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Thread: Active Chlorine Formula

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    Donldson's Avatar
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    Active Chlorine Formula

    Does anybody have the formula for figuring out Active Chlorine level? I'm hoping for something that I can plug the FC, CYA, and temperature in and get a PPM reading for the active chlorine level.

    Thanks in advanced for humoring my obscure request
    JD - 28' Round Above Ground Pool, 17,000 Gallons. Dual speed Jacuzzi pump with cartridge filter. Dual speed 1 HP pump, Hayward S210T sand filter
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    Re: Active Chlorine Formula

    I'm not sure what you're asking for exists, or could exist in a formula that would be entirely accurate. It would have to calculate a factor for demand, and I don't know how that would work. Unless I misunderstand you....
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    Mod Squad jblizzle's Avatar
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    Re: Active Chlorine Formula

    Somewhere there is a link to a table that chem geek created that has the active level as a function of FC and CYA. I do not have it on my phone, but know I have posted it many times.
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    Re: Active Chlorine Formula

    We need Chem Geek to the rescue!
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    Re: Active Chlorine Formula

    Thanks guys. Jason, after taking "formula" out of the search term the link you mentioned was in the second link I clicked. http://troublefreepool.com/~richardfalk/pool/HOCl.htm

    This is pretty close to what I was hoping for. The idea is that when I am helping someone that isn't up for a full course on TFPC then I can use the term Active Chlorine and use a smaller number. Say their FC is 10 at 50 CYA, I can tell them their Active Chlorine level is 0.2. Not the best solution I know, but something that can help calm the nervous. The reason I was hoping for temperature is this chart is for 80F and I know spa temperatures will lead to higher numbers, but this is exactly where I need to start.
    JD - 28' Round Above Ground Pool, 17,000 Gallons. Dual speed Jacuzzi pump with cartridge filter. Dual speed 1 HP pump, Hayward S210T sand filter
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    Mod Squad pooldv's Avatar
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    Re: Active Chlorine Formula

    Yeah, people freak out when they are in my pool commenting on how clear the water is and how the water doesn't burn their eyes at all and they ask me what my chlorine level is or how it is so clear with no chlorine in it. And I tell them the chlorine is between 12 and 15 ppm because I cranked it up for the weekend pool party. Or even better I tell them it is 28 ppm because I am trying to kill some algae.
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    Re: Active Chlorine Formula

    In the Pool Water Chemistry thread that Richard wrote is a mathematical formula for the approximate HOCl as a function CYA. The table you linked to assumed a pH of 7.5. The actual HOCl level is pH dependent. If one wanted that table at a different pH, then you would have to use Richard's spreadsheet to get the exact values.

    See here - Pool Water Chemistry


    Last edited by JoyfulNoise; 01-29-2016 at 10:47 AM. Reason: Edited for clarity and added link
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    Re: Active Chlorine Formula

    Quote Originally Posted by JoyfulNoise View Post
    The actual HOCl level is pH dependent
    Doesn't the difference become negligible in the presence of CYA?
    JD - 28' Round Above Ground Pool, 17,000 Gallons. Dual speed Jacuzzi pump with cartridge filter. Dual speed 1 HP pump, Hayward S210T sand filter
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    Re: Active Chlorine Formula

    Quote Originally Posted by Donldson View Post
    Doesn't the difference become negligible in the presence of CYA?
    No. The combined [HOCl] + [OCl-] concentration is moderated by CYA but the ratio of [HOCl]/[OCl-] remains roughly the same. For example, at 0ppm CYA, 1ppm FC and a pH of 7.5, you would have ~ 0.5ppm each of [HOCl] and [OCl-]. At higher CYA, you'd have less overall [HOCl] + [OCl-] but the fractional split is roughly the same, 50/50. The only time you get mostly HOCl is when the pH is very low, below 7.0. If your pH is very high, above 8.0, then the dominant active chlorine species is hypochlorite (OCl-).


    The charts in post #2 of this thread - Pool Water Chemistry show the variation on a semi-log plot.
    Matt
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