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Thread: Understanding the FC/CYA chart

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    Understanding the FC/CYA chart

    Quote Originally Posted by chem geek View Post
    Their claim that 5 ppm FC is a problem may not have any understanding of the FC/CYA relationship because it should be the active chlorine level that is relevant towards the degradation of the biological material. 5 ppm FC by itself means nothing. It has to be understood in the context of the CYA level. So I would not assume that a higher FC, when the CYA is proportionately higher, will be any problem for this product.
    I am either miss-understanding your statement or I have missed the boat in my understanding of the FC/CYA chart provided here. My understanding is that FC is FC no matter how much CYA is in the water. HOWEVER the more CYA that is in the water the more FC is required in order to be effective. Therefore if they require 5ppm FC or less is doesn't matter how much CYA is in the pool because 5ppm FC is 5ppm FC regardless. Please let me know what i am missing.

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    Re: thoughts on bio-active cya reducer

    Quote Originally Posted by tim5055 View Post

    I would have been reticent to let my FC drop during the summer to use the product. But, at the same time the pool store told me that 200 CYA was fine and just keep the FC between 3 - 5. It was right on the nice computerized printout they gave me along with a list of stuff I should buy.
    200 CYA? The pool company said 3-5ppm was Ok? My chart doesn't even go up to 200 ppm but it does say 12ppm @100ppm is the target and 7 is the minimum. So based on that and a CYA of 200 shouldn't the minimum be 14ppm. I hope I'm not hijacking this thread.

    Moved into a new topic. JasonLion

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    Re: Understanding the FC/CYA chart

    The higher the CYA level, the higher you need to keep FC to provide the same effective algae killing/sanitizing level of active chlorine.

    And yes, pool stores are crazy and do tell people that FC at 3-5 is fine regardless of your CYA level.
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    Re: thoughts on bio-active cya reducer

    Many many people come here asking for assistance and tell us similar stores as this. Unfortunately, people come here asking for help with what the pool store made worse.

    We have learned that just because someone works in a pool store does not mean they know anything about pool chemistry. So this is one of the reasons we do not trust pool store advice. Its my personal experience (what little I claim to have), that the pool store folks level of experience is what they are told by slick talking chemical salesmen and a computer program.

    other pool store comments told to forum posters
    - "Never use bleach in a pool. Use liquid chlorine, shock, or pucks."
    - "You have chlorine lock, you need to drain the pool and start over" (in actuality, the CYA is way too high and FC is not sufficient. The suggested drain is because they dont know what else to do)
    - "Its normal to have to drain your pool at least once a summer because its so hot in Texas" (I personally heard the guy at the Pool store that starts with "L" say this to a customer).

    Disclaimer: I didnt buy anything at the pool store, I was there on a RECON mission

    And the list of less than correct pool store advice goes on...and on...and on...






    Quote Originally Posted by Lux Man View Post
    200 CYA? The pool company said 3-5ppm was Ok? My chart doesn't even go up to 200 ppm but it does say 12ppm @100ppm is the target and 7 is the minimum. So based on that and a CYA of 200 shouldn't the minimum be 14ppm.
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    Re: thoughts on bio-active cya reducer

    Quote Originally Posted by Lux Man View Post
    200 CYA? The pool company said 3-5ppm was Ok? My chart doesn't even go up to 200 ppm but it does say 12ppm @100ppm is the target and 7 is the minimum. So based on that and a CYA of 200 shouldn't the minimum be 14ppm. I hope I'm not hijacking this thread.

    Moved into a new topic. JasonLion
    as others have pointed out, the lack of good information coming from pool stores is limited most times.

    If you look at the printout from the "computerized" water testing in some stores, the "good" range for FC is 3 - 5 and the CYA is 30 - 200. The folks working there said keep up what you are doing, it's working fine.
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    Re: Understanding the FC/CYA chart

    Quote Originally Posted by Lux Man View Post
    I am either miss-understanding your statement or I have missed the boat in my understanding of the FC/CYA chart provided here. My understanding is that FC is FC no matter how much CYA is in the water. HOWEVER the more CYA that is in the water the more FC is required in order to be effective. Therefore if they require 5ppm FC or less is doesn't matter how much CYA is in the pool because 5ppm FC is 5ppm FC regardless. Please let me know what i am missing.
    Perhaps when I wrote that "5 ppm FC by itself means nothing" I was being too extreme. It does mean something -- it tells you the total amount of chlorine that is available so its capacity, but not its active level. Since the FC level is being maintained, this capacity doesn't mean anything with regard to the degradation rate of their product. It is the active chlorine level, which is proportional to the FC/CYA ratio, that determines the degradation rate and the rate of all chemical reactions involving chlorine.

