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Thread: Understanding CYA and FC

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    Understanding CYA and FC

    I think I've read enough to understand the basics of FC vs CC vs TC, and how your CYA levels dictate your target FC. What I'm having trouble understanding is if/how CYA "gives up" it's hold on FC when it's needed.

    It seems from what I've read, that CYA basically holds some of your FC hostage so that the sun wont break it down, but that this also makes that FC not truely "available" for sanitizing purposes. It also seems from what I've read that without CYA, the sun would rapidly cut your FC down by about half every hour or so? If this were all "absolutely" true, then wouldn't it be impossible (with a CYA level of let's say 40), for your FC reading to drop to zero? Does CYA protect your FC from sun, but then "give it up" when it's needed for sanitizing?

    Thanks for any help you can give explaining this.

    Ron
    19,000 IG Vinyl, sand filter, liquid chlor, 1HP pump

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    Mod Squad Bama Rambler's Avatar
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    Re: Understanding CYA and FC

    You're right, CYA bonds to chlorine but it's a weak bond and it realeases it as it gets used up.

    There are a few very detailed posts about it from Richard (Chem Geek) in the Deep End and Chemistry 201.
    Dave J. TFP Moderator
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    Re: Understanding CYA and FC

    I'm new to all of this, and currently going through some stuff with my pool due to a power outage. I can try to explain this though. CYA doesn't hold your chlorine hostage. It acts like a sunscreen, preventing it from being broken down AS QUICKLY as it would without cya. Chlorine lock happens when the CYA level is too high and the FC has to be brought up to extremely high levels to be effective. You can have high CYA and still have 0 FC. Check out some of the posts about water balancing, and check out Pool School. Both are very helpful.
    And if any pros out there tell you something different than I did, please listen to them and not me!!
    14k gallon oval agp, sand filter, 1hp pump, BBB method, K-2006 Test Kit
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    JasonLion's Avatar
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    Re: Understanding CYA and FC

    Quote Originally Posted by karmazrevenge
    CYA doesn't hold your chlorine hostage.
    While "hostage" isn't the word I would use, it is an apt metaphor. CYA grabs on to some (most) of the chlorine and prevents it from actively participating in sanitization, which is more or less what hostage means in this context.

    Quote Originally Posted by karmazrevenge
    Chlorine lock happens when the CYA level is too high and the FC has to be brought up to extremely high levels to be effective.
    There is no such thing as "chlorine lock", it is a term pool store people use when they either don't understand what is happening, or don't feel like taking the time to explain what is happening.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    Re: Understanding CYA and FC

    I stand corrected! This is why I'm the newbie. I was under the impression though that if the cya gets too high then the fc would also have to be very high (if you're not draining the water ). Did I misunderstand that?
    14k gallon oval agp, sand filter, 1hp pump, BBB method, K-2006 Test Kit
    Roanoke, VA

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    Re: Understanding CYA and FC

    It's just a chemical equilibrium which means that some portion, most in this case, of the chlorine is bound to CYA while some, a relatively small amount, is unbound and is active to disinfect and oxidize bather waste. There is a back and forth that goes on very frequently -- bound chlorine getting released from CYA and unbound chlorine attaching to CYA. If needed (i.e. if the active chlorine gets used up very quickly), half of the FC can be released from being bound to CYA every 1/4 second.
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