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Thread: OTO Chlorine Test vs FAS-DPD Chlorine Test

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    Join Date
    Aug 2014
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    Saint Albans, Vermont
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    OTO Chlorine Test vs FAS-DPD Chlorine Test

    I have the TF-100 test kit which has both the OTO Chlorine Test & the FAS-DPD Chlorine Test. I use the OTO test daily to monitor chlorine and PH and the FAS-DPD test about weekly or so. Lately I've noticed the OTO chlorine test reading starts to creep up 1 or 2 ppm after the sample has set a few minutes.

    From the notes in the Extended Test Kit Directions this is said to be an indication of the presents of CC. ("This test measures TC, but if you watch closely it is possible to get a sense of the presence of CC. When you first add the drops of R-0600, the sample will immediately show the FC level. Then, over the next minute or so, it will drift up to the TC level. If you see the color changing, CC is present. Using the test in this way requires careful attention and a good ability to distinguish shades of yellow quickly.")

    However when I use the FAS-DPD Chlorine Test I get a CC reading of zero. After getting the FC results and Adding 5 drops of R-0003 the sample remains clear indicating the CC level is zero.

    I don't think I was seeing this last season with the OTO test. Could it be that my reagents have gone bad? They are the original reagents that came with the test kit purchased around the 1st of August of last year. If not that, any ideas of what may cause the different reading between these two tests, and which one should I believe?

    Thanks,
    Dave.
    Dave
    18x36 ~25k gal, IG vinyl, Hayward Super pump VS (IS2600VSP), Hayward sand filter S244T, Hayward CL200 auto chlorine feeder, Zodiac G3 cleaner, Pool install July 2014, TF-100 test kit.

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    Propbndr's Avatar
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    Mar 2015
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    Charles City, Va
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    Re: OTO Chlorine Test vs FAS-DPD Chlorine Test

    Last year, I had a different test kit with instructions that explained that the immediate color change is the FC and the color after 5 minutes is the TC.

    With the FAS-DPD test after adding reagent 003, sometimes I also notice that the sample starts to turn a slight pink again after a few minutes (anywhere from 2 to 5 minutes) of adding the final drop of R-871. The color disappears if another drop of R-871 is added. It does not happen all the time. What I wonder is does this mean the "final, final drop" of reagent should be added to get the actual CC.
    Steve
    32 X 16 inground, 17260 gallons, Vinyl Liner, Triton II TR-60 filter,
    Sta-Rite 1hp pump Model #MPRA6E-147L, CircuPool SI-30 Plus SWG, TF-100 Test Kit

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    ship of fools's Avatar
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    Nov 2011
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    Albany, NY
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    495

    Re: OTO Chlorine Test vs FAS-DPD Chlorine Test

    Quote Originally Posted by vtdhc View Post
    I have the TF-100 test kit which has both the OTO Chlorine Test & the FAS-DPD Chlorine Test. I use the OTO test daily to monitor chlorine and PH and the FAS-DPD test about weekly or so. Lately I've noticed the OTO chlorine test reading starts to creep up 1 or 2 ppm after the sample has set a few minutes.

    From the notes in the Extended Test Kit Directions this is said to be an indication of the presents of CC. ("This test measures TC, but if you watch closely it is possible to get a sense of the presence of CC. When you first add the drops of R-0600, the sample will immediately show the FC level. Then, over the next minute or so, it will drift up to the TC level. If you see the color changing, CC is present. Using the test in this way requires careful attention and a good ability to distinguish shades of yellow quickly.")

    However when I use the FAS-DPD Chlorine Test I get a CC reading of zero. After getting the FC results and Adding 5 drops of R-0003 the sample remains clear indicating the CC level is zero.

    I don't think I was seeing this last season with the OTO test. Could it be that my reagents have gone bad? They are the original reagents that came with the test kit purchased around the 1st of August of last year. If not that, any ideas of what may cause the different reading between these two tests, and which one should I believe?

    Thanks,
    Dave.
    My OTO test results have never been accurate so i always do the FAS-DPD test. It only takes a minute anyway and is far more accurate.
    21,000 gallon inground
    Haywood Sand Filter
    Vinyl Liner

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    JasonLion's Avatar
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    May 2007
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    Silver Spring, MD
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    37,879

    Re: OTO Chlorine Test vs FAS-DPD Chlorine Test

    The OTO test is simply not at all precise, while the FAS-DPD test is quite precise. You should always trust the FAS-DPD results over the OTO test with only one exception. If FAS-DPD reads zero, and OTO reads non-zero, then FAS-DPD is wrong (almost always because you used too little R-0870 powder). Other than that, the FAS-DPD results are always much more reliable.

    FAS-DPD will always turn pink if you leave it sitting for a while. This is normal and does not mean anything.

    The OTO color shift is never a particularly reliable indicator. In some cases the color can change for other reasons besides the presence of CC. Not to mention that distinguishing shades of yellow above 2 ppm is nearly impossible.
    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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    Join Date
    Aug 2014
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    Saint Albans, Vermont
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    19

    Re: OTO Chlorine Test vs FAS-DPD Chlorine Test

    Thanks for the responses. I considered the FAS-DPD test to be the most reliable test and your answers gave me the reassurance I was looking for.

    Dave.
    Dave
    18x36 ~25k gal, IG vinyl, Hayward Super pump VS (IS2600VSP), Hayward sand filter S244T, Hayward CL200 auto chlorine feeder, Zodiac G3 cleaner, Pool install July 2014, TF-100 test kit.

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