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Thread: 5ml FC test verses 10ml

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    wjr75's Avatar
    Join Date
    Jun 2013

    5ml FC test verses 10ml

    I know this has been debated before but just want to get a clearer answer in my mind. Right now I have a CYA of 70ppm and keep my FC at a 8ppm target (Use the TF-100). I want to use less of the regent so instead of 10ml I want to use 5ml of water sample. Can I use 5ml and count each drop as 1ppm? I tested until it was slightly pink still and figured the reading was slightly highier than that value. The next drop turned it clear. Thanks in advance!
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    Join Date
    Jul 2013

    Re: 5ml FC test verses 10ml

    I'm no expert but I believe that's fine as long as you're not doing an overnight test to check for loss of FC
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    Re: 5ml FC test verses 10ml

    Yes, you can. Obviously, it's not as precise. I wouldn't try to guess intermediate, slightly pink values, though. If it turns clear at drop 8, then read it as 8 ppm. If it turns clear at 9, then 9 ppm. If you want something more granular, go back to a larger sample.
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    Patrick_B's Avatar
    Join Date
    Jun 2011
    Midland TX

    Re: 5ml FC test verses 10ml

    You can run a 5ml in a pinch, but you lose accuracy. I would not suggest it, unless you are waiting on new reagent. A reduced sample size always sacrifices too much for a long term solution.
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    Join Date
    Mar 2007
    San Rafael, CA USA

    Re: 5ml FC test verses 10ml

    The accuracy loss comes from the +/- 1 drop and from the estimate of the water volume. The one drop goes from 0.5 ppm FC to 1 ppm FC. An error of, say, 1 ml out of 5 ml is 20% error while out of 10 ml it is a 10% error. So for a given FC level, the proportional error from volume measurement of the sample is doubled and the absolute error from drop size relative to sample size is also doubled. When measuring 8 ppm FC, the error goes from 8*0.1 + 0.5 = 1.3 ppm to 8*0.2 + 1.0 = 2.6. So it is still linearly proportional in error.
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