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Thread: CYA ... is 80ppm too high?

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    CYA ... is 80ppm too high?

    [Split by moderator from nexa-chlor-generator-t52175.html - jblizzle]

    Cya is too high for one thing. Need to drain some water and refill with fresh. No more stabilized tablets or cya for a while.

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    Re: Nexa Chlor generator

    Quote Originally Posted by aguashui
    Cya is too high for one thing. Need to drain some water and refill with fresh. No more stabilized tablets or cya for a while.
    A CYA of 80ppm is fine for a salt water (SWG) pool:
    pool-school/recommended_levels
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    Re: Nexa Chlor generator

    I hear what you are saying, but the higher the cya level the less effective the oxidizer. That is why the higher the cya is the higher the fc needs to be kept. This rule is non negotiable regardless of method of chlorination. Cya of 80 is maximum for utility and at that level or above can render chlorine ineffective. I am not just going by the book here. Service calls i have made had similar problems as the op and often times the cya was the culprit. After adjusted the problem ceased. I concede with a swg one can keep a higher cya level, but the rule is still a rule. As cya increases, the oxidation effectiveness decreases.

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    Re: Nexa Chlor generator

    I would disagree with your "rule". There are plenty of posts in the Deep End discussing this in detail. As long as the FC is maintained high enough in relation to the CYA, the chemistry will be fine and the pool will stay clear. If the FC drops too low, and algae develops, then the problem becomes the very high FC levels required for the shock process ... it is at this point that reducing the CYA is the best course of action.
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    Re: Nexa Chlor generator

    Quote Originally Posted by aguashui
    As cya increases, the oxidation effectiveness decreases.
    This rule is only true IF you keep the Free Chlorine (FC) the same. That is, for any given FC, it is true that as CYA increases the active chlorine level drops so the rate of disinfection and oxidation and algae prevention drops. However, if you raise the FC proportionately with the CYA level to keep the FC/CYA ratio constant, then you get the same active chlorine level. 1.5 ppm FC with 30 ppm CYA has the same active chlorine level as 3 ppm FC with 60 ppm CYA which is the same as 4 ppm FC with 80 ppm CYA. It is the active chlorine (hypochlorous acid) level that determines the rate of disinfection, oxidation and the ability to kill algae faster than it can grow.

    Our recommendations are never an FC level alone in isolation with the CYA level. If you look at the Chlorine / CYA Chart you will see that the recommended FC level rises proportionately with the CYA level (ignoring rounding).

    This is known science since at least 1974 as described more fully in the "Chlorine / CYA Relationship" section in the first post of the Certified Pool Operator (CPO) training -- What is not taught thread and has been proven in tens of thousands of pools on this and other forums. These FC/CYA ratios of roughly 7.5% for manually dosed pools and 5% for SWG pools work regardless of algae nutrient (phosphate, nitrate) level to prevent green algae with a failure rate of less than 1 in 5000 pools.

    So when you go out on calls where the CYA is high, what is the FC level that was maintained (before problems -- I assume the FC got even lower once the SWG couldn't keep up and algae started to grow even if not yet visible)? If the FC was the SWG manufacturer recommended 1-3 ppm range but the CYA was 80 ppm, then that is certainly consistent with what we have seen on the forum and is why the minimum FC is 4 ppm for 80 ppm CYA and that minimum means minimum, so measured before the SWG comes on again in the morning (or during peak sun and bather load, if the FC is unable to be maintained due to an undersized SWG).

    The main reason for the higher CYA recommendation in SWG pools (with proportionally higher FC levels) is to lower the loss of chlorine from sunlight. Such loss is lower at higher CYA level in spite of the higher FC levels because there is a CYA shielding effect of UV on lower depths that is non-linear. This lower chlorine loss lets one turn down the SWG on-time which not only has the cell last longer but results in a slower rate of pH rise due to less hydrogen gas bubbles (which can increase carbon dioxide outgassing) and less undissolved chlorine gas outgassing.

    By the way, there are residential service companies that operate over 1000 pools at 100 ppm CYA in hot and desert areas with FC starting at 14 ppm using chlorine gas and/or chlorinating liquid and dropping to around 4 ppm when they visit again (so in some sense are "shocking" the pool) and they don't get algae and don't need any supplements such as algaecides, phosphate removers, clarifiers, flocculants, enzymes, etc. Even my local pool store that operates pool service out of two locations servicing over 2000 pools uses a 4.5 ppm FC target maintained with Trichlor pucks/tabs and they do partial drain/refill when the CYA gets to 100 ppm. Now in this case they are below our recommendations so in fact do get algae in some small percentage of their pools and they first shock and if that doesn't work long-term then they use a phosphate remover. They don't fully understand the chlorine/CYA relationship so have just stumbled on an intermediate solution, but they did figure out that a higher FC DOES work with higher CYA, just not perfectly because they didn't go as far as they should in raising the FC target (more like 7.5 ppm FC with 100 ppm CYA).
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    Re: Nexa Chlor generator

    Maintaining excessively high fc levels become impractical.

