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Thread: Does high FC effect an accurate CYA reading?

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    DonW's Avatar
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    Jun 2008
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    Central IL.
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    Does high FC effect an accurate CYA reading?

    I don't know if this is the correct forum for this question, forgive me if it's not. I'm in the middle of my 2nd bout with mustard algea and before I started my CYA level was all over the place, from 42 on July 25, 55-60 on Aug. 17 to 60 -65 this past Friday when I started to shock.
    I settled on shock value of 35 -40 ppm according to Chemgeek's Mustard shock chart. I just checked my FC at 1:30p and it was 41 ppm. On a whim I decided to check my CYA level, and according to the tube, I have 20ppm or LESS. I could still see the dot. Does high FC effect an accurate CYA reading?
    This second bout of mustard algae is kicking my butt. I threw in the towel and mustard algae kicked it back out. I've thrown 51 gallons of 6% bleach at it in going on my 4th day. I've called in sick once for work, and if need be will do it again. I'm pretty disheartened. I let my FC level fall below 3 once. Evidently that's all it takes. Just when you start thinking, "hey the pool's looking pretty good. I really haven't used that much bleach this year either".
    18' x 33' AGP
    16,715 gal.
    200 lb. Sand Filter w/Hayward PowerFlo MATRIX 1.5 H.P. 2 Spd. pump
    Stenner Injection Pump System

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    JasonLion's Avatar
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    May 2007
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    Re: Does high FC effect an accurate CYA reading?

    That sounds like testing error of some kind to me. High FC levels don't have any significant effect on CYA test results. CYA can go down over time, but it can't go up unless you add some.
    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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    DonW's Avatar
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    Jun 2008
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    Central IL.
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    Re: Does high FC effect an accurate CYA reading?

    I don't doubt it's an error, I've made a few. But my measurements are exact, each and every time. The only variance in the test is the lighting conditions. From overcast to sunny. I guess from now on I'll have to test in the bathroom. No windows and the lighting is constant. Thanks Jason for your reply.
    18' x 33' AGP
    16,715 gal.
    200 lb. Sand Filter w/Hayward PowerFlo MATRIX 1.5 H.P. 2 Spd. pump
    Stenner Injection Pump System

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