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Thread: The infamous DPD powder question...

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    y_not's Avatar
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    The infamous DPD powder question...

    I have read and read and read some more that the amount of DPD powder you add to the sample doesn't matter and WILL NOT affect the results, so long as it turns pink and stays pink.

    OK, so 2 questions and then a semi brief elaboration.

    1. If it turns pink, but it's only a light pink, not vibrant neon pink. Does that count?
    2. What happens if you add too much? Say a really heaping scoop that makes it super dark, compared to a less heaping scoop. Will this throw of the test for the former by requiring more drops to reach endpoint?

    I ask the 1st question because I have noticed that if I use a full heaping scoop when shocking, at my CYA it's 12ppm+, then I get a nice dark pink. But if I use a level scoop, I still get pink, but it's much lighter.

    Also, granule size, there are big huge granules, in relation to the others that is and there are sizes ranging down from there, all the way to a fine dust.
    So I have also seen when adding a heaping scoop of just the big granules, that the water doesn't turn pink at all!! Recently I had to add almost 2.5 scoops to get it to go pink and FC was only at about 12/13ppm.
    I did let the SS run for a full cycle after each addition.

    On question #2. Wouldn't more powder, thus making a darker sample color, take more titrating reagent to neutralize the dye?

    Thanks everyone.
    Thanks for reading... - Tony
    Da' Pool: Intex 15'x42" 3284gal AGP EasySet (Inflatable Ring) - (Summer 2014: 27' round EW /w 6.5' deep end @ 22,500gal)
    Pump & Cart Mod: 1000gph Cart. 5ft² - 2 nylons, 24/7 OP. Traps bugs/bits, lasts longer/cleans easier = Happier Pool Owner!!
    The Bible for a "Trouble Free Pool" life = PoolSchool, the BBB method a TF100 test kit(Recommend Kits Compared). - Cleaning a Sand Filter
    Water looks like GLASS, if yours doesn't...SLAM IT! Feels nice and never been happier!!! :D

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    Re: The infamous DPD powder question...

    The directions make it sound like it's not an issue putting a bit too much. I have noticed, especially with high chlorine levels, that the water will only turn a light pink sometimes but when you add a drop or 2 from the bottle (I forget the number) then it turns really dark before more drops start to lighten it up. I don't worry about the granules, it pretty much always matches what I think it should be. You could try testing both heaping and non-heaping to see if it comes out the same.
    Mike
    2007 1st full year with AG pool~15k gallons 18'x33' JW
    6/2010 added 2 button Intex SWG-painless except for when reset
    Switched to BBB from Baquacil (couldn't pay me to go back)

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    Re: The infamous DPD powder question...

    The instructions say to use two level scoops, most of us use one heaping scoop that is about equal to 2 level scoops. You need to have 'more than enough', particularly when at higher FC levels, as in maybe above 5 ppm. You should expect to have some granules left undissolved in the bottom of the vial. If they are all dissolved, it could be that you did not add enough. You are not harmed by having more than enough, but not enough is a problem.
    23,000 gallon in ground pool with rock waterfall and spillover spa, Aqualink control system, Polaris 380 cleaner, Purex Triton Clean&Clear Plus cartridge filter. Located in The Woodlands, Texas.

    Pool owner since Nov 2008, Trouble Free since April 2009. Happy to help when I can.

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    Re: The infamous DPD powder question...

    The purpose of the DPD powder is only to provide a pink/red to clear transition point at the end of the titration. So long as the chlorine reaction with dye (including continued reaction to the colorless state) is reversible when you add titrant, the amount of powder doesn't matter so long as it is enough for a distinct transition at the end. See this document for more details about the DPD reaction (see Figure 2.1) and the FAS-DPD test (see Figure 2.3).

    If the chlorine level is very high, then the DPD powder can get bleached out, but technically that's reversible and what should happen is that as you add more titrant drops the pink/red color will show up and get more intense before it starts to decline in intensity again. The main reason for the instruction to add more powder if the pink/red doesn't stay initially is just to make things simple by 1) seeing that there is indeed chlorine in the water and 2) to simplify the instructions as going from pink/red to clear (rather than sometimes being clear to pink/red to clear).
    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: The infamous DPD powder question...

    I have read and read and read some more that the amount of DPD powder you add to the sample doesn't matter and WILL NOT affect the results, so long as it turns pink and stays pink.
    That is correct. Far too many folks overthink this very accurate test. While there are some remote exceptions, simply put enough powder in the pool water sample to turn it pink and then perform the test......"don't keep trying to fix it if it ain't broke"
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: The infamous DPD powder question...

