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Thread: Trying to understand

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    Trying to understand

    Bear with me...some of you may wish you hadn't read my wonderings/wanderings once you get started....

    If I understand correctly, the pro of CYA is that it helps keep the chlorine from burning off so quickly - especially when sunlight is present. Consequently, the higher your CYA, the more chlorine you need in your water to maintain clean, sanitized water. This is because some of the cholorine is "held" by the CYA? So, you have to add more cholorine to the water because some of the cholorine is basically rendered useless while it's being held by the CYA? So you first have to "fill up" (or saturate?) the CYA in your pool (ie have enough chlorine to bond with all the CYA) to the point that there is more chlorine than can bond with the CYA...leaving chlorine to kill the baddies in the water...?

    But, if that is all true, does the chlorine that is being held with the CYA ever get released? And if not, it seems like CYA wouldn't be necessary - that doesn't seem like a stabilizing thing if it just grabs the chlorine and renders it useless... I'd think that it would be more cost effective to have no CYA so that all the chlorine you add is beneficial. Which is obviously not true. Which makes me think that the chlorine must get released by the CYA...which leads me to believe that I've got the process backwards. You "saturate" the water with chlorine and then the CYA holds extra chlorine? As the chlorine is used/breaks down in the water, chlorine is released from the CYA? BUT if that were true, than maintaining a higher chlorine load (when you have a high CYA) would be optional if the chlorine is held with the water first. I should have put a disclaimer - those Chem classes and labs were a looonnng time ago. I'm sure I'm not thinking about this correctly. And probably talking you in circles.

    If there's a thread somewhere explaining all this, point me in the right direction. Or maybe I missed it in pool school. I'm going to wander back over there now that I have it clear in my head what I'm asking/thinking.

    Also, I assume that the FC tests count even the chlorine that is held by the CYA? So if it still tests as FC, why is it not effective as FC (back to why the level has to be so high with high CYA).

    Sheesh. Good luck to anyone who can make sense of that mini discussion with myself.

    Either way - higher CYA is recommended for SWCG pools...but lower FC. Can someone explain why you want to have the ability to hold more chlorine (high CYA) yet produce/maintain less chlorine with a SWCG than a non-SWCG pool with an even lower CYA?

    All this has been bugging me.
    19x43 Inground Vinyl Pool, approx 22 years old, no main drain, 2 (lopsided!) skimmers, 2 returns.
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    JasonLion's Avatar
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    Re: Trying to understand

    CYA holds on to a fixed percentage of the chlorine, the actual percentage depending on the CYA level. If the active chlorine gets used up, the CYA releases some of the bound chlorine to be active again. Effectively the bound up chlorine is held in reserve.

    Yes, the FC test counts chlorine held by CYA. It does so because the FC test uses up the chlorine, so the CYA releases the remaining bound chlorine during the course of the test.

    What happens with a SWG pool is unrelated to everything else mentioned so far. The SWG has two effects that are different from adding chlorine manually. One is that it maintains a much more uniform chlorine level, adding chlorine slowly the entire time the pump is running. The other is that some of the water that passes through the SWG cell is super-chlorinated, completely sterilized. That keeps some things that can occasionally cause problems from ever building up very high. The net effect is that a SWG pool can run at a lower FC level than a manually chlorinated pool even when both have the same CYA level.
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    Mod Squad woodyp's Avatar
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    Re: Trying to understand

    Is the minimum number per the CYA/Chlorine chart the actual amount "held in reserve" for a given CYA level? I just assumed that if my minimum level is 4 and (target of 8), if I go below that down to even 3 I would be at risk for an algae breakout. How much can you fudge the minimum number by and still be "safe"?
    16x32x52" Steel Cornelius Miramar AGP Vinyl liner 13,100 gal. Buried 2 ft.
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    JasonLion's Avatar
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    Re: Trying to understand

    Remember the amount held in reserve is a percentage, so the active chlorine level never goes to zero until all of the chlorine is gone (technically some strange things can happen for short time periods, but ignoring that).

    It takes a certain amount of chlorine to reliably kill any algae introduced into the pool. That is where the minimum levels come from. A little below that, algae will often get killed off, you just can't count on it. So you could go below the minimum for a time, even weeks, and be fine but then one day get algae. The minimums were selected by trying to find something that works reliably and doesn't have that fails occasionally problem.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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