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Thread: fc vs. ph

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    fc vs. ph

    Please forgive me if this is readily available or obvious... somewhere I remember that the lower I keep my ph the more effective fc is in cleaning up (keeping ph at or above 7). When I clean up in the spring I try to keep at 7.2 and am astonished every time at how fast the filth clears up. I try to keep at 7.2 to 7.4 when swimming. No cya just keep dripping in bleach. Anyway I am trying to locate the ref on fc effectiveness at different ph levels...

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    JasonLion's Avatar
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    Re: fc vs. ph

    That is only true when you don't have any CYA in the water. As soon as there is CYA in the pool the difference in effectiveness becomes so small that you can just ignore it completely.
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    Re: fc vs. ph

    I don't normally use CYA. I want max effect from fc with lowest
    level fc possible.

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    JasonLion's Avatar
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    Re: fc vs. ph

    I strongly recommend against that approach. The actual effect will be the exact opposite of what you are trying to achieve. You need certain minimum FC levels to assure sanitation, based on how much chlorine gets used up when people get into the pool. Using levels that high (FC around 2) without CYA leads to very very harsh active chlorine levels. Adding CYA holds most of that chlorine in reserve and gives you much lower active chlorine levels, while dramatically reducing the amount of dangerous chlorine side products.
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    Re: fc vs. ph

    My limited understanding is that CYA binds up FC; so that while the FC readings may be higher, the FC is less effective. I also understand that indoor pools shouldn't use any CYA - that CYA is intended to stabilize FC against sunshine. I guess I'm looking for the science resources that show that more FC with CYA is better than less FC without CYA. If those resources are available, I'd love to look at them. And I'd be delighted to take that question on as a separate topic. My initial intent on this topic was to query fellow users to see if I can locate the charts (science) behind the relationship of FC and pH.

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    JasonLion's Avatar
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    Re: fc vs. ph

    The charts you are asking about are in this topic.

    My limited understanding is that CYA binds up FC; so that while the FC readings may be higher, the FC is less effective.
    That is true as far as it goes. The counterpoint is that you can raise the FC level so that it is just as effective as before, so this isn't a bar to using CYA.

    I also understand that indoor pools shouldn't use any CYA
    That is the common wisdom, but it appears to be wrong and isn't what we recommend. Using CYA can cut down on the harmful chlorine byproducts, so it is worth using indoors as well as outdoors (though at different levels).

    The science behind this is scattered around the site. The link I gave you at the start of this post leads to a lot of it, but the specific questions you asked are answered elsewhere, places I can't find to link to just at the moment. Chem geek tends to know where to find all of those posts (he wrote most of them). Hopefully he will show up with some additional helpful links.
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    Re: fc vs. ph

    The Chem geek signal is on the fritz again, we really need to upgrade it to LED.

    You will get a lot of over-my-head information on the effects of CYA from the topic Jason posted and any additional links Chem geek has. If that is your style then you have a lot of good reading ahead. I will try to sum up how I most easily understand it though. I keep about 40 ppm CYA in my pool and keep the FC between 3-7. As I understand it this gives it the equivalent of between 0.5-1.0 ppm unstabilized FC. To do this in my outdoor pool without CYA I would need to CONSTANTLY add chlorine to now allow it to drop below 0.5 but not too much so it does not go above 1.0. When factoring in the burn off from the sun it would literally have to be constant addition during the day. This causes you to lose far more chlorine to sunlight than you "waste" locked up in proper CYA levels. For an indoor pool you don't have the sun to worry about but you do have all of the other negative effects. As an example, my daughter takes swim lessons at the YMCA. My wife had two swimsuits when she had to swim with her, one for home and one for there. The one she uses at home looks just fine after a few seasons while the one for the YMCA pool was noticeably bleached out after a couple months of 1 hour a week swimming. This is due to the complete lack of stabilizer in the pool water.

    So there is some great scientific info on this forum explaining the why, it's some good reading. For me though, I can see the difference very clearly from experience.
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    Re: fc vs. ph

    Here is a post that I just made in another thread about the active chlorine level as FC and CYA vary. This is more about the buffering effect than the sun protection:
    Quote Originally Posted by jblizzle View Post
    If you really want to wow her, show her this chart which is the active chlorine level: http://richardfalk.home.comcast.net/.../pool/HOCl.htm

    At a CYA of 80ppm with a FC of 10ppm, the active chlorine level is ~0.058
    At a public indoor pool with NO CYA, the FC may be 2ppm ... with an active level of 0.967

    So swimming at the public pool would be 16 times more active chlorine and cause 16 times more damage to suits and hair than in your pool with 10ppm of FC.
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    Re: fc vs. ph

    You can look at the "Chlorine/CYA Relationship" section in the first post of the thread Certified Pool Operator (CPO) training -- What is not taught and you can look at posts in the thread Pool Water Chemistry. You need to think of CYA as being a hypochlorous acid (active chlorine) buffer which means it not only protects its degradation from sunlight, but also buffers its level against changes in pH and significantly moderates chlorine's strength (which is a good thing if not overdone). You need to consider that having chlorine in the water with no CYA is over-chlorinating in that the chlorine is too strong. This is why in Europe the DIN 19643 standard that does not use CYA has the chlorine level at 0.3 to 0.6 ppm with no ozone or 0.2 to 0.5 ppm when ozone is used (in the circulation system, not in the bulk pool water). In the U.S., the minimum FC level in nearly every state is 1 ppm FC and in most pools its even higher.

    Fortunately, it takes a relatively low active chlorine level to kill pathogens quickly. You can see kill times for an FC that is roughly 10% of the CYA level in this post. For an indoor pool, I'd recommend 4 ppm FC with 20 ppm CYA which is roughly equivalent to 0.2 ppm FC with no CYA in terms of active chlorine level. Indoor pools usually need some form of supplemental oxidation such as from UV or ozone since there is no exposure to sunlight. It would be very, very difficult to maintain a low 0.2 or 0.1 ppm FC level with no CYA consistently in the pool. Think of CYA as holding onto chlorine releasing it wherever it is needed, but this reserve or reservoir of chlorine lets you not run out locally.
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