    Quote Originally Posted by duraleigh
    I have read and read and read some more that the amount of DPD powder you add to the sample doesn't matter and WILL NOT affect the results, so long as it turns pink and stays pink.
    That is correct. Far too many folks overthink this very accurate test. While there are some remote exceptions, simply put enough powder in the pool water sample to turn it pink and then perform the test......"don't keep trying to fix it if it ain't broke"
    Trust me, I'm not unnecessarily overthinking it. It may seem like it, with the questions I'm asking, but they are 2 fold.

    The reason the questions came about is because I started seeing variances. When testing back to back, I would see variances of more than one drop. So then I began to wonder about the color and the ammt. of DPD powder required to shade the sample, since sometimes at the same or similar FC levels, I'd require more powder to get it to tint. Never got a bleach out effect, just wouldn't turn pink or it was so light that it was barely there.

    So it seemed to me that when you used mostly or all large granules, that you had to add about 2-3x as many scoops to get it to turn a solid pink.

    I did test last night with a smaller scoop, such that it was just slightly pink, but not much. No go! I added the full 20 drops for the 10ppm FC I'm at right now as I come down off shock and it never once got any more pink, just cleared out after a drop or so then that was that. Totally invalid test.

    Which tells me you really do need to have a good, solid pink in there, doesn't have to be vibrant, just a good ammt. of pigmentation, say like watered down pink lemonade. That is, assuming your FC level isn't something, say <1.0ppm. That aside, the sample just doesn't work for proper testing if the pink is waayyy too light.

    I have never once seen a bleach out where it flashes pink then back to clear. So I can't attest to any results with that, but I thoroughly trust what Richard (ChemGeek) said on this. So if that ever happens, I'll know what to do, either add more DPD powder till it holds, or sacrifice more drops till it comes to pink, then goes back to clear. I think the former is a more prudent coarse of action, as DPD powder seems to be used less, as opposed to R-0871 re-agent, which runs out sooner.

    With everything I had read on here before getting the kit and Jason's fabulous ETD pg. I knew once I got the kit how to do it and just went for it and never questioned the method, and knowing how "not to do it".
    That aside, sure, I wanted to know a bit more about how the chemistry of the test itself worked, but saving that for a later date and probably a separate thread.

    Hopefully that makes sense where I'm coming from.
    Thanks for reading... - Tony
    Da' Pool: Intex 15'x42" 3284gal AGP EasySet (Inflatable Ring) - (Summer 2014: 27' round EW /w 6.5' deep end @ 22,500gal)
    Pump & Cart Mod: 1000gph Cart. 5ft² - 2 nylons, 24/7 OP. Traps bugs/bits, lasts longer/cleans easier = Happier Pool Owner!!
    The Bible for a "Trouble Free Pool" life = PoolSchool, the BBB method a TF100 test kit(Recommend Kits Compared). - Cleaning a Sand Filter
    Water looks like GLASS, if yours doesn't...SLAM IT! Feels nice and never been happier!!! :D

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    Re: The infamous DPD powder question...

    Quote Originally Posted by chem geek
    The purpose of the DPD powder is only to provide a pink/red to clear transition point at the end of the titration. So long as the chlorine reaction with dye (including continued reaction to the colorless state) is reversible when you add titrant, the amount of powder doesn't matter so long as it is enough for a distinct transition at the end. See this document for more details about the DPD reaction (see Figure 2.1) and the FAS-DPD test (see Figure 2.3).

    If the chlorine level is very high, then the DPD powder can get bleached out, but technically that's reversible and what should happen is that as you add more titrant drops the pink/red color will show up and get more intense before it starts to decline in intensity again. The main reason for the instruction to add more powder if the pink/red doesn't stay initially is just to make things simple by 1) seeing that there is indeed chlorine in the water and 2) to simplify the instructions as going from pink/red to clear (rather than sometimes being clear to pink/red to clear).

    WOOOOO!! Information overload in that document. But I have read worse things in the IT realm. UUGGHHH!! HAHA
    I skimmed it, I'll definitely have to print it out and read it soon.

