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Thread: Please explain relationship of CYA and sun; chlorine loss

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    Question Please explain relationship of CYA and sun; chlorine loss

    I've hunted around on the site and have not found an explanation of the exact nature of the relationship between the CYA in the pool and the loss of chlorine levels when the sun is out. I read that if my CYA is at 50, chlorine won't be lost to the sun.

    And I see this on one of the PoolSchool pages:

    "If your CYA level is too low, most of your chlorine will be lost to sunlight, instead of fighting algae"

    Can someone tell me exactly how the chlorine is lost to the sunlight? A chemcial reaction? If so, what?

    And second question:

    If my CYA is at 50, is the only other possible explanation for chlorine loss that organics in the pool (algae or other) are consuming it in a chemical reaction?

    And third (and follow-up) question:

    If organics are consuming chlorine in a chemical reaction, would I necessarily see the results of this in the form of some kind of precipitate on the bottom of the pool? Is there any way that large amounts of chlorine would be consumed, but there would be no evidence of precipitate on the bottom of the pool when the water is clear?

    Thank you in advance for shedding light on this whole subject!
    ~27,000 gallon IG kidney-shaped vinyl pool, built in early '80s, about 8' in deep end
    Hayward Sand filter, Hayward pump, Taylor K-2006 FAS-DPD

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    Mod Squad jblizzle's Avatar
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    Re: Please explain relationship of CYA and sun; chlorine loss

    I will not go into the details, since I do not know them off hand

    The higher the CYA, the more FC that is protected from the sun.
    At a CYA of 50ppm, you may lose about 50% of your FC to the sun everyday.
    At a CYA of 0ppm, you will lose all your FC to the sun in a few hours.

    There is always some FC loss to the sun ... following the recommended levels for CYA and FC, you can expect to have to add 2-4ppm of FC everyday.

    You can certainly lose a lot of FC to something in the water and not see anything ... one example would be ammonia in the water.
    Jason, TFP Moderator
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    Re: Please explain relationship of CYA and sun; chlorine loss

    Thank you for your quick response. I'm actually hoping for the details.

    I don't think ammonia is an issue here--no CC.

    I'm curious: Where did you find the information regarding 50% loss of FC to the sun every day? And are you saying 50% of whatever amount happens to be in the pool?
    Thanks again!
    ~27,000 gallon IG kidney-shaped vinyl pool, built in early '80s, about 8' in deep end
    Hayward Sand filter, Hayward pump, Taylor K-2006 FAS-DPD

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    Divin Dave's Avatar
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    Re: Please explain relationship of CYA and sun; chlorine loss

    basically what happens is that the UV from the sun breaks chlorine down very quicly.

    Im borrowing the following from previous posts on this subject.
    2OCl- --> O2 + 2Cl-

    Liquid Chlorine produces Hypochlorite ions, which do the work of destroying organics. However, they're relatively unstable, and bombardment by UV light splits them into O2 (Oxygen molecules) and a Chlorine ion...which then, yes, outgasses.

    There is much more technical detail in FC breakdown and use of CYA as a buffer in this post. This should hopefully do it for you on the technical details
    Pool Water Chemistry
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    IG Vinyl, 15' x 30', 3 1/2' - 6' deep, Oval, ~15K gal, Intelliclor IC40, Intelliflo VS pump, Clean and Clear 420 Filter, auto-fill-disabled, Retrofit LED Color Light, Dolphin Nautilus Robot, TF100 Test Kit, Taylor K1766 Salt Test Kit, Tftestkit Pressure Gauge.
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    Re: Please explain relationship of CYA and sun; chlorine loss

    Thank you for this part! I'll look it over carefully.
    ~27,000 gallon IG kidney-shaped vinyl pool, built in early '80s, about 8' in deep end
    Hayward Sand filter, Hayward pump, Taylor K-2006 FAS-DPD

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    Re: Please explain relationship of CYA and sun; chlorine loss

    Reply to jblizzie,

    Ah, the link provided by Divin' Dave below provided an answer to my question regarding your post about the percentage of chorline loss and the CYA:

    "A rough rule of thumb that applies is that at a pH of 7.5, the effective chlorine level is reduced by a factor about equal to the ppm of the CYA. So, a CYA of 30 ppm reduces the disinfecting chlorine (HOCl) level to about 1/30th of what it would be with no CYA."

    So, as I read this, at a pH of 7.5 and a CYA level of 50ppm, the disinfecting chlorine level would be reduced to 1/50 of the level with no CYA, not allow the sun to reduce the chlorine by 50%.
    Last edited by contrite&reformed; 05-30-2015 at 01:56 PM. Reason: To clarify to whom reply was directed
    ~27,000 gallon IG kidney-shaped vinyl pool, built in early '80s, about 8' in deep end
    Hayward Sand filter, Hayward pump, Taylor K-2006 FAS-DPD

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    Mod Squad jblizzle's Avatar
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    Re: Please explain relationship of CYA and sun; chlorine loss

    You are mixing 2 different pieces of information. I stand by my statement as a rough ball park figure.
    Jason, TFP Moderator
    18k IG pebblesheen pool, Hayward ProLogic P4 w/ T-15 SWG, Pentair 1HP 2-speed Superflo, Hayward 6020 DE filter
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    Richard320's Avatar
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    Re: Please explain relationship of CYA and sun; chlorine loss

    Scroll down to Chlorine/CYA Relationship in this thread
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
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    Re: Please explain relationship of CYA and sun; chlorine loss

    Thank you!
    ~27,000 gallon IG kidney-shaped vinyl pool, built in early '80s, about 8' in deep end
    Hayward Sand filter, Hayward pump, Taylor K-2006 FAS-DPD

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    JoyfulNoise's Avatar
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    Re: Please explain relationship of CYA and sun; chlorine loss

    See this Thread for a detailed discussion of chlorine chemistry in water
    Matt
    16k IG PebbleTec pool, 650gal spa, spillway and waterfall, 3HP IntelliFlo VS / 1.5HP WhisperFlo, Pentair QuadDE-100 filter, IC40 SWCG, MasterTemp 400k BTU/hr NG heater, KreepyKrauly suction-side cleaner Dolphin S300i robot, EasyTouch controls, city water, K-1001, K-2006 and K-1766 test kits, Mannitol test for borates

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    Re: Please explain relationship of CYA and sun; chlorine loss

    Thank you!
    ~27,000 gallon IG kidney-shaped vinyl pool, built in early '80s, about 8' in deep end
    Hayward Sand filter, Hayward pump, Taylor K-2006 FAS-DPD

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