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Thread: Acceptable overnight FC loss and testing error margins

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    saraiks's Avatar
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    Jul 2007
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    Fremont, CA
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    Acceptable overnight FC loss and testing error margins

    While trying to test for FC loss overnight what should be an acceptable number ?

    I have tested to a 0 ppm to 2 ppm drop overnight without any other issues.

    Can this be considered within the error margin of the test ?
    16,000 G in ground Gunnite (plastered) with attached Spa, Gemini 3 NG Heater, Sta-rite cartridge filter, Aquabot Mark V, TF test kit

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    JasonLion's Avatar
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    May 2007
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    Silver Spring, MD
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    37,887
    The FAS-DPD test is +- 0.5 on each test. So a difference of 1 ppm is within the error range of the test and can be ignored. It isn't that difficult for there to be 1 ppm of chlorine lost to pollen or leaves falling in the pool. Both of those together could account for a difference of 2 ppm. Still, if watching FC overnight to decide when to stop shocking I would still go an extra day if the difference was more than 1 ppm. It is better to be sure you have gotten all the algae.
    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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