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Thread: Chlorine/CYA relationship

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    mikemass's Avatar
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    Chlorine/CYA relationship

    I am new to this forum, but certainly not new to pool care. I have to say I have never seen so much attention to maintaining different Cl levels based upon one's CYA level. I am not really sure what I am missing, but these charts seem outrageous to me. I mean there are areas of the chart that mention 40, 50, 60!!! ppm shock levels. I've never heard of such a thing. I have never had a severe algae bloom. "IF" I see the first sign of algae, I throw 3 bags of Calcium Hypo into my 38k gallon pool and I am good.

    Most test kits don't test north of 5.0 for your Cl level. When I shock I am slightly above that. So maybe a 7.0, maybe even 8.0 to give the benefit of the doubt.

    As far as CYA levels.... If the purpose of CYA is to "hold" the free Cl from loss from the sun, why do I need to maintain a higher level Cl if I am reading say 70 as opposed to 40 on my CYA? The reason I want CYA in the first place is so I don't lose my FC. Now this forum is telling me to add more Chlorine the higher my CYA is.

    Does this seem backward to anyone else?

    And again this is coming from someone who has rarely had problems with my pool. And when I do, they are minor and simple to correct.
    25 + year pool owner. Current pool (going on 11th year) - 38K gallon in ground Gunite/White Plaster, Spillover Spa, Hayward Cartridge Filter, Hayward 450k BTU NG Heater, Hayward Goldline Aqua Logic control with T-Cell-15 Salt Water Cell, Dolphin Explorer.
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    harleysilo's Avatar
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    Re: Chlorine/CYA relationship

    Do you know what your CYA level is? Calcium level?

    If it's not broke why fix it?

    Have you read this? http://standards.nsf.org/apps/group_pub ... nt_id=5891
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    Mod Squad Bama Rambler's Avatar
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    Re: Chlorine/CYA relationship

    Once you read through some of Richard's (Chem Geek) posts you'll begin to understand why.

    It's like covering up when running the A/C. The more you cover (CYA) you have the more A/C (FC) you have to have to stay cool.
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    Re: Chlorine/CYA relationship

    I have read A LOT of Richard's posts. Very knowledgable. He's someone I would hire if I needed a person to maintain my pool. But I must say, the more I read on this topic, the more confused I am.

    Harley, thanks for the link. Going to take a look now. CYA is currently 45 (altho I think higher) according to my local pool store's "accurate" computer test. I will test myself when I get a reagent refill for my kit. (on it's way). And my Calcium is at 300. And I couldn't agree more with the "don't fix what isn't broke"
    TA = 90
    pH = 7.5
    Cl = 3.0
    25 + year pool owner. Current pool (going on 11th year) - 38K gallon in ground Gunite/White Plaster, Spillover Spa, Hayward Cartridge Filter, Hayward 450k BTU NG Heater, Hayward Goldline Aqua Logic control with T-Cell-15 Salt Water Cell, Dolphin Explorer.
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    Re: Chlorine/CYA relationship

    He's someone I would hire if I needed a person to maintain my pool.
    I am speechless.
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    Re: Chlorine/CYA relationship

    In your reading, or in searching, i'd read about Calcium levels and SWG's, i recall there is a point where issues start, but i don't recall at what level.
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    Re: Chlorine/CYA relationship

    Quote Originally Posted by harleysilo
    In your reading, or in searching, i'd read about Calcium levels and SWG's, i recall there is a point where issues start, but i don't recall at what level.
    My first cell lasted almost 6 1/2 seasons, from April 2006 - June 2012. Most guys tell me you're lucky to get 5 years out of them. So I think I'm doing something right.
    25 + year pool owner. Current pool (going on 11th year) - 38K gallon in ground Gunite/White Plaster, Spillover Spa, Hayward Cartridge Filter, Hayward 450k BTU NG Heater, Hayward Goldline Aqua Logic control with T-Cell-15 Salt Water Cell, Dolphin Explorer.
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    Re: Chlorine/CYA relationship

    For some other good discussion of CYA / FC levels that is somewhat technical, but in general at a lower level than that paper try this thread:

    chlorine-cya-chart-t2346.html

    Ike

    p.s. I too maintained a pool for many years with only basic testing / pool store testing before discovering this site, and rarely had what I then considered as problems, since switching my water is much clearer, has much less odor, does not burn eyes as much, etc. Now I always get people commenting about the water, how nice it is, etc. As to the reason you are not having more problems than you are, there are many factors, perhaps you have water that is naturally low in algae nutrients, maybe your just lucky, as chance of algae blooming in a chlorinated (under chlorinated) body of water is somewhat statistical, etc.
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    Re: Chlorine/CYA relationship

    Ike,

    That is the exact link I am talking about. So if I get my CYA level up to where this site suggests for a SWG (80), Then I should shoot for a FC cl level of 9.1? I just find that humerous as I get to that level after a shock treatment and wouldn't let my kids swim in it till the next day. I dunno, maybe it's me, but if we were to shoot for Cl levels that high, woudln't we expect the basic test kit to show a range larger than 0.0-5.0?

