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Thread: strange FC readings

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    iam4iam's Avatar
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    strange FC readings

    First of all, thanks to TFP, I have had a truly TFP all summer long!

    Background: I've been faithfully measuring FC levels all summer with no problems. A few unexpected readings that may be the result of two successive inaccuracies in opposite directions, but for the most part I've gotten to where I can predict what the reading will be with decent accuracy. I've noticed that as the temperature has decreased, so has the FC dissipation rate. For the last few weeks, I've averaged about 48 oz/day of bleach to maintain FC around 5 with CYA of 70. Since 32 oz of bleach raises the FC level ~0.5 ppm, this translates to a depletion rate of FC of about 1.5 ppm/day, which seems very reasonable (and quite a relief compared to the 3+ ppm/day during the hotter weeks of the summer). For much of the summer I was measuring every night, but lately I've been confident enough to only measure every 2-3 days. I've meticulously kept a spreadsheet of my pool stats all summer long.

    Quandary: Lines 355-363 of attached spreadsheet [attachment=0:1e8t3clc]pool stats.xls[/attachment:1e8t3clc]
    This period of 5 days indicates FC depletion of only 0.3 ppm/day on average, which is acceptable overnight FC loss in 24 hours! When I got the reading on line 360, I figured maybe the previous reading had been low and this one was high, but even if the line 355 reading was low by 0.5 ppm and the line 363 reading is high by 0.5, that would still be only 0.5 ppm/day of FC loss over this 5-day period! I didn't change anything in my testing method. I might not be that taken aback if the water were only 50 degrees, but at 70? Can anyone think of a possible explanation?
    Attached Files Attached Files
    ~18,000 gal (16x32)
    Hayward sand filter
    Circupool RJ45 SWG
    Hayward Max-Flo XL SP2305X7 pump (0.75 HP)

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    Re: strange FC readings

    Shorter days? Lower angle of the sun? More incidental shade?
    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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    Richard320's Avatar
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    Re: strange FC readings

    I would say shorter days, cooler water, and less bather load. I doubt anyone's swimming in 70 degree water.

    The other possibility is that you didn't notice that Chlorox upped the strength of their bleach from 6 to 8.
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
    Troublefree does not mean Maintenancefree. It's like brushing your teeth: You can spend a couple minutes a day and pennies a week or go to the dentist once a year and spend several thousand dollars.
    A pool is like a pet - you have to feed it every day, even the days you don't want to play with it!

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    iam4iam's Avatar
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    Re: strange FC readings

    Quote Originally Posted by Richard320
    The other possibility is that you didn't notice that Chlorox upped the strength of their bleach from 6 to 8.
    Is that true? That seemed like a very plausible explanation, but I checked and my bottles are still 6%. I'm going to take another reading today when the pump comes on and provide an update. Still puzzled. Sun and temperature variation don't seem to explain such a dramatic decrease in dissipation rate. I'd expect -0.3 ppm/day if there were no sun, i.e. 24 hours of night!
    ~18,000 gal (16x32)
    Hayward sand filter
    Circupool RJ45 SWG
    Hayward Max-Flo XL SP2305X7 pump (0.75 HP)

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    msgtdan's Avatar
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    Re: strange FC readings

    I doubt anyone's swimming in 70 degree water.
    My daughters and I were in about a week ago at 69. Only stayed in about 15 min and I had to force them out even though they couldn't talk without stammering every word. "Dadadadddddddyyy wiwiwillllll yoyoyouuu gggigivvve mmmmeeee a pipipipiggggyyyyy bbbbaaccck rrrriridddde". Pool has been down to 66, but is back up to 70 last night, might get another swim in yet.

