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Thread: Common myth re: pool chemistry- not sure I understand this?!

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    watersplash's Avatar
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    Common myth re: pool chemistry- not sure I understand this?!

    I don't fully understand the explanation for myth #11 from this article: http://ppoa.org/?page_id=418

    11. When a DPD chlorine test flashes a little pink then turns crystal clear, your water has almost no chlorine in it.

    The opposite is true! More spas are fried and bathing suits bleached because of this error than just about any other. As you know, the universally accepted DPD test for chlorine turns progressively more pink as higher residuals are encountered in the sample… that is until the indicator is itself reverts to a clear resultant. This phenomenon occurs somewhere near 20 ppm chlorine, the actual value varying with different pH or total alkalinity conditions. Many, many untrained operators or “helpers” have dumped excessive amounts of chlorine in their water (most often in hot whirlpools or spas), thinking all along that for some crazy reason they just hadn’t yet put enough in to get a reading! In a small body of water this testing error has resulted in a few hundred parts per million chlorine residual, creating some very unsafe, damaging or at least unpleasant conditions.

    While my pool is stable, now, as I set up my pool 2 weeks ago I had to add excessive amounts of chlorine to keep my pool at the proper levels. At first, I attributed the disappearing chlorine to the really hot days without a pool cover and not enough cya and then thought maybe I had an algae issue that was lurking in the pump/equipment. Since the pool was taken out of storage and reset up with fresh water I don't even know if that is possible. I added chemicals approx. 4 or 5 hours after the pool finished filling. Now after reading the above information which I don't quite understand, I wonder, if somehow, I had high levels of chlorine even though the pool water in the vial would stay clear after I added the dpd testing powder. (I should mention that the oto test was also showing clear!) I, also, noticed a pair of shorts I swam in that were black faded to grey really fast. ( Although these are not meant for swimming and might not be very colorfast.) Is the article referring to the same test using the dpd testing powder? I use the Taylor R-0870 powder, then if it turns pink I use the R-0871 FAS-DPD titrating reagent and then I use the R-0003 DPD reagent and if it turns pink then I use the R-0871 FAS-DPD titrating reagent once more.

    What do you more knowledgeable fellow pool lovers think?
    6450 gallons
    Cartridge filter
    Above ground round metal frame quick set - 18'Diameter, 4'High
    Polygroup SFS1000 filter system w/ F1000c pump and in-wall skimmer - 1075 GPH
    2nd year - re-assembled 5/30/13 and finish filling on 6/1/13

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    linen's Avatar
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    Re: Common myth re: pool chemistry- not sure I understand th

    Don't confuse the DPD chlorine test with the FAS-DPD chlorine test (the one you have). The myth is only for the DPD chlorine test.
    TFP Expert who uses Pool School and my TF100 test kit along with PoolMath for my: Round 11K gallon AGP with deep end, 20" sand filter, Matrix 1hp 2spd, 6 2ftX20ft solar panels (and solar cover!), Intex SWCG (copper bars disconnected) and a Rubadub hot tub (chlorine). The SLAM process is not finished until: 1. CC < 0.5 ppm, 2. An OCLT < 1.0 ppm and, 3. The water is crystal clear.

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    watersplash's Avatar
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    Re: Common myth re: pool chemistry- not sure I understand th

    Ah, I had thought I had heard of a DPD test. Since I've never used that one I didn't know if it used the powder which is just marked DPD powder. It's good to know that my test won't show clear when there is high levels of chlorine. I was worried that I had my family swim in high levels of chlorine! Thanks for clearing that up and so quickly, too.
    6450 gallons
    Cartridge filter
    Above ground round metal frame quick set - 18'Diameter, 4'High
    Polygroup SFS1000 filter system w/ F1000c pump and in-wall skimmer - 1075 GPH
    2nd year - re-assembled 5/30/13 and finish filling on 6/1/13

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    Re: Common myth re: pool chemistry- not sure I understand th

    FAS-DPD is accurate to 50 ppm
    18x36 inground vinyl lined, Hayward sand filter and tablet chrolinator, approx 18k gallon

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    JasonLion's Avatar
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    Re: Common myth re: pool chemistry- not sure I understand th

    The DPD test can bleach (go clear) out at mildly high FC levels, typically anything above 10 ppm will do it. When this happens there will be a brief flash of pink as you are adding the DPD reagent.

    The FAS-DPD test can also do something related, but at far higher FC levels. When FC is well above 20 and you don't use enough R-0870 powder the sample can turn pink right when you add the powder, but then go clear again. When that happens you need to add more powder.

    The OTO chlorine test is the most reliable for determining chlorine vs no chlorine. If the sample stays clear with the OTO test there really isn't any chlorine.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    watersplash's Avatar
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    Re: Common myth re: pool chemistry- not sure I understand th

    Thank you, both, for clearing that up. I so appreciate the fast responses and the detail in your answer, Jason, really helps me understand what to watch for when I test. The people on this forum are awesome!
    6450 gallons
    Cartridge filter
    Above ground round metal frame quick set - 18'Diameter, 4'High
    Polygroup SFS1000 filter system w/ F1000c pump and in-wall skimmer - 1075 GPH
    2nd year - re-assembled 5/30/13 and finish filling on 6/1/13

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