    So 3 ppm FC with 30 ppm CYA has the same active chlorine level as 6 ppm FC with 60 ppm CYA and is the same as 10 ppm FC with 100 ppm CYA and they would all react with the product at the same rate (within reason -- at high FC levels the chlorine bound to CYA has some reactivity that starts to become important). This is why I believe they are wrong when they say that 5 ppm FC is a problem. If they determined this from testing, then it needs to be in the context of the CYA level. If their test was with no CYA, then 5 ppm FC has a huge amount of active chlorine and no reasonable pool with CYA would be a problem for their product. If they did their test with 100+ ppm CYA, then that would be a problem as their product would degrade in pool environments where algae could grow because the active chlorine level is too low.

    I think you are treating FC as being separate and that the CYA somehow makes it ineffective in an indirect way. That's not how it works. It's very direct. Most of the FC in a pool is bound to CYA forming a completely separate chemical (technically a group of chemicals called chlorinated isocyanurates) that has very little reactivity and virtually no disinfection. Only the unbound chlorine is effectively active. The bound chlorine is released fairly quickly which is why it all gets measured in the time of a chlorine test.
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    Re: Understanding the FC/CYA chart

    chem geek-good stuff for me to absorb- thank you. Now a question or two. Could you send me the test kit which will allow me to test for "active" chlorine, which is equal to FC minus chlorinated isocyanurates? Since it would seem that is what we are really needing to do- is to maintain a "safe" level of "active/useful" chlorine. I realize this test kit does not exist but I was just using it to make a point.

    What I have absorbed is that high CYA, 100 plus is not necessarily a bad thing in fact it is my friend I just need to understand it. I say this because of what you wrote in another thread that @ 100 ppm CYA I will only loose approx. 15% FC in the mid day sun. Am I still on track?

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    Re: Understanding the FC/CYA chart

    There is no need for a test for active chlorine level. Knowing the FC and CYA, the "active level" can be calculated and it is given in this table: http://richardfalk.home.comcast.net/.../pool/HOCl.htm

    You are correct that at higher CYA levels you will lose a lower % of the FC to the sun. But there are other problems with this line of thinking. 1. If the FC needs to be above 10ppm, the pH test no longer is accurate. And 2. If you develop algae, the required FC levels for the SLAM process are VERY high.

    The levels we recommend are a balance of the benefits of higher CYA levels and the risks of problems developing.
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    Re: Understanding the FC/CYA chart

    EDIT: Jason beat me to an answer

    Cya 100 is a problem for two reasons. It requires high FC to keep you out of trouble and this gets impractical and expensive in a pool with any size to it.

    Most importantly it becomes a major burden if you do get into trouble and need to SLAM a pool. The 15% loss "benefit" is completely overshadowed by the enormous demand for FC in both these scenarios.
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    Re: Understanding the FC/CYA chart

    One realistically needs to set a cutoff somewhere because higher CYA levels become more risky if and when anything goes wrong. We've set that upper limit to 80 ppm. It's not a hard limit. If you want to use 100 ppm, you can, but just understand that doing a SLAM at such levels (if ever needed) uses a LOT of chlorine (roughly 39 ppm FC for the SLAM level). The SLAM level for 80 ppm CYA is around 31 ppm so is already pretty high.
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    Re: Understanding the FC/CYA chart

    Gentlemen(the 3 of you) thank you for your insight, I understand completely. Just a thought....what about an addendum of sorts on the FC/CYA chart page which would cover these few additional points and maybe one or two more that I didn't bring up. I realize this info. may all be here on the sight but it may be scattered throughout numerous threads. If it was all in one place you could just send a guy like me there and say, "read up". I might state that when it comes to water management and keeping the pool clear and algae free that understanding this relationship is paramount. I realize this is stated clearly but due to my personality trait flaws like to take things just one or two more steps so I really understand the why's and wherefores.

