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Thread: FC level surprise rise - need a mathmetician?

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    MiaOKC's Avatar
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    FC level surprise rise - need a mathmetician?

    Hello,
    I'm in the final phases of clearing our rain-mudded pool. It is mostly clear (slight cloudiness to the water in the deep end) and I wanted to pass the OCLT test and call it done since we are having a pool party Saturday. Our CYA had been at 30 for part of the swamp session but I was working to raise it to closer to 40-50 - we are in a full sun, extreme heat location and 30 did not seem optimal for us last year. So part of my shocking process was done targeting a CYA of 30 and part of it was done at 50 as of a positive test result of CYA at 50 on Saturday.

    Two nights ago, I tested just after sundown and found the FC at 9.5, CC was 0. Did not add any chlorine, was going to see if the previous 11 days at shock levels of FC was enough and hadn't attempted the OCLT until now. Got up early and my husband tested that morning and found FC of 11.5, CC 0. We assumed tester error (mine or his, who knows) to explain the rise in FC even though we didn't add any and hadn't floated any pucks in the ducks or anything (now that our CYA is where we want it, ducks are grounded).

    So last night, we test together after work so there are no discrepancies . Came up with 7.5 FC, 0 CC. (I thought that jived with a loss of 2ppm during the day if in fact my test the night before was correct but DH's that morning was off, but being a good wife, I didn't rub it in ... too much). Especially since I don't know definitively that I'm right... I try not to let that stop me.

    We decided to bring the FC up to the shock level of 20 based on our new CYA of 50. We used the newer pool calc and calculated our 35000 gal pool to need 5.5 jugs of 8.25% 121 oz. We didn't have 5.5 jugs on hand, we had 4 available, so put them in. Messing with the data in the calculator, it seemed that adding four jugs should take our pool to 16.75 FC. We thought if we pass OCLT, it wouldn't matter terribly if we are at 16.75 or 20. I have experienced that the cloudiness left behind can take a long time to completely clear with our sand filter and large pool.

    Weeeeelll, when DH tested this morning, he found FC of 21.5! What the what?! Two days where the FC readings are higher than we thought they should be. We are baffled. We have a few theories and would love your input and ideas:

    A. Our pool volume is smaller than we thought. This is possible, as various experienced pool guys have looked at it and guessed anywhere from 30-40K gallons, and I've tried to measure and calculate the volume myself but with our steeply pitched deep end it's a little difficult for my remedial geometry skills and have come up with somewhere in the neighborhood of 33K (picture me lying flat on the end of the diving board with a tape measure). Playing with Pool Calc I figured that four jugs of our bleach would take a 23K pool from 12.5 to 21.5 FC. Not sure I would buy that our pool is only 23K.

    B. Our jugs of laundry chlorine from Wal-Mart have super powers and/or are higher concentration than listed

    C. Testing errors, although I don't think this is the case. We tested together last night and are confident that we are following the instructions well WITH THE EXCEPTION of adding the powder. I'd read here that one scoop was sufficient if the water turns hot pink and so would conserve the powder, and so we've added one heaping spoonful of the powder instead of 2, and it's REALLY pink, and we add the drops from there. Using the 10ml sample method.

    I would appreciate any thoughts the people here can provide. I know you are smarties! Thanks!
    35,000 gallon gunite pool, semi-T-shaped, 29x15 shallow end and 16x22 deep end, built in the mid-1970s and renovated 2013-2014 // Polaris 9300 Sport robotic cleaner // Taylor K-2006 ... Learning as we go and so grateful to everyone here for their help! Our reno thread

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    JasonLion's Avatar
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    Re: FC level surprise rise - need a mathmetician?

    Walmart chlorine tends to be 8.25% these days, to match the increase in concentration that Clorox recently made. Check the label on the bottles to be sure.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    Mod Squad Bama Rambler's Avatar
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    Re: FC level surprise rise - need a mathmetician?

    I get that 484 oz (4ea 121oz) of 8.25% would raise the FC by a little less than 10 ppm in a 35,000 gallon pool. Either your jug size or the concentration or both were possibly off.
    Dave J. TFP Moderator
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    Patrick_B's Avatar
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    Re: FC level surprise rise - need a mathmetician?

    Pool volume, and bleach percent error entered into pool calc are two guesses. Nothing is WAY off, but it's interesting though...and I know my nerd is showing by saying that. Yours is too but it is fun!
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    Patrick_B's Avatar
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    Re: FC level surprise rise - need a mathmetician?