    Good info in your post though, good to have in the back of my mind for when I need it. I always like to know the "what we don't tell everyone" stuff. Like when a mechanic tells you to never do that, but he does it all the time. As he knows what he's looking for and how to not mess it up by experience. HEHE
    CYA stuff mostly, and no, not the stabilizer.
    Thanks for reading... - Tony
    Da' Pool: Intex 15'x42" 3284gal AGP EasySet (Inflatable Ring) - (Summer 2014: 27' round EW /w 6.5' deep end @ 22,500gal)
    Pump & Cart Mod: 1000gph Cart. 5ft² - 2 nylons, 24/7 OP. Traps bugs/bits, lasts longer/cleans easier = Happier Pool Owner!!
    The Bible for a "Trouble Free Pool" life = PoolSchool, the BBB method a TF100 test kit(Recommend Kits Compared). - Cleaning a Sand Filter
    Water looks like GLASS, if yours doesn't...SLAM IT! Feels nice and never been happier!!! :D

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    Re: The infamous DPD powder question...

    Quote Originally Posted by chem geek
    The purpose of the DPD powder is only to provide a pink/red to clear transition point at the end of the titration. So long as the chlorine reaction with dye (including continued reaction to the colorless state) is reversible when you add titrant, the amount of powder doesn't matter so long as it is enough for a distinct transition at the end. See this document for more details about the DPD reaction (see Figure 2.1) and the FAS-DPD test (see Figure 2.3).

    If the chlorine level is very high, then the DPD powder can get bleached out, but technically that's reversible and what should happen is that as you add more titrant drops the pink/red color will show up and get more intense before it starts to decline in intensity again. The main reason for the instruction to add more powder if the pink/red doesn't stay initially is just to make things simple by 1) seeing that there is indeed chlorine in the water and 2) to simplify the instructions as going from pink/red to clear (rather than sometimes being clear to pink/red to clear).

    I understood how the FAS-DPD test woked, but I never looked into the actual chemistry behind it.

    Now that I have, I have a question. The titrating agent uses an iron reducing compound to reverse the reaction of the DPD poweder with the Cl.

    Now, it has been years (decades actualy) since I took, or even used, courses in wastewater management, but I do recall that ferrous reducing agents are (or maybe were) used in waste water treatment to increase flocculation. Is it possible that ferrous reducing agents already exist in my pool water, and are thowing off the test? Or is the answer that if there were any such agents in the water, they would have already reacted with any free cholrine and already reduced it, thereby making that chloine unavailable and so then what I am measuring with the test is the actual free chlorine?

    I think I just answered my own question by actualy thinking it through as I typed it out.

    -dave
    27' Round ABG (17,000 gal) - +/- 11 hours of full sun per day
    Hard plumbed Hayward Perflex EC-40 DE filter w/ 1 HP pump (Got a new one June 2015)
    Filered 4 hours per day (on a timer)
    Pentair Aqualuminator - now working - and now dead once again, this time for good.
    Zodiac Wahoo

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    Re: The infamous DPD powder question...

    Quote Originally Posted by duraleigh
    I have read and read and read some more that the amount of DPD powder you add to the sample doesn't matter and WILL NOT affect the results, so long as it turns pink and stays pink.
    That is correct. Far too many folks overthink this very accurate test. While there are some remote exceptions, simply put enough powder in the pool water sample to turn it pink and then perform the test......"don't keep trying to fix it if it ain't broke"
    Here's my confusion. When shocking we have seen countless threads where people were told they hadn't put enough powder in the tube at the start of the test - despite the water turning pink at the onset. How does that square with what we are reading in this thread - or is that one of the exceptions?
    Inlaws Pool Boy since June 14th 2012, Pool built ~ 2003, In-Ground, 16'x32'
    13500 gal, Vinyl Liner, Fiberglass Slide, TF-100 Test Kit, Hayward 210T
    sand filter, A.O. Smith 1.5HP main pump motor (C48L2N134C1),
    Hayward SuperPump (model ?), Polaris 380 & PB4 Booster Pump

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    Re: The infamous DPD powder question...

    Why were people told that they didn't add enough powder at the start? Did they get a different result or was this just common advice? I thought the only time they were told to add more powder was when they saw a flash of pink or the color was very weak.
    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: The infamous DPD powder question...

    Quote Originally Posted by chem geek
    Why were people told that they didn't add enough powder at the start? Did they get a different result or was this just common advice? I thought the only time they were told to add more powder was when they saw a flash of pink or the color was very weak.
    I have seen different advice where the expected value came out low and people were told to add more powder. I've seen instances of this myself where it seemed like the solution turned clear prematurely when I used less powder. This was with a 20+ shock level and I guess it could just be due to inconsistent speed drop size etc.


