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Thread: FAS-DPD Interference question

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    FAS-DPD Interference question

    Hello all,

    I received my TF-100 Testkit about two weeks ago and have been doing a full set of tests every five days or so. So far my weekly results seem to be precise, (I have been also taking a water sample to the pool store, to compare results, since I'm not completely confident with my testing ability yet. The poolstore I go to uses a Lamotte Waterlink Express), with the exception of the CC test

    Initially I chalked it up to just learning how to do the test, Now I'm not so sure. Before I delve into the test results, here is some background on my pool.

    25,000 gallon 18x36 IG Pool with a liner. Currently, I am using the Sustain system for chlorination (Unfortunately these chemicals were bought in advance, so i can't justify switching over to BBB quite yet (though I'd like to)). Thanks to you guys the water has been sparkling clear for the past three weeks.

    EDIT: the following tests were taken over the course of two weeks, Test #3 is the most current taken 6/29
    Test #1 (TF-100 / Store results)
    FC: 2 / 1.7
    CC: 3.5 / .82

    Test #2
    FC: 5.5 / 5.7
    CC: 4 / .13

    Test #3
    pH: 7.5 / 7.4
    Alkalinity: 110
    Calcium: 330/ 310
    CYA: 30 / 30
    FC: 3.5 / 3.72
    CC: 4 / .13

    My testing procedure for the last test was

    1) Rinsed out sample container marked chlorine only using pool water repeatedly
    2) Obtain sample by filling container marked chlorine only to the 10 mL
    3) Performed FC test by adding a scoop of R-0870 DPD powder, mixing. Solution turned pink
    4) Swirled and added drops of R-0871 until the solution was clear. This took 7 drops.. recorded FC as 3.5
    5) To the same solution added 5 drops of R-0003. Solution turned bright pink/red. Again added and swirled drops of R-0871 until solution was clear. This took 8 drops. recorded CC as 4.0.

    Is this procedure correct or am I missing something? Are there any interference issues that I should be aware of? The only thing I've found by googling was

    FAS-DPD is available in stand-alone kits to measure chlorine or bromine, and in combination with other common
    tests. Supplement this test with Deox Reagent when testing chlorine in the presence of monopersulfate shocks or
    interference will cause a false-high
    combined chlorine reading.
    -- http://www.taylortechnologies.com/produ ... opTest.pdf

    Are there any issues with Sustain and the DPD-FAS test. As far as I know the sustain tabs are cal-hypo based, not MPS. Not sure about the Summer Shield aspect of the system.

    Can anything else cause false highs in CC?

    On a completely unrelated note: my R-0010 reagent is labeled as an Alkalinity test, but it is only used in the calcium hardness test. Is this correct?


    Thanks for your time,
    radne
    18X36 IG vinyl 22100 gal. 1.0 HP Hayward Pump w/300# Hayward sand filter.

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    Re: FAS-DPD Interference question

    I don't have his kit but have bought some reagents from Dave, can't remember off the top of my head but if the reagent for the CC test is what Dave mentions in this thread, perhaps that's the problem?

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    Re: FAS-DPD Interference question

    My guess is that your CC's are really 4.0. That test is pretty accurate and your method appears correct. Shock your pool (you don't post CYA so I can't tell you how much Cl to use) and I think you will see a drastic reduction of CC's.

    On a completely unrelated note: my R-0010 reagent is labeled as an Alkalinity test, but it is only used in the calcium hardness test. Is this correct?
    Yes....simply a misprint.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: FAS-DPD Interference question

    Quote Originally Posted by Rangeball
    I don't have his kit but have bought some reagents from Dave, can't remember off the top of my head but if the reagent for the CC test is what Dave mentions in this thread, perhaps that's the problem?
    Ignore me. Apparently I don't know how to read

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    Re: FAS-DPD Interference question

    I don't have his kit but have bought some reagents from Dave, can't remember off the top of my head but if the reagent for the CC test is what Dave mentions in this thread, perhaps that's the problem?
    Rangeball, the drops come out in very controlled controlled manner and are uniform in size. Looks like that issue was also regarding the reagent for the alkalinity test (R-0009). Currently my solution turns pink on the first drop of R-0003 and is vibrant pink by fifth.

    My guess is that your CC's are really 4.0. That test is pretty accurate and your method appears correct. Shock your pool (you don't post CYA so I can't tell you how much Cl to use) and I think you will see a drastic reduction of CC's.
    Dave, sorry, I was very unclear about the test results that I posted. Those were three separate test sessions over the course two weeks. I only briefly eluded to this, and had meant to remention it before I posted the results but must have left that part out on accident.

    Test #3 is the most recent and has CYA listed. I've shocked several times since then, fearing that my CC results were in fact accurate. Again, I use the Sustain system, which uses Cal-hypo pellets (1" fast dissolving). For superchlorination and 3 inch cal-hypo pucks for regular chlorination, and one dosing of something called Summer Shield at the beginning of the summer (No idea what is in this).

    I've read that a CC above .5 is considered high. Wouldn't there be some noticeable water quality issues with a CC of 4? The water is clear, odorless, haven't had any issues with itching or red eyes.

