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Thread: FAS DPD test

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    FAS DPD test

    I'm curious to know if anyone has ever gotten a false positive on the FAS DPD test? I have bought 3 different pool test strips this year and none of the strips register any free chlorine in my pool. The FAS DPD test has consistently measured free chlorine. I'm doing the bucket test to see how much chlorine I would have to add in order to see a free chlorine results on the FAS DPD test because I do know that once enough is added it will show up on the strip. I've read some articles online that say that high combined chlorine can interfere with the DPD chemistry and show up as free chlorine. Any thoughts? My water has been dull all year. I closed the pool last year with extremely high CYA and opened to none. By the way I know that no one likes strips and the strips aren't expired either. I'm looking for someone with insight to this issue not a knee jerk reaction. Thanks.

    drop test strip test
    FC 5 0
    CC 0 5
    TA 240
    CH 250
    CYA 30

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    JasonLion's Avatar
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    Re: FAS DPD test

    It is possible, though very very rare, to get false positives (or negatives) on the FAS-DPD test. The OTO chlorine test is the most reliable for chlorine/no-chlorine determination.

    CC can "bleed" through into the FC portion of the FAS-DPD test, but only if you wait too long after mixing in the powder. The higher the CC level the faster it can bleed through, though I've never seen it happen more quickly than 30 seconds, and usually it is minutes before there is any bleed through.

    Test strips can easily bleach out when FC levels are very high, showing a zero reading, when FC is above 10 (sometimes 15 depending on brand). Whatever is actually happening it is dramatically more likely that the problem is with the test strips, which can also be interfered with due to PH out of range and a couple of other factors.
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    Mod Squad jblizzle's Avatar
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    Re: FAS DPD test

    If you have the FAS-DPD, why in the world are you messing with inaccurate test strips? ... ok that is out of the way.

    The FAS-DPD is just about as fool proof as it gets. You are correct though that at high FC levels the DPD test (pink color matching) can get bleached out ... BUT ... that test is completely different than the FAS-DPD test that we recommend.

    To confirm, you are adding a powder to you water and adding drops until it goes clear to test the FC and then adding 5 drops of the R-0003 to see if it turns pink again and then use the R-0871 drop to clear it for the CC ... right? That is the FAS-DPD test.

    And not, adding 5 drops to then try to match a shade of pink which is the DPD test.
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    frustratedpoolmom's Avatar
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    Re: FAS DPD test

    I have seen the CC bleed through very quickly, within seconds - when dealing with ammonia causing the high CC.
    I would suggest buying an ammonia test kit from the pool store for around $9 instead of wasting money on strips.
    Helpful links: Pool School; CYA/Chlorine Chart
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    Re: FAS DPD test

    I am doing the FAS DPD test as instructed. I believe frustratedpoolmom is onto something. I have a cheap ammonia test from walmart that only goes to 5ppm and it measures high ammonia of at least 5ppm in my pool. I tested regular tap water awhile back and it showed a high reading also so I just started to disregard the results thinking they were wrong. I will try to find a different ammonia test tomorrow. Will ammonia smell in a pool? Water is clear but hazy.

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    Re: FAS DPD test

    My bucket test finally yielded a free chlorine reading above 10ppm on the strips at a cumulative chlorine of 128ppm added to 5 gallon of pool water. Interesting to also note that now my ammonia strips read .25ppm on this test water. I retested the the pool water for ammonia and it's over 5ppm (the highest my test goes). So based on this do you think I am getting a false positive on my FAS-DPD test? I'm thinking it's going to take another 24 gallons (minimum) of 12.5% chlorine to wipe out the ammonia and get a true reading for FC. Am I on the right track?

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    Mod Squad jblizzle's Avatar
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    Re: FAS DPD test

    In the amount of time you have been messing with this bucket, you could have added 10-15ppm of chlorine to the pool, waited 30 minutes, retested FC and CC and added more chlorine, etc ... you would have almost been done by now.

    No one here is going to say, sure go ahead and add 24 gallons all at the same time.
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    Re: FAS DPD test

    I thought I read on here that you could do a bucket test to get an idea of how much chlorine it will take. Since I've already spent a small fortune this spring getting things cleaned up I thought it might be a good idea to do that. That way I could decide if it was even worth it or if I would need to drain or partly drain instead. In my original post today I asked for no one with a knee jerk reaction to respond, but of course someone did. Thanks Jason and FrustratedPoolMom for your thoughtful responses
    Last edited by jblizzle; 05-13-2014 at 06:37 PM. Reason: More name calling ...

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    Administrator Leebo's Avatar
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    Re: FAS DPD test

    I would start by using the OTO test to verify if there is any chlorine in the water. I believe the "bucket test" you're thinking of is described in this thread.

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    Re: FAS DPD test

    Yes thanks Leebo.



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    frustratedpoolmom's Avatar
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    Re: FAS DPD test

    Problem is the OTO test could show the CC. Its showing TC.
    I'd suggest a partial drain and refill to lower the ammonia level. Problem is your tap water. I have a client on a well who has ammonia present in the tap water. So you're not necessarily wrong on the ammonia test when you tested the tap. Do you have any bottled or distilled water? Test that for ammonia, that will rule out test malfunction/error.

    I've never done the bucket/FC demand method, I'm not about to tell anyone to dump in 24 jugs at once. I do a rough estimate, know I'll need a minimum certain amount of jugs on hand. On a particular pool last Spring, I added chlorine up to the calculated "shock" level of liquid every 30 minutes for the first hour and half. Did separate tests in various areas of the pool to see if it was registering anywhere and it wasn't, so I figured it's not circulation its just being consumed instantly. Want to say that pool had 4ppm ammonia. I stepped it up, adding shock level every 20 minutes. Did this for about 3 more hours. It started to register at 3FC, then 4 FC and the CCs were dropping down to 2, so I knew I was making progress. On the ammonia pools I've done it's taken a minimum of 5 cases of 12.5%, worst case was 10 cases and that pool had an 8ppm of ammonia...the owner didn't want me to do a partial drain, water here is expensive. Then there is always the need for a few additional jugs to kill remaining algae.
    Helpful links: Pool School; CYA/Chlorine Chart
    24' round AG pool, 52" high, Raypak heater; Waterway 2 spd Pump;
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