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Thread: Test Kit inaccuracies ?

  1. Back To Top    #1

    Join Date
    May 2016
    Los Altos, CA

    Test Kit inaccuracies ?

    Hi, I wonder if you guys can shed some light on a question or two that I have please ?

    I have been using the dichlor / bleach method for a couple of months in my SPA. The numbers are pretty good and things act as I expect. I have a Taylor K-2006, but I also have a Taylor K-10XX kit. The K-2006 uses DPD-FAS, the K-10XX uses the DPD method. In many posts I see here, everyone says the DPD-FAS method is better, but I have not seen a good description of why. Is it just that it's more accurate or is it because you can easily get a CC reading ?

    I would expect both kits to give similar reading for FC, but the K-2006 always reads higher. Once when I read FC=2 with the DPD kit, the DPD-FAS kit read FC=5. Another time, when the DPD read FC=5, the DPD-FAS read FC=8. It often seems to be about 50% higher. I am doing the 0.5ppm version of the test and diving the number of drops by 2, with the DPD-FAS test.

    Anyway, because of this, I changed the reagent in the K-2006 for a new bottle, but the new one was exactly the same as the old. So then I changed the reagent #2 in the DPD test and the old and new reagents there gave the same results too.

    So, I'm wondering why the DPD test reads a quite different reading from the DPD-FAS test. Can anybody explain that please ? Do I need to go get a control sample to measure to calibrate the test (seems weird to have to do that) ?

    Any advice ?



    Jaccuzzi J-375 SPA

  2. Back To Top    #2

    TFP Guide

    Join Date
    Jul 2015
    Central Valley CA

    Re: Test Kit inaccuracies ?

    The FAS-DPD is prefered for a few reasons 1.) more accurate since you are adding drops until you get a reaction (sample goes clear ) Vs a comparator that can be misinterpreted 2.) can test higher ppm than DPD and lastly can get down to .2 ppm if you have a need for that granular of a result. You said "I am doing the 0.5ppm version of the test and diving the number of drops by 2, with the DPD-FAS test" If I am interpreting what you wrote correctly that could be your problem. for a .5 ppm test you should be using 10 ml of sample water 1 scoop of DPD powder then the R-0871 1 drop at a time until the sample turns clear. multiply the drops needed to clear the sample by .5 as a example if it takes 5 drops that is 2.5 ppm FC

    never mind I am being dense, divide by 2 is same as multiply by .5.

    Have you tried the .2 ppm test with a 25 ml sample? I have also seen a suggestion to use slightly more power and be sure you have the dropper bottle completely inverted when adding.
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  3. Back To Top    #3
    Divin Dave's Avatar
    Join Date
    Oct 2013
    Longview, Texas

    Re: Test Kit inaccuracies ?

    the DPD reads total chlorine. FC+CC=TC.
    the FAS DPD reads FC only, which is the good kind of chlorine.
    You can even dilute the FAS DPD and get even more accurate if you wanted to, but usually there isnt any need to.

    As attech mentions, the DPD comparitor can be difficult to detect small differences in changes of color. I have a hard time with that thing.

    As a matter of general rule, the DPD is fine to use to get a general feeling of your FC level. Most of us just use that daily, or ever couple days, and then use the FAS DPD once a week to make sure all is ok. Im speaking of pools of course, I dont have a spa so my comment may not be so relevent.
    Divin Dave,
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  4. Back To Top    #4
    Mod Squad pooldv's Avatar
    Join Date
    Aug 2012
    DFW, TX

    Re: Test Kit inaccuracies ?

    Welcome to TFP!

    FAS-DPD is the best and most accurate test for chlorine. I have a hard time reading the pink gradations on the DPD comparator. I find that I can read it better in full sun with a white or light background, PH too. I do better with the yellow OTO chlorine test if you want a quicker test.
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