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Thread: Magnetic Stirrer Inconsistent

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    todd.scallions's Avatar
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    Magnetic Stirrer Inconsistent

    So I ordered a magnetic stirrer last week to speed up the testing process and so far I'm loving it. The problem I am encountering is inconsistent chlorine tests. For example, last night I tested the chlorine levels and came up with 4.5ppm. I then tested it manually and the result was 6.5. I then tested again using the magnetic stirrer and get a result of 5. The night before I added enough chlorine to bring my levels up to 6. So the manual process always seems to show me closer to my goal where as the magnetic stirrer has me adding chlorine on a nightly basis. Has anyone else experienced this using a magnetic stirrer? The TA test always come out exactly the same no matter what. It's only with the chlorine test that come out wrong. I don't know which one to trust more.
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    Re: Magnetic Stirrer Inconsistent

    From what I have read here, is if your sample is not the same everytime, you could have varied results. Also another option is that the magnetic stirrer is mixing your sample better than when you do it manually. Also from I have read the amount of powder you put in should not really matter all that much as long as it turns pink. Also when you do these test or you doing them back to back or what?
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    JasonLion's Avatar
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    Re: Magnetic Stirrer Inconsistent

    Are you using different test vials for the three tests? There was a batch of test vials that had the labeling off a little bit, so they held a slightly different amount of water than the labeling said they held. If the vial you are using for the FAS-DPD test by hand was off and the vials used for the TA/CH tests and the SpeedStir were right, that could account for what you are seeing.
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    Re: Magnetic Stirrer Inconsistent

    I've noticed some discrepancies between the volume of the 9198 vials that ship with the stirrer and other labware I have including other vials, graduated cylinders, burettes and class-a volumetric flasks. I also did notice that when illuminated, the SpeedStir tends to wash out the pale pink color that may develop while titrating, possibly giving false low readings for chlorine. But yes always use the same sample size, stir or swirl enough to allow the reagent to totally incorporate.

    And SpeedStir is a great product, highly recommended.

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    todd.scallions's Avatar
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    Re: Magnetic Stirrer Inconsistent

    Yes, I tested back to back last night. I have been using the test vial that came with speed stirrer for both the chlorine test and the TA test. Tonight I will test using the vials that came with my TFT-100 test kit. I hear you on that light. I don't see myself ever using it.
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    Re: Magnetic Stirrer Inconsistent

    I just got an early birthday present, the Speedstir, and used it for the first time tonight. Pretty darn cool! I did notice that the TFT100 vial and the vial that comes with the Speedstir are vastly different. 25 ml seems to be much higher up on the Speedstir vial. The vials seem to be the same diameter??
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    Re: Magnetic Stirrer Inconsistent

    On mine, the TF100 vial is a tad wider than the Speedstir vial. Also the TF100's bottom is clearly flatter than the Speedstir.

    However, a dead-even 25ml in the Speedstir, poured into the TF100 vial, reads 27ml ??
    --paulr
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    Richard320's Avatar
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    Re: Magnetic Stirrer Inconsistent

    Quote Originally Posted by PaulR
    On mine, the TF100 vial is a tad wider than the Speedstir vial. Also the TF100's bottom is clearly flatter than the Speedstir.

    However, a dead-even 25ml in the Speedstir, poured into the TF100 vial, reads 27ml ??
    --paulr
    Now that's interesting!

    I have the vial that came in my first, dinky Taylor kit, and I like it better than the pill bottles in the TF100. Thicker walls and the measurements are molded into the plastic, not hot-stamped on. I have a speedstir on order, I'm curious to see if it matches now.
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    Re: Magnetic Stirrer Inconsistent

    Remember that some liquid is left behind when you pour into another vial. To compare vials reliably you should get both vials wet before you start, so they both have equal amounts of residual water in them, and try pouring in both directions to see if you get consistent results in both directions.

    Ideally you want the vials to be exactly the same, but in practice a difference of less than 10% really doesn't matter. Testing in the same way each time, so any error is the same each time, is far more important than having an absolute calibration.
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    Re: Magnetic Stirrer Inconsistent

    You don't get THAT much liquid left behind. I did pour the sample back and forth between vials a couple times and it was consistent; #9198 25ml, TF100 27ml.

    I agree consistency matters more than a Platonic Ideal of accuracy, although I think 10% is enough difference to be annoying.
    --paulr
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    Re: Magnetic Stirrer Inconsistent

    I poured water back and forth in the vials. 25 ml of water in the #9198 showed up as 23 ml in the TF100 vial. I think the TF is a little narrower because the white tops that come with the kit are too big for the #9198 vial. I don't think the 2 ml difference should matter.
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    Re: Magnetic Stirrer Inconsistent

    Ok, I think I figured out what was throwing off my test. When I filled the vial that came with my TF-100 kit to 10ml and then poured it into the one that shipped with my magnetic stirrer, it came out at 11ml with the magnet in. So before, I was leaving the magnet in the vial and filling it to 10ml, thus I had been testing with on 9ml of water. I was not filling it to 11ml to compensate for the magnet. When I tested tonight, my results were exactly the same as doing it manually. So FYI: no matter what test vial you want to use,be sure to remove the magnet first, add your sample, and then drop the magnet in. I took an Ambien about an hour ago and i think the keys on my keybuurd are starting to stick to my fingers.... must get sleep. Fuunt how these thought d hit you when youcan bearely seenm the scvreenn . Goodniht all!
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    JasonLion's Avatar
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    Re: Magnetic Stirrer Inconsistent

    Good point! It is fairly easy to make that mistake.
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    Re: Magnetic Stirrer Inconsistent

    I have received 2 different test vials from TFT and both were inaccurate. One was over and one was less. Discussed in this thread: this-normal-for-the-fas-dpd-test-tube-t20976.html ChemGeek said it does cause inaccuracies.
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    mickey4paws's Avatar
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    Re: Magnetic Stirrer Inconsistent

    Speaking of test vials, does anyone know if I can use any CYA mixing bottle, or for the TF100, does it have to be the CYA mixing bottle that comes with it? The reason I ask is I lost the top to mine (fell between the boards on the deck and I'm not crawling underneath to get it - we have snakes living under there). I am hoping to just buy a replacement CYA bottle at the local pool store.

    Todd, I hope you got a good sleep
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    Re: Magnetic Stirrer Inconsistent

    You don't have to have the CYA mixing bottle. All that's required are equal parts of pool water and R-0013 reagent.

    In fact, since I know that my CYA is close to 50 I just use 10ml of reagent and pool water and that'll fill the view tube above the 40 ppm mark so I don't waste reagent that way.
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    Re: Magnetic Stirrer Inconsistent

    Thanks so much for the reply, Bama Rambler. So I can just do a half and half mix in any container? I have such a hard time with this test. After reading PaulR's instructions, I discovered that I was doing the mixing wrong. I would shake it up for about a minute, let it stand for 30 seconds, shake a few more times. But according to his instructions, after mixing it, you should let it stand 2 to 3 minutes, then shake again. Not sure if it makes a difference, but I'm going to try it this way.
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    Re: Magnetic Stirrer Inconsistent

    I'm not sure the 2-3 minutes makes that much difference either, but it gives the reagent a little more time to react. Probably matters more when the water is colder. (My instructions are a collection from many sources, not all of them described the why of things.)
    --paulr
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