    I will definitely shock it tonight, and verify that the chlorine levels reach at least 15ppm, and retest tomorrow.

    Just wasn't sure if there were any interference issues the fas-dpd, besides the MPS. Haven't heard of any issues with the TC/FC test for the La motte waterlink test that my store uses (Only issues with alkalinity and calcium) and given my newness to the whole testing system, I immediately questioned my own results.

    Thank you kindly for your responses, Rangeball and duraleigh
    radne
    Jason
    18X36 IG vinyl 22100 gal. 1.0 HP Hayward Pump w/300# Hayward sand filter.

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    Re: FAS-DPD Interference question

    Radne:
    On this forum, most tend to disregard pool store tests, based on bad experience. Your FC pool store tests, however, are very consistent with what you're getting yourself with the test kit. This should give a bit more credence that we normally would give to the pool store CC test.

    Try this: Let your free chlorine go down to around 2, and perform a FAS-DPD test. If you again get results similar to your test #1 - FC around 2, CC around 4 - retest using your OTO test. The OTO tests for total chlorine, free plus combined. The test is not particularly precise, but you should be able to easily tell the difference between TC of 2 and TC of >5. If the OTO test gives a result around 2, this would be verification of the pool store test, and would be strong evidence that you have some problem with your FAS-DPD CC reading, some interference. On the other hand, if the OTO test is dark yellow, indicating TC around 5 or higher, that's evidence that your pool store's CC test is inaccurate, you've got a CC problem and must deal with it, despite the clarity of your water.

    Keep us updated. I'm curious to know what happens.
    22,000 gal, IG vinyl, 3/4 hp pump, 300lb sand filter (switched from DE), converted from baquacil (thanks to Ben), BBB since 2004

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    Re: FAS-DPD Interference question

    I'll say that I had an issue with an unusually high CC reading 2 days ago. I thought maybe I got some rogue water drops in the test somehow. I attribute it to using the little blue scooper to "stir" the mixture. Now, I just dump the mixture in and swirl the cylinder around manually until it dissolves. I re-ran the test immediately after and I had .5 CC. No more stirring with the blue scooper for me.
    18ft Round x 4ft deep AGP. Approx 7800 gallon.

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    Re: FAS-DPD Interference question

    Quote Originally Posted by radne
    Again, I use the Sustain system, which uses Cal-hypo pellets (1" fast dissolving). For superchlorination and 3 inch cal-hypo pucks for regular chlorination, and one dosing of something called Summer Shield at the beginning of the summer (No idea what is in this).
    Does the "Summer Shield" list an ingredient on the label? That sounds suspiciously like CYA or Stabilizer.

    Also, keep an eye on your CH reading as you have a vinyl pool, and the recommended range is 200-400 using Cal=Hypo continuously will raise your CH.

    How much powder are you adding. I know it says "heaping" but use enough to turn it pink. IF there is alot left undissolved that can begin to dissolve and again turn pink, causing your CC's to read higher, perhaps?
    Helpful links: Pool School; CYA/Chlorine Chart
    24' round AG pool, 52" high, Raypak heater; Waterway 2 spd Pump;
    150 Sq ft. Clearwater Cartridge Filter; Former and DISSATISFIED "Pool Frog" owner
    http://www.PerfectlyClearPoolService.com

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    Re: FAS-DPD Interference question

    Wow, thank you for all the replies. This is going to be a wall-o-quotes. For that I apologize

    On this forum, most tend to disregard pool store tests, based on bad experience. Your FC pool store tests, however, are very consistent with what you're getting yourself with the test kit. This should give a bit more credence that we normally would give to the pool store CC test.
    tcpii, Yep, if all the values were off I'd be inclined to disregard the pool store results (I already disregard most of the "dosing" advice they give me unless it falls inline with what I read on here.), however, it is just the CC that is off.

    I'll say that I had an issue with an unusually high CC reading 2 days ago. I thought maybe I got some rogue water drops in the test somehow. I attribute it to using the little blue scooper to "stir" the mixture. Now, I just dump the mixture in and swirl the cylinder around manually until it dissolves. I re-ran the test immediately after and I had .5 CC. No more stirring with the blue scooper for me.
    HunterDT, Interesting, though I swirl the mixture until everything is dissolved and swirl some more after that

    How much powder are you adding. I know it says "heaping" but use enough to turn it pink. IF there is alot left undissolved that can begin to dissolve and again turn pink, causing your CC's to read higher, perhaps?
    frustratedpoolman, I use a heaping scoop of the powder. As far as I can tell that powder is completely dissolved. It usually dissolves fairly quickly after a few vigorous swirls. After that I swirl a few more seconds.

    Also, keep an eye on your CH reading as you have a vinyl pool, and the recommended range is 200-400 using Cal=Hypo continuously will raise your CH.
    will do, at the advice of JasonLion, I've been keeping a close eye on that and my pH, and am monitoring my approximated CSI.

