond on this one if you have questions so that you know what we are referring back to. Thanks!

    cya-chlorine-and-a-question-t34257.html
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    Whatrymes's Avatar
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    Re: Please help me understand FC/CYA

    Quote Originally Posted by RobbieH
    Mind if I send you to a recent link? Respond on this one if you have questions so that you know what we are referring back to. Thanks!

    cya-chlorine-and-a-question-t34257.html
    Thanks for the link. I read that one, and many others, but that is not the point of my question. Why are higher levels of FC now safe because of CYA. Conventional wisdom says 1 - 3, no more than 5 and CYA is never mentioned as a factor. Except when, of course, CYA gets so high that chlorine can't work.
    15 X 30 X 3.6 Ester williams AGP
    10,800 gallons
    Haywood EC40C90 DE filter 1 HP pump
    TF-100 Test Kit

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    Re: Please help me understand FC/CYA

    When cyanuric acid is in the water, chlorine forms a bond with it. The chlorine that is bound to the cyanuric acid is not very active at all.

    Chlorine will unbond from the cyanuric acid for very short periods. Only during the brief periods where the chlorine is not bound to the cyanuric acid will it be able to react.

    This makes the overall effect similar to having a very low level of chlorine (less than 1 ppm).

    At any particular instant, there is only a very small percentage of the chlorine in the form of hypochlorous acid, which is the active form of chlorine.

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    Re: Please help me understand FC/CYA

    Quote Originally Posted by Whatrymes
    Except when, of course, CYA gets so high that chlorine can't work.
    Either the CYA relationship is gradual or it is sudden. If it is gradual, then the "not working" progresses from working to not working well to not working at all. Or else it is not important up to some level, then it matters a lot after some threshold.

    I doubt that there is a threshold level, fine up to this and terrible afterward. More likely it is gradual, working less and less well as concentrations increase.

    The articles I've seen here point to research that says that even up to very high levels of CYA, chlorine can still kill pathogens in a certain period of time. That time period is rather long at high CYA levels. Fine if you clear the pool when any event happens like a fecal accident but not so good if there is snot floating in the pool unseen.
    23,000 gallon in ground pool with rock waterfall and spillover spa, Aqualink control system, Polaris 380 cleaner, Purex Triton Clean&Clear Plus cartridge filter. Located in The Woodlands, Texas.

    Pool owner since Nov 2008, Trouble Free since April 2009. Happy to help when I can.

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