    - Sent using Tapatalk
    Inlaws Pool Boy since June 14th 2012, Pool built ~ 2003, In-Ground, 16'x32'
    13500 gal, Vinyl Liner, Fiberglass Slide, TF-100 Test Kit, Hayward 210T
    sand filter, A.O. Smith 1.5HP main pump motor (C48L2N134C1),
    Hayward SuperPump (model ?), Polaris 380 & PB4 Booster Pump

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    Re: The infamous DPD powder question...

    The FAS/DPD test found in both the TF-100 and Taylor K-2006 test kits is a primary reason for owning these kits. It has an ability to ACCURATELY measure very high levels of chlorine (50+) and this test alone separates these two kits from less useful kits.

    To someone reading this thread, it may appear the FAS/DPD test is hard to perform and inconsistent. Nothing could be further from the truth.

    The test is performed daily by thousands of people all across the world with consistent and accurate results down to .2ppm. Those that use this test know of it's dependability and precision.

    Naturally, I have some agenda here in that I sell this test separately and inside the TF-100 as well.; I think it is important to point out that the VERY reason it is included in the K-2006 and the TF-100 is it's dependability and precision.

    There is simply no reasonably priced test available that approaches it's precision and consistent results. Most of us that practice the BBB procedures taught on this forum would agree that this test is an essential part of how we manage our pools.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
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    Re: The infamous DPD powder question...

    LOL. I love the test - I'm just a logical guy and have a deep need to understand stuff.
    Inlaws Pool Boy since June 14th 2012, Pool built ~ 2003, In-Ground, 16'x32'
    13500 gal, Vinyl Liner, Fiberglass Slide, TF-100 Test Kit, Hayward 210T
    sand filter, A.O. Smith 1.5HP main pump motor (C48L2N134C1),
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    Re: The infamous DPD powder question...

    I couldn't agree more.
    The test is sound, I don't question it's validity at all. Other than when I was being a total 'tard and couldn't agitate it properly without spilling some of the sample out, then I began to question myself and it. Certainly didn't blame the test though, just mad at me, but I did get kinda mad at the test I must admit. But didn't ultimately blame it.

    That's why I got the speed stir.
    TTTSSSSHHHH AAAAHHHHHH!!! Relief!! Easy!! Love it!!

    I'm with UnderWaterVanya, I just like to know more about what I'm doing and how things work.
    I got caught taking the hair dryer apart when I was like 7 or somewhere in there. Wasn't the 1st time either. LOL
    I didn't do it to be malicious or to break it, I wanted to know how that puppy worked.... BAAADDDD!!

    If you want, move this to the deep end. That way there's no confusion with the newbies. I just figured it'd be helpful to discuss it here.
    As I know there's already tons of confusion on the powder issue and I feel ChemGeek and yourself Jason, have been a big help already in answering my questions quite satisfactorily.

    I guess the only real final question that remains, one that I pretty much know the answer to, but just need clarification on.
    That is, when y'all say "...so long as it turns pink" in regards to adding the DPD powder. The question still looms, how pink is "pink"?
    Yeah, yeah, I know. But seriously!
    Pink lemonade, hot pink bikini, pink classic Cadillac, hot pink neon, a teaspoon of pink lemonade in a cup kinda tinge, what?

    Well, I guess there are 2 questions. The second and part 2 of that being.
    What's the deal with the big huge granules?
    They don't work as well as the smaller ones, or the dust which is best.
    You just have to use more of the big ones. That does complicate things, a lot!
    Always wondering if that scoop will be enough, or if it isn't how much more do I need to get to pink..?
    Lastly, what on earth is this R-0870 stuff that they have to put it in powder form and why so varied in size?
    Thanks for reading... - Tony
    Da' Pool: Intex 15'x42" 3284gal AGP EasySet (Inflatable Ring) - (Summer 2014: 27' round EW /w 6.5' deep end @ 22,500gal)
    Pump & Cart Mod: 1000gph Cart. 5ft² - 2 nylons, 24/7 OP. Traps bugs/bits, lasts longer/cleans easier = Happier Pool Owner!!
    The Bible for a "Trouble Free Pool" life = PoolSchool, the BBB method a TF100 test kit(Recommend Kits Compared). - Cleaning a Sand Filter
    Water looks like GLASS, if yours doesn't...SLAM IT! Feels nice and never been happier!!! :D

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    Re: The infamous DPD powder question...

    I say quit over-thinking and most important, don't be stingy!

