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Thread: Question on FC levels

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    Question on FC levels

    I've read the pool school stuff a few times and checked out the recommended levels page as well.

    I just have a question on when its "safe" to use the pool. Most place (other than here) say not to use the pool if your FC level is above 4.0 ppm. According to the recommended levels and the CYA chart, the target FC level for most is above 4ppm.

    When shocking the pool to levels of 10+, when are you actually "safe" to use the pool again after shocking. What level do you need to let the FC get back down to before swimming and why are the target FC levels all above 4ppm if other places say not to swim in anything above 4ppm?
    18' round AG vinyl | avg. 3'9" deep | ~7,600 gallons
    Hayward Pro Series Sand Filter, Model S166T | Hayward 1hp Pump

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    dmanb2b's Avatar
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    Re: Question on FC levels

    most here agree anything below shock level is safe
    24'x52" AGP (13,500 Gallons), Intex SWG, (2)Solar Bear 4x20 panels, Hayward S220T Filter, 1/2hp Pentair Superflo

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    Re: Question on FC levels

    Quote Originally Posted by dmanb2b
    most here agree anything below shock level is safe
    that's what I've gathered from reading through all the posts here, but I guess my real question is why.

    Why do most here feel that anything below shock level is safe?

    And, if most agree that anything below shock level is safe, then why is there the general warning regarding not using a pool above 4ppm?
    18' round AG vinyl | avg. 3'9" deep | ~7,600 gallons
    Hayward Pro Series Sand Filter, Model S166T | Hayward 1hp Pump

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    reebok's Avatar
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    Re: Question on FC levels

    the pool industry does not subscribe to the belief that cya affects what level of chlorine is safe. their numbers are based on pools with 0cya like indoor pools. but the cya level does affect it because it's bound to some of the chlorine to protect it, which in turn makes that chlorine ineffective for the time being. (that last part is my understanding, I could be off)
    16x32 21,000 gallon in-ground exposed aggregate, 1.5hp pump, 120 sqft catridge filter, birdcage, solar panels, aquavac tigershark qc robot.

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    Re: Question on FC levels

    Reebok got the gist of it.

    With apologies to chemgeek....a 16ppm FC pool with say around 40ppm CYA has as much effective chlorine as a 1-2ppm FC pool with no CYA. That's not very exact at all....search chemgeek's posts for the true relationship.

    The point is that other sources of info do not take CYA buffering into account.

    We are lucky to have some of the very finest pool minds in the world who share their information with us here on the forum.

    That may sound a little over the top (and I am certainly not one of them) but if any of you know of any website that provides better, more accurate information about virtually any aspect of residential swimming pools, I'd sure like to visit that site.

    BBB - knowledge is power
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
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    Re: Question on FC levels

    thanks reebok and durleigh. that makes sense now when factoring the CYA into account
    18' round AG vinyl | avg. 3'9" deep | ~7,600 gallons
    Hayward Pro Series Sand Filter, Model S166T | Hayward 1hp Pump

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    Re: Question on FC levels

    reebok and I are rookies (sorry for putting you in my category, reebok) compared to some of the others who figure this stuff out.

    If Richard (chemgeek) sees this thread, I'm sure he'll follow up with a more accurate comparison of effective chlorine in CYA and non-CYA environments. It's eye-popping. (well, for me. anyway. I am very easily entertained )
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
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    reebok's Avatar
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    Re: Question on FC levels

    pfff...if you're a rookie then I haven't even been born yet.

    chlorine-cya-chart-t2346.html
    16x32 21,000 gallon in-ground exposed aggregate, 1.5hp pump, 120 sqft catridge filter, birdcage, solar panels, aquavac tigershark qc robot.

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    Re: Question on FC levels

    as a follow up, if a pool has 0 CYA in it, then should you wait until the FC is below 4.0 or still anything below shock level is fine ( less than ~10 )?
    18' round AG vinyl | avg. 3'9" deep | ~7,600 gallons
    Hayward Pro Series Sand Filter, Model S166T | Hayward 1hp Pump

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    reebok's Avatar
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    Re: Question on FC levels

    you added cya right? just because it doesn't show up on a test doesn't mean it's not in there. the wait a week to test is advice so you don't waste reagent and also don't add more cya thinking that you didn't add enough. JasonLion says assume it is in there after 24 hours for shocking purposes. whether that applies to swimming, I'm not sure. I can tell you if your cya is truly 0 you'll probably be looking at some bleached swim trunks at 10ppm. I would go naked instead.
    16x32 21,000 gallon in-ground exposed aggregate, 1.5hp pump, 120 sqft catridge filter, birdcage, solar panels, aquavac tigershark qc robot.

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    Re: Question on FC levels

    well, before I got my TF100 test kit, the pool store test said my CYA was 20.

    When I got my TF100 test, my CTA registered as 0 - meaning I was able to fill the tube completely to the top and the black dot at the bottom of the tube never went away.

    I haven't added any CYA yet since I'm not sure what my real reading is or how much I should add. I do have 4 lbs. of the CYA powder, but I haven't added any yet.
    18' round AG vinyl | avg. 3'9" deep | ~7,600 gallons
    Hayward Pro Series Sand Filter, Model S166T | Hayward 1hp Pump

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    Re: Question on FC levels

    Trust your test results and use the pool calculator to tell you how much to add. if you tested zero, you have zero.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
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    Re: Question on FC levels

    ok, I'll go whip out an old sock and start adding some CYA
    18' round AG vinyl | avg. 3'9" deep | ~7,600 gallons
    Hayward Pro Series Sand Filter, Model S166T | Hayward 1hp Pump

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    frustratedpoolmom's Avatar
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    Re: Question on FC levels

    I would add 2-3 lbs, then wait a week, retest and then add more if you need to.
    Helpful links: Pool School; CYA/Chlorine Chart
    24' round AG pool, 52" high, Raypak heater; Waterway 2 spd Pump;
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    Re: Question on FC levels

    Quote Originally Posted by frustratedpoolmom
    I would add 2-3 lbs, then wait a week, retest and then add more if you need to.

    You don't have to wait a week to go swimming. just wait on testing etc.
    18ft x 48in Round Intex Ultra frame. 6,700 gallons.

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    Re: Question on FC levels

    Quote Originally Posted by cpjolicoeur
    as a follow up, if a pool has 0 CYA in it, then should you wait until the FC is below 4.0 or still anything below shock level is fine ( less than ~10 )?
    Yes, if you don't have CYA in the pool then an FC of more than 4 ppm would be quite high. Even 1 ppm FC would be over 10 times higher in "active" chlorine (hypochlorous acid) concentration than normal levels recommended on this forum (which is roughly equivalent to 0.1 ppm FC with no CYA). Shock levels are roughly equivalent to an FC of around 0.6 ppm with no CYA -- still lower than found in most indoor pools that don't have CYA in them.

    I write more about the technical details and where the 4 ppm FC comes from (the EPA) and why here plus a link to the scientific literature that definitively determined the chlorine/CYA relationship in 1974 and links to numerous scientific papers that show the drastic reduction in chlorine's strength when CYA is present. The only relevance of FC separate from CYA in terms of safety would be if you drink a lot of pool water every day since in that case the reaction rate isn't as important as the total chlorine exposure.

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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