    Don't get me wrong. I think this site is great and have since "borated" my pool just cause it sounds cool. And I am liking the results!!!
    25 + year pool owner. Current pool (going on 11th year) - 38K gallon in ground Gunite/White Plaster, Spillover Spa, Hayward Cartridge Filter, Hayward 450k BTU NG Heater, Hayward Goldline Aqua Logic control with T-Cell-15 Salt Water Cell, Dolphin Explorer.
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    Re: Chlorine/CYA relationship

    Basic test kits are design with industry recommended levels in mind and the industry has been practicing the ostrich (head in the sand) model of dealing with what happens to active chlorine effectiveness as the CYA level rises.

    Also the methods used by the industry assume you use the shock weekly model and buy algicides etc. if you like that model you can keep FC low and ignore CYA. You will likely at some point experience trouble if you do that. The name of the forum here is trouble-free.


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    Re: Chlorine/CYA relationship

    The science behind the chlorine / CYA relationship has been known definitively since at least 1974 when in this paper the equilibrium constants between chlorine bound to CYA and chlorine that is unbound were definitively determined. What makes the chemistry complex is that there are many different chemical equilibria involved, but one can understand the principle by just looking at the most dominant chemical species at pool pH which are the following:

    HClCY- + H2O <<<---> HOCl + HCY-
    "Chlorine bound to CYA" + Water <<<---> "Active Chlorine" + Cyanurate Ion (a form of CYA)

    When CYA is present, most of the chlorine is bound to it -- that is, the chemical equilibrium shown above is way to the left. Chlorine bound to CYA is barely reactive and for practical purposes can be ignored except as a reserve or reservoir of chlorine. It is hypochlorous acid, shown on the right, that is the active chlorine that disinfects, kills algae, and oxidizes bather waste (see the "Chlorine/CYA Relationship" section in this post for more details).

    So the first thing you need to get out of your mind is the idea that CYA acts like a "sun shield" to protect chlorine. That's what many in the industry call it, but it is terribly misleading since it makes it sound like CYA and chlorine are kept separate which is not true. CYA binds to the chlorine to produce a new compound that, for practical purposes, does not react to disinfect, prevent algae, or oxidize bather waste. When CYA is present, most of the FC is inactive.

    In order to prevent algae growth, one needs a minimum active chlorine level. So the question is how does one determine that level given that all one knows are the pool water chemistry parameters we measure (FC, CC, pH, TA, CH, CYA, temp)? The answer is derived technically in this post where due to the chemical equilibrium I showed above, the active chlorine level is proportional to the FC/CYA ratio. So 3 ppm FC with 30 ppm CYA has the same active chlorine level as 6 ppm FC with 60 ppm CYA and as 10 ppm FC with 100 ppm CYA.

    As for the actual active chlorine level itself, it is very low since most of the chlorine is bound to CYA. At a pH of 7.5 and 77F (to be conservative), an FC that is at the minimum FC we recommend for manually dosed pools which is around 7.5% of the CYA level has the same active chlorine level as a pool with only 0.06 ppm FC and no CYA. That is not a typo. Fortunately, it takes a low active chlorine level to prevent algae growth.

    However, to kill off already established algae, one needs a significantly higher chlorine level since it gets used up locally quickly and must penetrate algae clumps which takes longer to do so in order to get ahead of algae growth (reproduction), one needs a higher chlorine level. Though technically something like 20% would probably be enough to kill algae faster than it grows even in a bloom, it would take longer to kill off so we use an FC that is 40% of the CYA level as the shock level for clearing a pool of existing algae. This FC/CYA ratio has the same active chlorine as an FC of 0.6 ppm with no CYA.

    So a pool with 32 ppm FC and 80 ppm CYA has an active chlorine level the same as only 0.6 ppm FC with no CYA. So these high numbers are just that, high numbers, and the only thing actually "high" is the amount of chlorine in reserve.

    Another way to think about this is considering soldiers fighting a war. The front-line soldiers have weapons for killing the enemy (though more technically a closer analogy would be that they sacrifice themselves in hand-to-hand combat with an enemy and only kill an enemy soldier when they themselves get killed). Whenever a front-line soldier gets killed, a replacement is made from soldiers in reserve. You can see that the rate of killing the enemy is related only to the number of soldiers on the front-line and that the number of soldiers in reserve is irrelevant in terms of the rate of kill. The reserve just tells you how long you can continue to fight.

    Does that make sense? If not, let me know what is still not clear and I'll see if there's another explanation that might help clarify.

    As for your worry about your kids swimming in a pool with high FC, again the FC number by itself is irrelevant towards chlorine's actual effects when CYA is present. My wife has personally experienced this difference when she has swum in an indoor commercial pool with 1-2 ppm FC and no CYA and her swimsuits would degrade (elasticity gets shot) in just one winter season of swimming and her skin was flakier and hair frizzier. In our own outdoor residential pool with 3-6 ppm FC and 40 ppm CYA, her swimsuits would last for 7 years and the effects on her skin and hair were substantially less noticeable.
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    Re: Chlorine/CYA relationship

    The problem is those tests and the industry suggested FC levels have went virtually unchanged for the last 50 years since shortly after CYA was first introduced onto the market around 1960. The same goes for the use instructions, just look at how many products label instructions still show to broadcast granular chlorine over the surface, which may be fine when all pools were plaster pools, but in an era when the vast majority pools have vinyl liners prone to bleaching it is no a good method. In this time knowledge about the interaction of CYA and FC has been gained, with initial studies going back nearly 40 years. The problem is the pool store / pool chemical industry can make more money if this is not revealed as they get to sell much more stabilized chlorine, algaecides, phosphate remover, etc. in order to achieve a somewhat balanced, somewhat safe pool.