    My chlorine usage has dropped off a lot lately as well, there were days when I didn't add any chlorine at all.
    Dan D
    Used 2003 Aqua Leader 27'x52", 17,800 w/10" hopper, SwimPro SW256T 250# sand filter, Hayward PowerFlo LX pump 1hp impellor, Emerson 1 1/2 hp motor, setup Aug 2012 Summer 2011 used Summer Escapes Ring pool 14' x 42", Intex 1600gph sand filter, HTH 6 way test kitTF-100 w/stirrer, Well water @ FC=0, PH=7.2, TA=290, CH=320

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    iam4iam's Avatar
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    Re: strange FC readings

    I jumped in (70 degrees) after a run earlier this week and couldn't get out fast enough. The only reason the pool is still open is I'm waiting for "closing temp" of 60. Anyway, based on readings yesterday and today, it seems that either the measurement process is unreliable (more than 10% error) or something very strange is happening in my pool. My conundrum is that a process that has been very reliable all summer wouldn't suddenly become unreliable. (I did have an occasional "huh?" reading, but they were few and far between. The majority of my readings over the last week have been of the "huh?" variety.)
    See attachment below.[attachment=0:3uyxrs7m]pool stats.xls[/attachment:3uyxrs7m]
    Given: CYA = 70 and 32 oz bleach = approx. +1 ppm FC
    Lines 355-363 ==> depletion at 0.3 ppm/day for 5 days*
    Lines 363-365 ==> depletion at 2.5 ppm/day for 1 day**
    Lines 365-367 ==> depletion of 0.5 ppm/day for 1 day

    *surprisingly low; led to original post in this topic
    **more than on days with temps near 100* (See lines 144-155 ==> depletion at 1.5 ppm/day for 3 days)

    Am I making a mountain out of a molehill? Maybe. I do tend to do that. Do I need more sleep? Definitely. Have I gone nuts? TBD
    Attached Files Attached Files
    ~18,000 gal (16x32)
    Hayward sand filter
    Circupool RJ45 SWG
    Hayward Max-Flo XL SP2305X7 pump (0.75 HP)

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    Re: strange FC readings

    All well within normal chlorine loss/demand IMHO. Wind, weather, sun, temp, ANYTHING getting into the pool, etc.

    Lawn treatment?
    18x36 inground vinyl lined, Hayward sand filter and tablet chrolinator, approx 18k gallon

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    Re: strange FC readings

    When the water is cooler, the chlorine loss that is due to temperature is much lower, but the loss due to sunlight is independent of temperature and depends only on the hours of sunlight, angle of sun, and if it is obscured (i.e. clouds). So were your days cloudy at all when the chlorine loss was low and did you have a sunny day when the loss was higher?
    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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    iam4iam's Avatar
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    Re: strange FC readings

    Quote Originally Posted by chem geek
    So were your days cloudy at all when the chlorine loss was low and did you have a sunny day when the loss was higher?
    Not particularly.

    I assume the standard deviation of FC level in a population of water samples of a particular size is very small--that is, FC distribution is very evenly distributed--right? (I was thinking maybe the "huh?" readings were taken on "outlier" water samples. I did get several very unexpected readings earlier during the summer. From now on, I wonder if I should just l take another sample to verify. The few times I've taken second samples, I've never had a variation of more than 0.5 ppm, I don't think, which doesn't necessarily even indicate variation in FC distribution, since the test itself is only reliable to +/- 10%.)
    ~18,000 gal (16x32)
    Hayward sand filter
    Circupool RJ45 SWG
    Hayward Max-Flo XL SP2305X7 pump (0.75 HP)

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    Re: strange FC readings

    The test is usually very consistent, but it is very sensitive to contamination due to the small water sample size. If you dipped your finger into the water sample, it would have an effect, or if the cylinder wasn't thoroughly rinsed. See this thread for an example of that. However, in your case, your unusual drop wasn't a one-time drop with the next measurement was back up again so this explanation is probably not what happened.

    Even with the variation you saw, you probably don't need to do another sample since it didn't look to be a single sample error, but rather an unexplained more rapid drop in one day.
    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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    iam4iam's Avatar
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    Re: strange FC readings

    What about the consistency of water samples? How evenly dispersed is the FC (in whatever form it is in that reacts with the testing reagent) throughout the pool?
    ~18,000 gal (16x32)
    Hayward sand filter
    Circupool RJ45 SWG
    Hayward Max-Flo XL SP2305X7 pump (0.75 HP)

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