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Thread: 90% Water Change & CYA

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    90% Water Change & CYA

    Hello all; new to the forum after getting bum steers from the local pool store on why my Cl level wouldn't hold the past two months. After a different answer (and purchase) every time I went in, I learned on this forum that the reason I was having issues was my CYA level of 100 was not allowing my Cl (between 6 and 0, every two days) to work. 15 minutes on this site convinced me to skip the pool store and go to Bleach. I've given up on powdered shock, but still use the pucks, bleach, and acid (the psuedo-BBB).

    To my point: I am perplexed by my TF-100 kit showing CYA at around 15 (hard to tell because 20 is the low limit), but the pool store told me it was 45 last week, and 60 today. TF-100 test seems pretty straight forward and has given me similar results twice (the only two times I've used it). Can I rely on the pool store for an accurate level? Is that test so subjective that it could vary by 30% in a week? They also told me that because plaster is porous, it can retain CYA, even after a water change and may "leech" back into the water. Sounds like baloney to me?

    Thanks in advance for any insight.

    12,700 gal IG plaster
    Ph 7.4
    CL 4.0
    CYA Somewhere between 15 and 60
    TA 110
    CH 210
    Phos 0
    TDS 750

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    Re: 90% Water Change & CYA

    Can I rely on the pool store for an accurate level?
    Absolutely not. As you continue to read, you will learn an important part of what we teach is to TRUST YOUR OWN TESTING. If you have filled the CYA View Tube up to the 20 ppm mark and you can still see the dot, then your CYA guesstimate of 15 ppm is just fine.

    But now I am puzzled....what did you do to reduce your CYA from 100? You don't mention that.

    Also, tell us how your water looks.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Mod Squad jblizzle's Avatar
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    Re: 90% Water Change & CYA

    If your CYA was truly 100 (that is the test max and a pool store may report 100 for anything over that), and you replaced 90% of the water as the title suggests, then logically the CYA should be around 10ppm.

    But, since likely your CYA was above 100ppm to start and it is hard to estimate 90%, 15ppm sounds very reasonable.

    The CYA test is the one the pool stores are the worst at, partly because they do not use the correct lighting ... as Dave said, trust yourself!!!
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    Re: 90% Water Change & CYA

    "But now I am puzzled....what did you do to reduce your CYA from 100? You don't mention that.

    Also, tell us how your water looks."

    I changed 90% of the water. Water looks better than it ever has!

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    Re: 90% Water Change & CYA

    OK, so are you looking for a next step or were you simply inquiring about your 15 ppm CYA?
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: 90% Water Change & CYA

    I was worried after a 90% water change my CYA was rapidly increasing (the porous plaster???), and how it could go from 30-45 in a week, or just trust my readings. I think I'll trust my readings.

    My Cl seems stable; I"m not inclined to add any stabilizer to my pool since I'm planning to continue using tri-chlor pucks.

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    Re: 90% Water Change & CYA

    I"m not inclined to add any stabilizer to my pool since I'm planning to continue using tri-chlor pucks.
    That's fine. Just monitor your FC (you'll probably need quite a few pucks) and then monitor your CYA build-up and your pH...Pretty easy to get pH too low with pucks.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: 90% Water Change & CYA

    Quote Originally Posted by IanB13
    I was worried after a 90% water change my CYA was rapidly increasing (the porous plaster???), and how it could go from 30-45 in a week, or just trust my readings. I think I'll trust my readings.

    My Cl seems stable; I"m not inclined to add any stabilizer to my pool since I'm planning to continue using tri-chlor pucks.


    If you add 2 ppm FC per day, which is pretty average, using trichlor, you will also be adding about 8.4 ppm CYA per week.

    In two weeks, you will have added over 16 ppm. In 12, over 100 CYA. If you're at 45 now, by Labor Day you'll be about 150. You better start worrying about your CYA now.
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
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    Re: 90% Water Change & CYA

    If you add 2 ppm FC per day, which is pretty average, using trichlor, you will also be adding about 8.4 ppm CYA per week.

    In two weeks, you will have added over 16 ppm. In 12, over 100 CYA. If you're at 45 now, by Labor Day you'll be about 150. You better start worrying about your CYA now.[/quote]

    Wow. Didn't realize the pucks added quite so much CYA. Will try going with "B" only. Suppose that's why my CYA has increased far faster than expected; thanks for the tip Richard320.

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    Re: 90% Water Change & CYA

    We switched to BBB about a year ago when a problem resulting from high CYA reared its ugly head, and have never looked back. I've not used a single puck since late last June. We didn't do a huge water replacement, but several times over last summer we'd drain an amount we knew we could refill within a reasonable amount of time. By the end of the summer we were below 100, and we're now around 70. Goal this summer is to get it to around 40. Testing CYA is my lowest confidence test, but I trust my reading more than the pool store's, so there's that.
    20,000 gal. free-form plaster on concrete
    Triton DE filter, 1.5HP pump
    Aquabot Elite pool cleaner

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