    Ike
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    Re: Chlorine/CYA relationship

    Richard,

    Thanks a ton. That actually does make a lot of sense. Like the Soldier analogy.

    So to summarize, where would you like to see my CYA levels with a SWG and in conjuction, my TC and FC?

    Actually while we're at it, where would you like to see all my levels? I am ordering the best test kit I can get my hands on and going to get this to perfection now. No more of the pool store testing my water.
    25 + year pool owner. Current pool (going on 11th year) - 38K gallon in ground Gunite/White Plaster, Spillover Spa, Hayward Cartridge Filter, Hayward 450k BTU NG Heater, Hayward Goldline Aqua Logic control with T-Cell-15 Salt Water Cell, Dolphin Explorer.
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    Re: Chlorine/CYA relationship

    The recommended levels for a plaster pool with a SWG are:

    FC 3-5
    pH 7.5-7.8
    TA 60-80
    CH 250-350
    CYA 70-80
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    Re: Chlorine/CYA relationship

    Quote Originally Posted by JasonLion
    The recommended levels for a plaster pool with a SWG are:

    FC 3-5
    pH 7.5-7.8
    TA 60-80
    CH 250-350
    CYA 70-80

    Bingo.... So why does the CYA/CL chart that's all over this web site ( chlorine-cya-chart-t2346.html ) say Target FC is 8.0 for 70CYA & 9.1 for 80 CYA?
    25 + year pool owner. Current pool (going on 11th year) - 38K gallon in ground Gunite/White Plaster, Spillover Spa, Hayward Cartridge Filter, Hayward 450k BTU NG Heater, Hayward Goldline Aqua Logic control with T-Cell-15 Salt Water Cell, Dolphin Explorer.
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    Re: Chlorine/CYA relationship

    The required FC levels are lower with a SWG partially due to the "super-chlorination" that occurs within the cell itself ... and since there is more of a guarantee that the FC is being constantly added and maintained.

    The higher levels for non-SWG have some "wiggle" room I think to account for human error while the SWG are a no-kidding better not drop lower than this ... seriously ... seriously.
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    Re: Chlorine/CYA relationship

    That chart has the following footnote:

    3Most saltwater chlorine generator (SWG) pools appear to prevent algae at a minimum FC level of 4.5% of the CYA level as compared with the roughly 7.5% of the CYA level shown in the "Min FC" column for manually dosed pools.
    and that chart is not the primary one we use. The chart in the Pool School is the Chlorine / CYA Chart that explicitly calls out SWG FC/CYA levels that are lower than that for non-SWG pools. Though it's based on my chart (which is based on the original chart at The PoolForum), the Pool School chart is rounded and simplified.

    SWG pools appear to not need quite as high an active chlorine level to prevent algae growth. Some of the reason may be due to the more continual dosing, some may be due to super-chlorination (high FC at low pH) in the SWG cell, some may be due to the higher salt level, but whatever the reason it seems to work so is what we recommend.
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    Re: Chlorine/CYA relationship

    Richard.....

    Now THAT makes sense. Thanks so much for the feedback. I got one more for you... A SWG for a 40k gal pool. What do you think a good %'age is to run it on as per the settings in the circuit board? Right now with the hot spell in NJ I have it set to 65%..... I like to run filter 9:00am - 4:00 pm. so Figure I am running 7 hours a day. But sometimes more because when I swim after 4:00pm I kick it back on cause I like to swim when the filter is on.
    25 + year pool owner. Current pool (going on 11th year) - 38K gallon in ground Gunite/White Plaster, Spillover Spa, Hayward Cartridge Filter, Hayward 450k BTU NG Heater, Hayward Goldline Aqua Logic control with T-Cell-15 Salt Water Cell, Dolphin Explorer.
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    Re: Chlorine/CYA relationship

    I think you run it at whatever percentage is required to keep your FC above the minimum. If 65% keeps it up, you could try dropping down to 60%. If 60% isn't enough, you go back up to 65%. But I don't have a SWG, so I am not positive about this.
    John
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    Re: Chlorine/CYA relationship

    John, you are totally correct. The issue is you know what we been having in NJ lately. One day 95 degrees, next day a deluge of rain. Then more heat, more ridiculous downpours, etc. This all effects how quick the CL gets used up.
    25 + year pool owner. Current pool (going on 11th year) - 38K gallon in ground Gunite/White Plaster, Spillover Spa, Hayward Cartridge Filter, Hayward 450k BTU NG Heater, Hayward Goldline Aqua Logic control with T-Cell-15 Salt Water Cell, Dolphin Explorer.
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