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Thread: How do CYA and FC levels affect overall amt. of bleach

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    Wildcat's Avatar
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    How do CYA and FC levels affect overall amt. of bleach

    As I am partially draining my pool to bring down CYA levels so I can safely maintain a lower FC level, a thought occurs to me.

    I understand that at normal CYA levels (60-80) FC can be much lower than at my 150 CYA levels. And as my FC drops a point or two I have to add X amount of bleach. Let's say the minimum amount of FC at my CYA is 14. If it drops to 12 I need to bring it up by adding X amount of bleach. What if my CYA is 80 and minimum FC is 8 (just a hypothetical example) and it drops to 6. Wouldn't I add the same amount to bring it up two points as I would to go from 12 -14?

    I know I could use the pool calculator but I am trying to understand the overall logic. Also doesn't mean I will use trichlor again.
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    Mod Squad woodyp's Avatar
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    Re: How do CYA and FC levels affect overall amt. of bleach

    2 ppm of FC is 2 ppm of FC based on your gallons. You're talking about minimum levels. You need to raise FC to the target levels so as it burns off, it never drops below the minimum.
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    JasonLion's Avatar
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    Re: How do CYA and FC levels affect overall amt. of bleach

    At higher CYA levels you need a higher FC level. But at the same time, higher CYA levels means less total chlorine added to maintain that FC level. Or, to say that another way, more CYA means less total chlorine added to the pool.

    However, higher CYA levels have some other problems associated with them, which make CYA levels above 80 or 90 impractical. First, the common CYA test reads CYA levels higher than 100 as 100, so there is no easy way to know what your CYA level actually is when it is above 90. There are some workarounds for this, such as doing the CYA test with dilution, but they suffer from significant loss of precision in the measurement. Higher CYA levels also mean higher FC levels, which eventually get to the point where they start to interfere with the PH test. PH tests are only reliable at FC levels up to about 10, and start to drift off from reality above that. There also start to be some more complex interactions that aggravate this further at higher CYA levels. For example if you ever need to SLAM the pool it becomes nearly impossible to maintain the PH, as the PH test stops working completely, yet adding chlorine tends to shift the PH temporarily out of range, reducing the effectiveness of the chlorine even further and making proper SLAMing almost impossible. There are a couple of other issues, more difficult to explain, but I think this is enough to give you an idea of the challenges at higher CYA levels.
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    Mod Squad jblizzle's Avatar
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    Re: How do CYA and FC levels affect overall amt. of bleach

    What you said is correct, 2ppm is 2ppm. Actually, at higher CYA levels, the daily loss of FC is actually lower. The issue is if something goes wrong, shock level gets to me quite high.
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    Re: How do CYA and FC levels affect overall amt. of bleach

    Quote Originally Posted by jblizzle
    What you said is correct, 2ppm is 2ppm. Actually, at higher CYA levels, the daily loss of FC is actually lower. The issue is if something goes wrong, shock level gets to me quite high.
    This answers the question I was asking, the other responses gave me good input as to while it may not take more chlorine, and could take less with high CYA, there are many other good reasons to get my CYA down. Which I am slowly doing. I know dilution is not as good as draining the 50% or more I need to drain, it is what seems prudent here in Tucson. Going into the 100s again this week after a brief respite of mid 90s.

    Thanks to all for the input. I can follow directions but I always like to know the "why"
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    Re: How do CYA and FC levels affect overall amt. of bleach

    Thats one of the main points of this site. Teaching the why. Rather than just telling to add x amount of this and x amount of that( like the pool stores do)
    Nathan
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