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Thread: FAS-DPD FC test inconsistencies

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    FAS-DPD FC test inconsistencies

    I'm starting a new post on this because one of my previous posts concerned discrepancies in the FAS-DPD FC test, using two different bottles of R-0871 reagent, one of which was probably too old. This post concerns discrepancies in the test, using the same bottle of R-0871, which has an expiration date of 9/2017. I'm wondering what mistakes I may be making in my procedures*.


    I do all my tests at about the same time in the morning, around 8 AM, using a 10 ml water sample for the FAS-DPD test
    On 8/14, I got FC=10.5/CC=0
    On 8/15, FC=8.5/CC=0
    On 8/16, FC=7.5/CC=0.5. I then did a second test and got FC=8.0/CC=0
    On 8/17, FC=6.0/CC=0.5. I then did three additional tests and got FC=8.0/CC=0, FC=8.5/CC=0, FC=7.5/CC=0


    I have added no chlorine during this period, because I was waiting for the FC level to come down from the 10.5 level on 8/14. There has been a good amount of sunshine all of these days. From 8/14 to 8/15, I lost 2 ppm of FC, which seems reasonable given past experience. From 8/15 to 8/16, I only lost 0.5 ppm. From 8/16 to 8/17, depending on which test I believe, I either lost 0.5 ppm or none.


    Each day, I rinse out the test tube at the end of my testing and put it back in the test kit box, close the lid, and put the box in an outside storage cabinet. However, the next day when I do the first FC test, I do not pre-rinse the tube, since I rinsed it out the day before. So I started thinking that the first reading may be false, and giving me some CC due to microscopic organisms left in the tube after the previous days rinse. But a sequence of tests using the same pool water sample, and rinsing thoroughly between tests, still gives me FC readings that can vary as much as 1 or 1.5. (I rinse with tap water which has no FC or CC since I have a water filter in the house).
    So I started wondering if my drop size is consistent. I hold the bottle vertically, and squeeze so that each drop comes out slowly (just a few seconds between drops).


    I wondered if I should release the pressure on the bottle after each drop, to let air get back into the bottle before I squeeze it again for the next drop? Would that affect the drop size? I am adding two dippers of R-0870 for each test, which should be enough, given that I am using 10 ml water samples.


    * In my college freshman chemistry class, I loved the theoretical part and got A's on all the exams. However, my lab work was apparently sloppy, and I got inconsistent and incorrect results on a lot of the experiments. My lab work actually dropped my overall grade to a B despite my grades on the exams. Maybe its my old curse, reappearing when I test my pool water.
    16,000 gallon outdoor in-ground 16'x32' plaster pool (built 2002); Pentair Intelliflo VF pump; Jandy Cartridge filter;Rainbow in-line chlorinator (used only when I am away); Hayward 2 wheel suction-side pool cleaner; LARS gas heater, Taylor K-2006 FAS-DPD test kit

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    Mod Squad woodyp's Avatar
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    Re: FAS-DPD FC test inconsistencies

    A drop is a drop is a drop. Just let it form and fall. No "repressuring" between drops required. Rinse the tube all U want, but just once will suffice. You'll be much better served by keeping your reagents in a cool environment.
    16x32x52" Steel Cornelius Miramar AGP Vinyl liner 13,100 gal. Buried 2 ft.
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    JoyfulNoise's Avatar
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    Re: FAS-DPD FC test inconsistencies

    Are you using a SpeedStir?

    Waiting several seconds between drops is not the proper way to do the titration. You should apply a constant pressure so that your drop rate is about 1 drop per second. Allowing the bottle to suck in air each time is not good as the titrating reagent is sensitive to oxidation. It also makes the droplet size somewhat variable as initial droplet sizes tend to be slightly larger than droplets that are the result of a steady state pressure. It's tricky and takes a little practice to get the droplet process just right.

    The good news is your numbers are not terribly bad. Your numbers tend to be within +/-1ppm. It's not like you're getting 5ppm, then 10ppm, then 4ppm. You just need to work on your consistency in droplet rate and your numbers will converge a little more.

    Another point is that a 10mL water sample size is definitely subject to worse variations in volume than a 25mL sample size. I also use a SampleSizer to get very consistent and correct water volumes.
    Matt
    16k IG PebbleTec pool, 650gal spa, spillway and waterfall, 3HP IntelliFlo VS / 1.5HP WhisperFlo, Pentair QuadDE-100 filter, IC40 SWCG, MasterTemp 400k BTU/hr NG heater, KreepyKrauly suction-side cleaner Dolphin S300i robot, EasyTouch controls, city water, K-1001, K-2006 and K-1766 test kits, Mannitol test for borates

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    Re: FAS-DPD FC test inconsistencies

    Quote Originally Posted by JoyfulNoise View Post
    Are you using a SpeedStir?
    Waiting several seconds between drops is not the proper way to do the titration. You should apply a constant pressure so that your drop rate is about 1 drop per second. Allowing the bottle to suck in air each time is not good as the titrating reagent is sensitive to oxidation. It also makes the droplet size somewhat variable as initial droplet sizes tend to be slightly larger than droplets that are the result of a steady state pressure. It's tricky and takes a little practice to get the droplet process just right.
    Thanks. i'll work on that. What is a SpeedStir and where can I get it?
    16,000 gallon outdoor in-ground 16'x32' plaster pool (built 2002); Pentair Intelliflo VF pump; Jandy Cartridge filter;Rainbow in-line chlorinator (used only when I am away); Hayward 2 wheel suction-side pool cleaner; LARS gas heater, Taylor K-2006 FAS-DPD test kit

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    JoyfulNoise's Avatar
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    Re: FAS-DPD FC test inconsistencies

    Matt
    16k IG PebbleTec pool, 650gal spa, spillway and waterfall, 3HP IntelliFlo VS / 1.5HP WhisperFlo, Pentair QuadDE-100 filter, IC40 SWCG, MasterTemp 400k BTU/hr NG heater, KreepyKrauly suction-side cleaner Dolphin S300i robot, EasyTouch controls, city water, K-1001, K-2006 and K-1766 test kits, Mannitol test for borates

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    Re: FAS-DPD FC test inconsistencies

    Thank you JoyfulNoise, we couldn’t have said it better ourselves!

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