    I find that the DPD powder gets nasty and sticky long before it runs out, so I am generous when testing FC. I use a big fat heaping scoop for regular testing, when looking for levels up to 7 maybe. For levels higher than that, I'd use two scoops. I think it is right when we are looking at hot pink neon, pink lemonade concentrate before you add water.

    Next time you order replacements, get a spare of the powder. Keep it sealed until you need it. Being wrong on the FC test is a big deal compared to the cost of a year's worth of DPD powder. BTW, the website says 25 tests in a $6.75 vial of powder -- that can't be right. I think mine lasts most of the year and I test 2-3 times a week in summer, maybe weekly in winter. Maybe I need to keep notes on this.
    23,000 gallon in ground pool with rock waterfall and spillover spa, Aqualink control system, Polaris 380 cleaner, Purex Triton Clean&Clear Plus cartridge filter. Located in The Woodlands, Texas.

    Pool owner since Nov 2008, Trouble Free since April 2009. Happy to help when I can.

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    Re: The infamous DPD powder question...

    Nope, no stinginess here on that powder.
    Once I saw how slowly it gets depleted, I scrapped that idea. I have been using it for a month straight. Testing 2, sometimes 3x/day just to get a handle on things and making sure my results were repeatable. I still have probably 4+ grams left and haven't even opened my other one.

    I have always used a heaping scoop, never more, even at 15ppm shock levels it turns mega hot, vibrant, vivid pink. Very, very pretty!! I might add.

    My whole point was the granule variance and such, that seems to throw off the "amount needed".
    If I use nothing but large granules at those shock levels, I have to add about 3, yes, 3 heaping scoops to get a proper pink.
    Re-read my post above yours and you'll see what I'm talking about.

    I must admit though, that at lower std. levels, say 5ppm. 1 level scoop, or even with just a "pit" missing seems just fine. Any more and you don't get any more pink.
    Thanks for reading... - Tony
    Da' Pool: Intex 15'x42" 3284gal AGP EasySet (Inflatable Ring) - (Summer 2014: 27' round EW /w 6.5' deep end @ 22,500gal)
    Pump & Cart Mod: 1000gph Cart. 5ft² - 2 nylons, 24/7 OP. Traps bugs/bits, lasts longer/cleans easier = Happier Pool Owner!!
    The Bible for a "Trouble Free Pool" life = PoolSchool, the BBB method a TF100 test kit(Recommend Kits Compared). - Cleaning a Sand Filter
    Water looks like GLASS, if yours doesn't...SLAM IT! Feels nice and never been happier!!! :D

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    Re: The infamous DPD powder question...

    Is it normal to test chlorine every day with the drop test?

    Don't really see much use for the other OTO colour chart thingy test....

    I enjoy the certainty of the drop test and do it every day, is that what most other people are doing?
    13,000 gallon; sand filter; inground; fibreglass; pump not sure yet (need to check); Installed over 12 months ago.

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    Re: The infamous DPD powder question...

    Quote Originally Posted by y_not
    My whole point was the granule variance and such, that seems to throw off the "amount needed".
    If I use nothing but large granules at those shock levels, I have to add about 3, yes, 3 heaping scoops to get a proper pink.
    If your point is that the large granules don't dissolve quickly so are effectively like not adding the powder to the sample at all, then that's an entirely different problem. It's not really a "how much should I add" problem as it is "if the granules are clumped, then they won't dissolve well" problem.

    I've used clumps without an issue because I either swirl until they dissolve or I break them up with my spoon before adding the powder. This is not a big deal. It's probably easiest to just tell people that if there are larger clumps, break them up with the spoon before adding them and swirl until most of what was added is dissolved and a rich color forms.
    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: The infamous DPD powder question...

    [quote=chem geek]
    Quote Originally Posted by "y_not":sq0l0t5o
    My whole point was the granule variance and such, that seems to throw off the "amount needed".
    If I use nothing but large granules at those shock levels, I have to add about 3, yes, 3 heaping scoops to get a proper pink.
    If your point is that the large granules don't dissolve quickly so are effectively like not adding the powder to the sample at all, then that's an entirely different problem. [/quote:sq0l0t5o]
    It's not so much of a problem that they don't dissolve quickly, as I run the SS on 1 full cycle before proceeding. I think it has more to do with the granules being of larger size and have jagged edges, just like with sand, that there's more air space between them. Thus less volume of actual chemical goes in per scoop. Taking more scoops to equal the same amount of chemical volume being added to the sample.

