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Thread: CYA Dilution Reality Check

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    sfbayjay's Avatar
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    CYA Dilution Reality Check

    Getting ready to do a partial drain and refill of my pool to address elevated CYA levels inherited from prior homeowner.

    When I did previous CYA testing myself a few weeks ago (Taylor kit), light conditions weren't right (didn't know then).

    Just repeated test on a very sunny day here in FL - sunshine at my back, test vial in the shadow of my own body and held at waist level. Concrete on pool deck is very light, almost white, but vial was held in a way where there was shade underneath - no direct sun.

    Since I knew the value was going to be high, and to be sure I had good resolution on the test, here's how I prepared the sample:

    Mix 10 mL pool water and 20 mL distilled water in a small glass (all measured with graduated syringe) - stir well.
    Take 7 mL of the mixed sample and put it in the CYA mixing bottle.
    Add 7 mL of CYA test reagent as usual.
    Perform test and multiply result by 3 to account for 1 part pool water and 2 parts distilled water used in test.

    When actually filling the black dot vial, near the endpoint, I would temporarily lose sight of the dot, but if I moved my head ever so slightly I would regain "focus" and could barely make it out again, so added a few more drops until it was really and truly "gone". I repeated the fill test about 5 times and got very similar results, ranging from 45 to 50 ppm each time. So actual CYA would be 135-150, call it 145.

    Does all this sound about right? Important for me to feel confident here as I am getting ready to drain the pool and I don't want to over- or under-shoot on volume adjustment.
    Pool: 12,000-gal IG Diamond-Brite, Test Kit: Taylor K-2006
    Filter: Pentair FNS Plus Fiberglass DE FNSP 36
    Pump: Sta-Rite SuperMax HP Single-Speed, 1 HP
    Skimmer: Pentair Pac-Fab "SkimClean"

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    Richard320's Avatar
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    Re: CYA Dilution Reality Check

    Yes, it sounds just perfect.
    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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    sfbayjay's Avatar
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    Re: CYA Dilution Reality Check

    Thanks for the quick reply!

    One nagging worry has to do with the inherent accuracy of the test, which I understand to be around +/-10 ppm. If my result is 10 ppm lower than actual, I would have to multiply the error by 3 (to account for dilution), so actual value could be something like 175 (145 + 30 points of possible error). The difference between 145 and 175 represents about a 1000 gallon difference for my 12000 gallon pool in terms of amount to drain to hit a new CYA target.

    I suppose it would be better to over-shoot, as I can always add some CYA back if I end up too low, but I'd have to drain again to remove more. Ugh - hate the pucks the prior owner was using!
    Pool: 12,000-gal IG Diamond-Brite, Test Kit: Taylor K-2006
    Filter: Pentair FNS Plus Fiberglass DE FNSP 36
    Pump: Sta-Rite SuperMax HP Single-Speed, 1 HP
    Skimmer: Pentair Pac-Fab "SkimClean"

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    Richard320's Avatar
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    Re: CYA Dilution Reality Check

    Once again, you got it.

    It is better to drain more than the minimum. Raising CYA is really easy if you overshot.
    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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    sfbayjay's Avatar
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    Re: CYA Dilution Reality Check

    Once I drain and refill I'm hoping to land at somewhere near the top of the TFP recommended range for CYA, around 50 ppm or so.

    Should I expect to see my chlorine use go up significantly since I'll be cutting my CYA basically by 2/3?

    My pool is relatively shallow and sees direct sunshine for about 10 hours each day. It is under a screened enclosure, which I've read can (maybe) slow down chlorine consumption due to sun. Right now I'm holding FC at about 14 as a conservative minimum and dosing with liquid chlorine every other day to hold it there. If I dose up to 17 ppm it takes about 2 days to drop back to 14, so I'm using maybe 1.5 ppm per day of chlorine due to sunshine. More on weekends when there are more swimmers.
    Pool: 12,000-gal IG Diamond-Brite, Test Kit: Taylor K-2006
    Filter: Pentair FNS Plus Fiberglass DE FNSP 36
    Pump: Sta-Rite SuperMax HP Single-Speed, 1 HP
    Skimmer: Pentair Pac-Fab "SkimClean"

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    Richard320's Avatar
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    Re: CYA Dilution Reality Check

    Maybe a little. Not so much that your wallet will feel the pinch.

    If you're of a scientific bent, study this graph from Pool Water Chemistry
    Note that CYA is the bottom scale. The graphs practically overlap above 50

    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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    sfbayjay's Avatar
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    Re: CYA Dilution Reality Check

    Hey that's an interesting chart - thanks for posting it. If I'm understanding it correctly, if I have 100 ppm CYA and start with 10 ppm FC, then I would expect to have only 5 ppm FC after about 7.5 hours? That's a much higher burn rate than I am seeing with my current situation. I know my CYA level is in excess of 100, maybe around 150, but the curve seems to level off at around an 8.5 hour half life.

    Maybe what I'm missing is that some of the FC that is bound to CYA is "released" to replenish the stuff that burns off? I guess I'm not sure whether chlorine that is bound to CYA is included in FC...

    At any rate, I like the chart, and I also understand from your reply that I might see some accelerated chlorine usage, but not so drastically that I'm going to break the bank. Right now I calculate that I'm spending about $0.21 per day for chlorine - not a huge sum. Hopefully it won't go up too much.
    Pool: 12,000-gal IG Diamond-Brite, Test Kit: Taylor K-2006
    Filter: Pentair FNS Plus Fiberglass DE FNSP 36
    Pump: Sta-Rite SuperMax HP Single-Speed, 1 HP
    Skimmer: Pentair Pac-Fab "SkimClean"

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