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Thread: OTO Accuracy and Difference from FAS-DPD

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    OTO Accuracy and Difference from FAS-DPD

    Hi Everyone,

    Got a few more questions that you folks will likely be able to shed light on. I have to the pool in a good balance. I have been testing TC with OTO daily during the week and then once a week I do a FAS-DPD for FC and CC. On the days I do FAS-DPD, I also do a OTO to compare. This is where the mystery starts. I notice that OTO is consistently lower than the FC from the FAS-DPD. Yes, I realise that OTO is testing TC and FAS-DPD is testing FC. However, the OTO results should be the same or higher than FAS-DPD. Now, I have read the extended instructions (thanks Jason) and noticed that OTO is not accurate above 2ppm (> 1ppm). But there seems to be a consistent low bias versus the FAS-DPD.

    My other issue is one of color matching. I use the OTO to test during the week because of the convenience factor. The pool is in balance so testing with FAS-DPD once a week to test both FC and CC and using OTO to keep tabs during the week seems logical to me. I run a CYA level of 60 and as such my target FC level is ~4. The 3-4 range is precisely the two colors on the OTO that are nearly identical. It is so difficult to tell the difference. Given that your target chlorine level is so critical, I am hesitant to use the OTO for an extended period without FAS-DPD

    BTW - I use a Taylor K-1000 for OTO and TF-100 for FAS-DPD

    Okay here is goes

    1) Am I crazy, or do others find that OTO always comes in lower than FC in FAS-DPD??

    2) I want an easy way to keep tabs in chlorine daily, and keep FAS-DPD for periodic checks. Are there better ways than OTO (or the K-1000)?

    ~18.5K gallon plaster - two level / spa to pool waterfall
    Pentair WhisperFlo WFE-28 Pump, Pentair Clean and Clear Filter
    Hayward Pool Vac Ultra Cleaner

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    Mod Squad jblizzle's Avatar
    Join Date
    May 2010
    Tucson, AZ

    Re: OTO Accuracy and Difference from FAS-DPD

    1 ... I have no idea
    2 ... A DPD test could be used for quick FC check up to 5ppm ... although it again is a color matching exercise and quickly looses accuracy. The FAS-DPD is the best test.

    Do you have a SWG? (adding pool info in your signature would help).
    For a CYA of 60ppm, the MINIMUM for FC is 5ppm (without a SWG) or 3ppm (with a SWG). Just wanted to make sure you were staying ABOVE these.
    Jason, TFP Moderator
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    Re: OTO Accuracy and Difference from FAS-DPD

    Yep, I have a SWG so my target FC is lower that 5ppm (4 according to the chart in pool school). Although, I am just considering keeping it at 5ppm anyway. This may help since the color scale seem to distinguish 5ppm much better.

    I'll work on the signature. That is a good idea. Thanks
    ~18.5K gallon plaster - two level / spa to pool waterfall
    Pentair WhisperFlo WFE-28 Pump, Pentair Clean and Clear Filter
    Hayward Pool Vac Ultra Cleaner

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    Richard320's Avatar
    Join Date
    Jan 2010
    San Dimas, CA (LA County)

    Re: OTO Accuracy and Difference from FAS-DPD

    Color matching is highly subjective.

    I use the OTO test just because I have the reagent, and I like to see if I'm getting any better at distinguishing the shades of yellow. The answer is no.

    What I think I'm reading can vary quite a bit depending on the light and the background and how far away I hold it. To me, 4 to 7 all looks like 5.
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
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    CUTiger78's Avatar
    Join Date
    May 2009

    Re: OTO Accuracy and Difference from FAS-DPD


    A bit off-topic, but are you the Tallahassee weather man?

    Just curious, from your username.

    36K gallon 42X22 gunite/plaster kidney-shaped IG;
    1 hp Hayward Super Pump (new in 2012);
    Hayward S244T sand filter w/ Zeosand (both new in 2009);
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    JasonLion's Avatar
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    May 2007
    Silver Spring, MD

    Re: OTO Accuracy and Difference from FAS-DPD

    I've never seen any problem with accuracy around 0-1.5. Above that things get kind of vague and anything 3-12 looks exactly the same to me.

    The way I use OTO is for a quick check to see if FC is 3+ or near zero. If I actually want to know a number then I use FAS-DPD. Once every week or two, or any time I suspect extra chlorine demand, I use FAS-DPD to get a read on how much chlorine I should be adding each day. The rest of the time I use OTO just to make sure nothing has gone terribly wrong (i.e. to check that chlorine isn't heading rapidly towards zero).
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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  7. Back To Top    #7

    Re: OTO Accuracy and Difference from FAS-DPD

    @ Jason and Richard - Thanks a ton. That is exactly the type of info I was looking for.

    @CUTiger78 - Very perceptive. You nailed it. Go NOLES!!
    ~18.5K gallon plaster - two level / spa to pool waterfall
    Pentair WhisperFlo WFE-28 Pump, Pentair Clean and Clear Filter
    Hayward Pool Vac Ultra Cleaner

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    frogabog's Avatar
    Join Date
    Jul 2010
    Portland, Oregon

    Re: OTO Accuracy and Difference from FAS-DPD

    I find that the FAS-DPD is considerably easier to do every day and the reagents aren't that expensive that I'd choose OTO for monetary reasons. The OTO is just a PITA in my opinion. (I was gonna do an IMO there, but that was one acronym too many for one sentence, IMO - oh I'm crackin myself up now... stop that! )

    Anyway, it's much harder to match colors, always a guess and then ask someone else to look to see what they think... blech! The FAS-DPD gives one a sure number every time and it's easy. 'nuff said...
    Where kids swim in 54 degree water, turn blue, and giggle happily cuz they got a POOL!
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