    Dangitt, Jason and Dave! You beat me to it.
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    MiaOKC's Avatar
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    Re: FC level surprise rise - need a mathmetician?

    These are Great Value (WalMart brand) 8.25% 121 oz. jugs, I'm almost certain - will eyeball the label again when I get home from work. That and the 8.25% 64oz jugs are all WalMart has carried in our area for a while now (these are both the 33% more concentrated that's been on the market for a while, which is why I was so glad that there was a new Pool Calc added here: calc.html and that's what I've been using).
    35,000 gallon gunite pool, semi-T-shaped, 29x15 shallow end and 16x22 deep end, built in the mid-1970s and renovated 2013-2014 // Polaris 9300 Sport robotic cleaner // Taylor K-2006 ... Learning as we go and so grateful to everyone here for their help! Our reno thread

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    Butterfly's Avatar
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    Re: FC level surprise rise - need a mathmetician?

    So last night, we test together after work so there are no discrepancies . Came up with 7.5 FC, 0 CC. (I thought that jived with a loss of 2ppm during the day if in fact my test the night before was correct but DH's that morning was off, but being a good wife, I didn't rub it in ... too much). Especially since I don't know definitively that I'm right... I try not to let that stop me.
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    MiaOKC's Avatar
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    Re: FC level surprise rise - need a mathmetician?

    UPDATE: Have been re-testing over the last few days and a new theory has emerged after we have A.) confirmed we are using 121 oz 8.25% jugs and B.) DH and I are using the exact same testing procedures.

    I think the problem that the circulation in the pool had not mixed the bleach up much from the evening dosing and testing and the morning testing. We run our pump overnight (and when shocking, as close to 24/7 as possible with our utilities schedule), but tend to dose most of our bleach by walking around a few edges of the pool (remember, I have a weird T-shape and so there are lots of twists and turns, and I run out of bleach before I run out of edges to walk). I think DH was pulling water from a fairly "dead spot" in one corner to test with, and that the bleach there had not sufficiently diffused, while I was pulling from an area nearer to a return.

    OCLT test passed with Wednesday night's FC 9.5 down to 8.5 and CC .5 on Thursday morning, and just to be extra sure, OCLT passed Thursday night with FC 6.5 down to 6.0 with CC 0 this morning. Yay! Thanks, TFP!

    A question about overnight chlorine loss... is there a typical ppm of loss to expect each night (i.e., is .5ppm the normal loss once your pool is healthy? Or should it be 0?) I'm trying to figure out our dosing schedule/bleach shopping schedule to make our pool care more streamlined this summer (and to keep it clear so no more shocking required!) Thanks!
    35,000 gallon gunite pool, semi-T-shaped, 29x15 shallow end and 16x22 deep end, built in the mid-1970s and renovated 2013-2014 // Polaris 9300 Sport robotic cleaner // Taylor K-2006 ... Learning as we go and so grateful to everyone here for their help! Our reno thread

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    Mod Squad Bama Rambler's Avatar
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    Re: FC level surprise rise - need a mathmetician?

    Ideally the overnight loss should be zero, but the .5 allowance is for testing errors and/or a little sunlight before or after the tests.

    If you have circulation issues, you need to do the best you can at aiming the eyeballs and running the pump to maximize the mixing. Even adding a cleaner to help may be something to consider.
    Dave J. TFP Moderator
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    MiaOKC's Avatar
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    Re: FC level surprise rise - need a mathmetician?

    Thanks for the info, Bama. I'll keep testing and hope that our loss goes down to 0 consistently. Some of our returns get wonky (a few of them have loose eyeballs due to the age of the pool, even after we've replaced the fittings) when we brush and they might not get re-aimed properly too quickly. I try to keep them pointed somewhat towards the skimmers to help push leaves over there. We also have a Polaris 9300 that gives the pool a daily or even twice daily scrub, I need to add that to my signature. He's not too bright, though, and doesn't go in that one corner where I want him to! We thought we didn't need the remote-controlled robot, but have since found I am super-picky about seeing a stray leaf here or there that the robot sails right by... remote control is on the must-have list for the next robot.
    35,000 gallon gunite pool, semi-T-shaped, 29x15 shallow end and 16x22 deep end, built in the mid-1970s and renovated 2013-2014 // Polaris 9300 Sport robotic cleaner // Taylor K-2006 ... Learning as we go and so grateful to everyone here for their help! Our reno thread

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