    Does the "Summer Shield" list an ingredient on the label? That sounds suspiciously like CYA or Stabilizer.
    You know, I'm not exactly sure what is in it. It has probably been close to a month since I added (only added once). At the time I was still relying on the pool store for tests, and I still had to add stabilizer to get it up to where it's at now. Unfortunately, I don't have the bottle anymore so not sure what it lists.[/quote]

    However, googling "Summer Shield" ingredients. I found some notsohelpful information
    Sustain Summer Shield Chlorine Extender is the brains of the Sustain System. It's not only smart, but unique to PPG, protected by a U. S. Patent. Summer Shield Chlorine Extender combines with some of the chlorine in the pool water and shields it, thereby creating a reserve tank of chlorines to give extra protection against the development of algae.

    As Free Available Chlorine (FAC) is depleted, the shielded chlorine releases into the pool water to control algae and bacteria growth. Sustain Summer Shield Chlorine Extender provides chlorine-killing power despite unpredictable increases ins chlorine demand brought on by debris from rain and wind storms; or when your nice quiet swim turns into a full-blown neighborhood pool party.
    Not much help, but.. further looking at the FAQ page I found
    Why can't I use an OTO test kit when using Sustain?

    OTO does not clearly show the difference between free and combined chlorine and therefore should not be used when the pool is treated with the Sustain Pool Care System. Sustain Summer Shield Chlorine Extender holds a backup reserve of chlorine that tests as combined chlorine. To avoid confusion, use DPD or 3-Way Test Strips to measure FAC without interference.
    -- http://corporateportal.ppg.com/NA/CalHy ... Questions/

    This may or may not explain why my CC tests high. However, why would the Lamotte waterlink test low? Does the La Motte chemistry differ from the Taylor chemistry when it comes to testing CC?

    I did take duraleigh's advice and shock my pool last night. Tested this afternoon before leaving for work at about 12:30 PM. The pool itself gets plenty of sun.

    pH: 7.9 [Disregarding this for now due to high FC, will test when I get home]
    FC: 10
    CC: 3.5
    TA: 120
    CH: 330
    CYA: 30
    Temp: 89
    18X36 IG vinyl 22100 gal. 1.0 HP Hayward Pump w/300# Hayward sand filter.

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    frustratedpoolmom's Avatar
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    Re: FAS-DPD Interference question

    Well that is interesting. I wonder if the Summer Shield lasts that long and if it is the reason for the CC's. Waterbear usually knows such things. Maybe he'll chime in.
    Helpful links: Pool School; CYA/Chlorine Chart
    24' round AG pool, 52" high, Raypak heater; Waterway 2 spd Pump;
    150 Sq ft. Clearwater Cartridge Filter; Former and DISSATISFIED "Pool Frog" owner
    http://www.PerfectlyClearPoolService.com

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    Re: FAS-DPD Interference question

    Sustain® Summer Shield has an MSDS you can find here though saving the file requires manual changing of it to ".PDF" (at least for me). It indicates the ingredients as 80-80% "Sodium Chloride Brine" (i.e. very saturated salt) and 15-20% "Sustain Shield (proprietary)". The pH is between 7 and 9. I am reasonably certain that this a monosodium cyanurate salt in a slurry, not that much unlike Liquid Instant Pool Conditioner except the latter does not have so much salt (brine) in it. I cannot find the patent they refer to, however. This basically means it's just CYA in a form that is more pH neutral and dissolves quickly in water.

    The Sustain® ¾-Inch Shield Energizer Tablets sound very much like Trichlor with the replenishment of the Summer Shield reserve bank of chlorine protection being nothing more than CYA added by the Trichlor.

    [EDIT] See this post for more up-to-date info. [END-EDIT]

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    Re: FAS-DPD Interference question

    Quote Originally Posted by frustratedpoolmom
    How much powder are you adding. I know it says "heaping" but use enough to turn it pink. IF there is alot left undissolved that can begin to dissolve and again turn pink, causing your CC's to read higher, perhaps?
    Just wanted to throw this out there. I have a test kit from Ben (poolsolutions), not one of Dave's kit but I do believe they are more or less the same, any way my instructions say "do not worry if all does NOT dissolve" So I would not think that would be source of the problem.
    27' Round AG, 17,200 gallons, sand filter

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    frustratedpoolmom's Avatar
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    Re: FAS-DPD Interference question

    Quote Originally Posted by Poolidiot
    Quote Originally Posted by frustratedpoolmom
    How much powder are you adding. I know it says "heaping" but use enough to turn it pink. IF there is alot left undissolved that can begin to dissolve and again turn pink, causing your CC's to read higher, perhaps?
    Just wanted to throw this out there. I have a test kit from Ben (poolsolutions), not one of Dave's kit but I do believe they are more or less the same, any way my instructions say "do not worry if all does NOT dissolve" So I would not think that would be source of the problem.
    Good to know. I wasn't sure.
    Helpful links: Pool School; CYA/Chlorine Chart
    24' round AG pool, 52" high, Raypak heater; Waterway 2 spd Pump;
    150 Sq ft. Clearwater Cartridge Filter; Former and DISSATISFIED "Pool Frog" owner
    http://www.PerfectlyClearPoolService.com

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