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    Re: Understanding the FC/CYA chart

    Well, the reason we wouldn't is so that we keep those sections as clean, simple, and clutter free as possible while still providing all the essential information.
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    Re: Understanding the FC/CYA chart

    The Further Reading section in the Pool School does have threads such as Advanced Pool Water Chemistry and Certified Pool Operator (CPO) training -- What is not taught. The problem is that when one gets more detailed, different people tend to have different questions. Also, most people just want a simple set of rules to follow and don't want to know the "why" or get into further details so the top level needs to be kept clean and simple.
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    Re: Understanding the FC/CYA chart

    chem geek- thank you for providing the additional links, I will be digesting them further. As I mentioned before I've been the industry for just 2 years but I've taken it very seriously to understand the deeper issues that we deal with. I've questioned our methods from early on. I took the CPO class in year 1 (spring 2013) and was mad at myself for getting 1 question wrong and that was because I just was kind of bored with it and just going too fast. I am not boasting but stating this because how little challenging it really is and as I read now how little pertinent information is provided. I do have a theory as to why that is but I'd rather not post it here. I would characterize our business this way, we run around putting out fires that we created, using methods that only work sometimes, because we really didn't know the cause of the fire in the first place. Our company will change since I have the ear of the owner.

    Thank you-

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    Re: Understanding the FC/CYA chart

    How about the fact that above 100ppm of CYA, the amount of chlorine you have to keep in the pool to keep it blue makes it more like swimming in chemical soup. My company works closely with one of the most reliable pool stores here in central Phoenix but we differ on our reccomendations on CYA levels. they say anything less than 200 is ok since you'll never be able to keep it any lower but the owners of 3 other service companies here (that have about 2400 accounts combined) manage to keep their pools at or below 80 ppm CYA here. Not the ideal 30-50 but at least it's below 100. We're finding more and more that our trouble pools during swim season are due to high CYA and were taking steps to head those problems off this year with water changes and liquid chlorine in the winter.
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    Re: Understanding the FC/CYA chart

    Quote Originally Posted by SwimmingpoolAZ View Post
    How about the fact that above 100ppm of CYA, the amount of chlorine you have to keep in the pool to keep it blue makes it more like swimming in chemical soup.
    That isn't the way the chemistry works. 20 ppm FC with 200 ppm CYA has the same active chlorine (hypochlorous acid) concentration as 3 ppm FC with 30 ppm CYA. The effect on swimsuits, skin, and hair, the rate of disinfection and prevention of algae, are all roughly the same. The pools are only trouble with high CYA because they are not being maintained with proportionally higher FC levels. While we don't recommend going to those high levels, there are quite a few pool owners on this site who started out with very high CYA levels and did not want to replace the water for major dilution (usually due to water restrictions) so operated their pools successfully with higher FC levels until eventually the CYA dropped from slower dilution over time.

    The higher FC level at a higher CYA level is only an issue if one were to drink large volumes of water since in that situation the total capacity of chlorine is relevant, not the active chlorine level (at least as far as cumulative effects). It is in no way a "chemical soup" implying that the high FC number means anything with regard to the potency of chlorine. FC is a measure of the chlorine reserve or total capacity, not its activity or "effectiveness". It is the FC/CYA ratio that is a measure of chlorine's active strength.

    See the thread FC - Concept of "Reserve" for more detailed info (also see Certified Pool Operator (CPO) training -- what is not taught). An analogy explaining the difference between the total capacity which is mostly the reserve vs. the much smaller amount of active chlorine is the following seen in the aforementioned posts:

    You can think of an analogy of people fighting a war where you have a group of "active" soldiers on the front lines fighting hand-to-hand combat with an enemy. You have many more soldiers in the rear that are not directly fighting and are in "reserve". When some soldiers in the front lines get killed or injured, you can replace them with some from the reserve, but the rate at which you will be able to wound or kill the enemy is only dependent on the number of soldiers you have on the front lines doing the hand-to-hand combat. It doesn't matter how many you have in reserve. The amount in reserve only tells you how long you can continue to fight -- not the RATE at which you can fight effectively.
    FC is the sum of active and reserve soldiers while the FC/CYA ratio is proportional to the amount of active soldiers (roughly half the FC/CYA ratio is active).
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    Re: Understanding the FC/CYA chart

    Some of the problems with CYA levels over about 80, perhaps 90, include:
    * There is no practical way to measure the CYA level at 90+. The usual test reports anything over 100 as 100, so starting somewhere in the 90s you have no idea what your CYA level really is. The test can be done with dilution to measure higher levels, but that reduces the precision so much that you only have a vague idea what your CYA level is.
    * Very high CYA levels require high FC levels. Once FC goes above 10 the PH test starts to have problems. This isn't a real problem when CYA is around 100, but by the time you get up to 150 it becomes much more difficult to ever know what your PH is.
    * Should you get algae, the amounts of chlorine you need to use to kill the algae rapidly get to be astronomical at higher CYA levels.
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