    I admit i have been out of the loop for a decade, but based on my past experiences what the op described i have remedied by bringing the cya back down to around 60 if using a swg. What you guys are describing is true in most cases even though as i said, maintaining excessively high fc levels is impractical to most i.e. cell deterioration and liner fading. I was offering a suggestion for the op to try that was based off my previous success. Even pool school explains there are exceptions. When it comes to pool chemistry sometimes things are in the grey area.

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    Re: Nexa Chlor generator

    (Moderators: please fork off the posts starting with the one from aquasushi saying "Cya is too high for one thing." through this post into a new thread in Agree to Disagree or The Deep End depending on where things end up.)

    This is again simply not true. For an SWG, it is not at all impractical to maintain a 4 ppm minimum FC with 80 ppm CYA. There is LESS cell deterioration in this case (with outdoor pools exposed to sunlight) because the SWG on-time is LOWER at the higher CYA level in spite of the proportionately higher FC level. This has been proven both in experiments by Mark and others on this forum and in the experience with real pools. As for liner fading, there is NO DIFFERENCE whatsoever in the active chlorine (hypochlorous acid) level between 1.5 ppm FC with 30 ppm CYA, 3 ppm FC with 60 ppm CYA and 4 ppm FC with 80 ppm CYA so NO difference in disinfection rate, NO difference in oxidation rate (including that of vinyl liners) and NO difference in rate of killing algae. The chlorine bound to CYA is very inactive (over 150 times slower in oxidation rate). Again, please read the detailed scientific links that actually MEASURED the oxidation rate of chlorine bound to CYA relative to that unbound (active) and the numerous studies showing that kill times track the active chlorine level, not the chlorine bound to CYA.

    When we talk about "exceptions" in the Pool School, we are not talking about the FC/CYA ratio rule. One should first start with the recommendations and then only as a last resort if they are not working then one can treat the pool as an "exception", which as I wrote is on the order of 1 in 5000 pools or less (literally a handful out of the tens of thousands of pools on this and other forums).
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    Re: Nexa Chlor generator

    These levels will work for most pools most of the time, but are not ideal in all situations. If you are using trichlor tablets for chlorine, follow the bleach recommendations but have TA around 100-120 and be sure it doesn't fall below 100. If you have high TA or high CH fill water, large amounts of aeration, lots of direct sunlight, or fresh plaster you should expect to need adjustments to these recommendations. For advice on what would be ideal for your pool please ask on the forum.

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    Re: Nexa Chlor generator

    If everything worked exactly as outlined in pool school then there would be no need for forums. Again, i was offering a solution that has worked for me on countless pools.

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    Re: Nexa Chlor generator

    I will bow out now, but i maintain that as cyanuric acid levels increase oxidation residual potential decreases. This is irrefutable.

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    Re: CYA ... is 80ppm too high?

    We don't say that a single TA level works for all pools and yes one has flexibility, but it doesn't change the chemical fact that higher TA has faster carbon dioxide outgassing and can lead to faster pH rise. The Pool School recommendations are for using chlorinating liquid or bleach (or other hypochlorite sources of chlorine) and not for using Trichlor. With Trichlor, a higher TA makes sense to have the carbon dioxide outgassing partially counteract the acidity of the Trichlor.

    So for an SWG pool that is indoors and not exposed to sunlight, we don't recommend 80 ppm CYA, but rather a low number such as 20 or 30 ppm CYA. However, you were making a blanket statement of it being too high in a thread about an outdoor pool exposed to sunlight. So yes, the Pool School doesn't cover every case and people should ask, but you made a blanket statement of 80 ppm CYA being too high for SWG and that is absolutely not a problem. It may have been a problem for your pools because you didn't know about the FC/CYA relationship so raised the CYA without proportionately raising the FC, but that doesn't change the fundamental irrefutable science.

    Again, you are making only a half-correct, or half-incorrect, statement about CYA and oxidation potential. It is only true that as CYA increases oxidation potential decreases IF THE FC IS NOT CHANGING. However, if the FC is raised proportional to the CYA level, then THE OXIDATION POTENTIAL DOES NOT CHANGE. That is an indisputable fact -- are you denying that and if so where are your sources, measurements, experiences?

    If you looked at the links in the CPO thread I already provided to you, such as this one showing ORP graphs IN REAL POOLS, you would see that ORP is not related to FC (of course), but it IS related to the hypochlorous acid concentration. But that concentration for that graph was calculated and is proportional to the FC/CYA ratio (in fact, initially I used the FC/CYA ratio and got a graph that was almost the same, but using HOCl is more accurate because the pH varies a bit in some of the pools). The Pool School recommendations are for a CONSTANT FC/CYA ratio so essentially a single HOCl value and therefore a single ORP value (though ORP is affected by things that don't actually affect oxidation rates -- it's a proxy for a thermodynamic value, NOT a reaction rate measurement). As for why the active chlorine (hypochlorous acid) concentration is proportional to the FC/CYA ratio, this is described technically in this post.

    Here's where we agree:

    2.5 ppm FC with 50 ppm CYA has higher oxidation potential than 2.5 ppm FC with 80 ppm CYA

    Here's where we disagree (where the following statement is what I am saying and what the chemistry and measured ORP and measured kill times and measured oxidation rates all say):

    2.5 ppm FC with 50 ppm CYA has the same oxidation potential as 4 ppm FC with 80 ppm CYA
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