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Thread: pH test discrepancy

  1. Back To Top    #1

    pH test discrepancy

    My pool water pH tests as 7.5 with the Taylor phenol red indicator kit or 7.6 with a similar BioGuard kit; however, the same water sample shows a pH of 7.0 when tested with a glass pH electrode and well calibrated pH meter from my research lab. Any idea why this discrepancy is occurring?

    Other test results for the pool water are:
    Free Cl 5.0 ppm
    Combined Cl 0 ppm
    Total alkalinity 110
    Ca hardness 180
    Borates 45 ppm
    CYA 30 ppm

    The pool is a 25K vinyl-lined pool in the southeastern US. Thanks for any insights!!
    25K gallon vinyl in ground pool, sand filter

  2. Back To Top    #2
    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: pH test discrepancy

    Welcome to TFP!

    I've definitely seen some variation between a glass bulb PH sensor and the Taylor PH test, with the difference increasing as FC levels rise. However, I have only seen differences as large as what you describe when the FC level is approaching 20 (give or take).

    What test did you use to measure your FC level? Some of the common FC tests will show FC levels well above 5 as if they were 5. I would also be interested to see how much variation you see when the FC level is near zero. You could take a water sample and leave it in the sun for a couple of days to get the FC level down to zero and then measure it both ways.

    All of the swimming pool PH tests include something to neutralize chlorine, so that it won't affect the phenol red. However the chlorine neutralizing process is never completely PH neutral. The more chlorine there is, the more the neutralization process will drive up the PH. The Taylor reagent does a better job at minimizing this PH change than the other brands I've seen, but they can't completely eliminate it.
    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

  3. Back To Top    #3

    Re: pH test discrepancy

    Hi Jason,
    Thanks for the welcome and for the information!

    I used the TF-100 FAS-DPD test kit to measure the free chlorine the morning that I tested the pH with the kits and pH meter. That evening, after a full day sunshine on the pool, the free chlorine was 2.0 ppm. The pH still tested at 7.5 with the Taylor kit, but I did not retest with the pH meter.

    As you suggest, I will set a water sample aside for a few days and test again with the kits and pH meter after the free chlorine drops near 0.

    Thanks again.
    25K gallon vinyl in ground pool, sand filter

  4. Back To Top    #4

    Join Date
    Oct 2007
    Valrico, FL

    Re: pH test discrepancy

    I routinely use a glass electronic pH meter to measure the pH in my pool. Every once in a while I will also measure the pH useing the pH test in the TFT100. Readings come out as close as you can read the TFT100 test. I normally run around 5 ppm FC in my pool.
    7,500 gal, IG pool, L shape 22' x 15', 1.5 hp pump, cartridge filter, AquaPlus SWG/Controller, Pebble-Tec liner.

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