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Thread: CYA dropping out of suspension

  1. Back To Top    #1
    TimS's Avatar
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    CYA dropping out of suspension

    This is another of my "I'm just curious" questions, so most of you can simply shake your heads and move on.

    This quote is from another thread:
    Quote Originally Posted by JasonLion
    CYA can crystalize somewhere and then redissolve later. This isn't common, but it does happen, usually after CYA levels have been astronomically high. I don't believe that it has been established exactly where CYA tends to crystalize, perhaps in the plumbing or within the surface of the plaster, but it does happen now and then. However, even if the hidden CYA is in the plaster, brushing doesn't help remove it.
    First, let me say that I would normally get the water sample from much deeper in the pool, and not while the pump had been off for any length of time, but...

    Last week, after the pool finally thawed, I pulled out some water to test. Since the pool was still covered, and the water level was still down, I took my sample from only about 3" below the surface by sticking my hand through the skimmer. (I did this mostly because I was bored and wanted to play with my chemistry set. ) When the pool is open, I get my samples from about 18" deep, but I couldn't reach that far this time. Keep in mind, that this is under a solid black winter cover, so there shouldn't be much UV getting to the surface of the water.

    At that time, my FC had dropped to 0.5, CC was <0.5 but was >0.0, and my CYA was down to 40. When I closed last fall, my FC was 6 and my CYA was 70. Since my FC was so low, and I didn't want to deal with an algae bloom later, I went ahead and filled the pool up again and got the pump running.

    After running the pump for about 6 hours, I tested again, again getting the sample the same way. This time, the FC = 2, CC = 0, and CYA = 50. Since I added 8" of water, that is a 20% addition, which should have diluted the chemicals by 20% as well, correct? So, my CYA should have been down to 32, and FC below the bottom end of the scale.

    OK. I added about 8" of water, so this means I replaced about 17% from my fall level. CYA 70 - 17% = 58, FC would slowly drop anyway, even with the pool covered, so we won't worry about it.

    Finally, after all that, here are my questions:

    Since the water sat dormant for 5 months could the CYA and chlorine drop partially (but not fully) out of suspension so that the top layer had lower levels than the rest?

    My water froze to at least 3" over the winter. When the water freezes, do the chlorine and CYA drop out of suspension? This would easily explain why the top few inches had lower levels before the water was stirred up again, and the numbers climbed back up after the pump was running. If that was the case though, I'd expect the numbers to be even lower, since the sample came from the top layer.
    24' AG Round (vinyl replaced 0909) - 13500 gal - Sand Dollar Filter (150lb) - Dynamo DYNII-N1-1 1HP - Hayward HP380 Heat Pump - TF-100.
    Central Missouri

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  2. Back To Top    #2
    JasonLion's Avatar
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    Re: CYA dropping out of suspension

    When the surface of the pool freezes, very few of the common dissolved chemicals go into the ice. Then when the ice melts it only mixes partially with water that is nearby, creating a layer on top that has lower levels than the pool would have if it were fully mixed. When you turn the pump on again everything mixes together again and becomes relatively uniform. How much mixing occurs before you turn on the pump depends on a variety of factors that are difficult to predict, but some mixing will always occur. Unless you sample at a large number of different depths, testing water that is not throughly mixed is fairly pointless.
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  3. Back To Top    #3
    TimS's Avatar
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    Re: CYA dropping out of suspension

    Quote Originally Posted by JasonLion
    When the surface of the pool freezes, very few of the common dissolved chemicals go into the ice. Then when the ice melts it only mixes partially with water that is nearby, creating a layer on top that has lower levels than the pool would have if it were fully mixed. When you turn the pump on again everything mixes together again and becomes relatively uniform. How much mixing occurs before you turn on the pump depends on a variety of factors that are difficult to predict, but some mixing will always occur.
    I thought that might be it.

    Quote Originally Posted by JasonLion
    Unless you sample at a large number of different depths, testing water that is not throughly mixed is fairly pointless.
    Yeah, but it's still fun to play with my chemistry set.
    24' AG Round (vinyl replaced 0909) - 13500 gal - Sand Dollar Filter (150lb) - Dynamo DYNII-N1-1 1HP - Hayward HP380 Heat Pump - TF-100.
    Central Missouri

    Before I speak, I have something important to say.

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