    I'm sure it does have some to do with the large ones not dissolving, but the water doesn't even turn pink at all with a heaping scoop of large only granules. As opposed to a heaping scoop of mixed granules and dust, yields a nice vibrant pink. Same water, same FC.

    Maybe they'll dissolve and turn the water pink if I start titrating?
    I have noticed some more dissolve when you add the R-0871 anyway. So maybe that's the trick.

    Quote Originally Posted by chem geek
    It's not really a "how much should I add" problem as it is "if the granules are clumped, then they won't dissolve well" problem.
    No problems with clumping, powder is too new.

    Quote Originally Posted by chem geek
    I've used clumps without an issue because I either swirl until they dissolve or I break them up with my spoon before adding the powder. This is not a big deal. It's probably easiest to just tell people that if there are larger clumps, break them up with the spoon before adding them and swirl until most of what was added is dissolved and a rich color forms.
    I couldn't agree more.
    Thanks for reading... - Tony
    Da' Pool: Intex 15'x42" 3284gal AGP EasySet (Inflatable Ring) - (Summer 2014: 27' round EW /w 6.5' deep end @ 22,500gal)
    Pump & Cart Mod: 1000gph Cart. 5ft² - 2 nylons, 24/7 OP. Traps bugs/bits, lasts longer/cleans easier = Happier Pool Owner!!
    The Bible for a "Trouble Free Pool" life = PoolSchool, the BBB method a TF100 test kit(Recommend Kits Compared). - Cleaning a Sand Filter
    Water looks like GLASS, if yours doesn't...SLAM IT! Feels nice and never been happier!!! :D

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    Re: The infamous DPD powder question...

    Quote Originally Posted by juzzie
    Is it normal to test chlorine every day with the drop test?

    Don't really see much use for the other OTO colour chart thingy test....

    I enjoy the certainty of the drop test and do it every day, is that what most other people are doing?
    At first one probably should, this way you know how your pool ebbs and flows with it's chlorine consumption/demand.
    But once you get the hang of how much it needs and how much to add daily, then you can just test every few days or so to verify things. In between, adding how much you "usually need" each night.
    OTO is a good, what we call "sanity check", to make sure that there is chlorine present and roughly how much. It's great for this, since it's pretty "bullet proof" that way. Any CL beyond trace amounts and it'll turn yellow, no doubt about it! You can count on that.

    My feeling is it's not accurate enough for anything other than a quick ballpark, to know you're staying in range. Followed up by a FAS-DPD to confirm things later.
    Not to say that it isn't capable of showing you semi accurate levels up to 5ppm and how much to dose thereof. But mainly that I'm always unsure of my reading of the color chart compared to my sample. So it really comes down to my uncertainty level more than anything.

    Anything above 5ppm FC and it's just a ballpark, as there is no chart in existence that shows those high levels and what they correspond to, not to mention accuracy decreases the higher up you go.
    BUT.. OTO is capable of indicating CL at very high levels, where the FAS-DPD and DPD will bleach out, turning clear.

    Here's a quote and snippet from Ben Powell regarding OTO. A person of whom is an extremely knowledgeable pool expert and the original creator of our own CL/CYA chart and many other contributions to this forum, if not the credited inventor of the BBB basic method.

    * Yellow to Dark Yellow => Ideal
    * Light Yellow => Add chlorine
    * Orange tinted => Swimmable, but too high, unless you are shocking or have high CYA
    * Orange => Old swimsuits only! Ideal for algae cleanup.
    * Brown => Too high for people and vinyl pools.
    (Not dangerous, but it will fade things, and sting eyes. You might get a little chapped where the sun doesn't shine, too.)
    Great for concrete pool cleanup.

    "Now, you can get meaningfully better results using the DPD-FAS, and tracking things more closely than that. But, using that scale alone, probably 1/2 the pool owners in the US could do better than they are now!"
    Thanks for reading... - Tony
    Da' Pool: Intex 15'x42" 3284gal AGP EasySet (Inflatable Ring) - (Summer 2014: 27' round EW /w 6.5' deep end @ 22,500gal)
    Pump & Cart Mod: 1000gph Cart. 5ft² - 2 nylons, 24/7 OP. Traps bugs/bits, lasts longer/cleans easier = Happier Pool Owner!!
    The Bible for a "Trouble Free Pool" life = PoolSchool, the BBB method a TF100 test kit(Recommend Kits Compared). - Cleaning a Sand Filter
    Water looks like GLASS, if yours doesn't...SLAM IT! Feels nice and never been happier!!! :D

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