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Thread: Timing of OCLT for FC

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    Timing of OCLT for FC

    The Pool School says to perform first at night, 30 minutes after sundown. Is the point to perform the test when there's been no direct sunlight on the pool for 30 minutes? I have a tree line that puts my pool in shade about 4 PM. Could I not do the evening side of the OCLT at 4:30PM??
    14,000G Gunite pool, 1-1/2 HP Challenger Pump + 1HP Booster Pump, Waterfall feature, Pentair 60 DE Filter, Polaris 360 Pool Sweep
    Manassas, VA

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    duraleigh's Avatar
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    Re: Timing of OCLT for FC

    Welcome to the forum

    Keeping your posts in one thread will get you better answers and help us better understand your questions.

    The purpose of the OCLT is to determine if you have properly shocked the pool. So, when you pool water is crystal clear, perform the FAS/DPD test that evening. Note your FC and CC results. CC's need to be .5 or less. If they are not, you need to maintain FC at chock value and you can stop the OCLT.

    Let's say, however, your water is clear and your CC's are .5ppm or less....you have met 2 odf the three criteria.

    Now you want to see how much chlorine you will lose during the hours of darkness. By eliminating loss of FC from the sun, any remaining loss can be attributed to algae still in your pool. Loss of 1ppm or greater (during the hours of darkness) of FC is too much so you fail the OCLT and should continue to maintain shock level.

    Or, more simply, the purpose of the OCLT is to measure the FC loss during dark hours.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    JasonLion's Avatar
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    Re: Timing of OCLT for FC

    Indirect sunlight in the early evening can still lower the FC level, so you might get a false indication of algae when there really isn't any if you do the first test too early. Usually this isn't a big deal, and if you still pass the test then you are fine. But if you fail the test, it won't be completely obvious why as it could still have been late afternoon indirect sunlight.
    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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    Re: Timing of OCLT for FC

    Thanks for the answers. Based on the topology of my site, I can do the evening test at least an hour before sundown, and there won't be any sunlight of any kind --- direct or indirect --- to distort results.
    14,000G Gunite pool, 1-1/2 HP Challenger Pump + 1HP Booster Pump, Waterfall feature, Pentair 60 DE Filter, Polaris 360 Pool Sweep
    Manassas, VA

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