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Thread: FC loss versus CYA level

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    FC loss versus CYA level

    Somewhere on this website I think I saw a chart that had %FC loss versus CYA level, but I can't seem to find it. I've seen posts that say things like 90% loss in 30 minutes in noon time sun with 0 CYA, but I'd like to see the table.

    Currently, I have 0 CYA. Before adding some, I wanted to get some data on FC loss at 0 CYA, at 10 ppm CYA, etc.

    Also, it seems like presenting FC loss as %FC loss versus CYA ppm is misleading. You need more FC for the same sanitation level as CYA increases. So, a given percentage FC loss means more chlorine is needed to replace the loss at higher CYA than lower CYA.

    An interesting chart would be FC loss in ppm versus CYA in ppm, or FC loss in ppm at the minimum FC level for sanitation versus CYA in ppm. That would show the relative amount of chlorine depletion.
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    Re: FC loss versus CYA level

    Joel - TFP Moderator - Minnesota - **Become a TFP Supporter!** Helpful Links: ABCs of Pool Water Chemistry - SLAM Procedure - Chlorine/CYA Chart
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    Re: FC loss versus CYA level

    Thanks for the link.

    So, if I'm reading the chart "Chlorine Use at Different Levels of HOCL", I think its telling me that the higher the CYA the MORE chlorine loss (in absolute terms) there is.

    Is that a correct interpretation?

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    Re: FC loss versus CYA level

    Quote Originally Posted by davethomaspilot View Post
    Thanks for the link.

    So, if I'm reading the chart "Chlorine Use at Different Levels of HOCL", I think its telling me that the higher the CYA the MORE chlorine loss (in absolute terms) there is.

    Is that a correct interpretation?

    ]
    i think Richard's original commentary with that chart explains it:

    The following graph combines the two concepts of needing more chlorine at higher CYA vs. the greater protection of chlorine by CYA. The graph shows the total chlorine (FC) loss rate in ppm/hour vs. CYA at different HOCl levels. Remember that this rate of loss will slow down as chlorine gets used up. Nevertheless, while in theory the absolute loss of chlorine is greater at higher CYA levels (keeping HOCl constant) and is the downside to a "high CYA & high Chlorine" approach, in practice there is some sort of CYA shielding effect such that higher FC and CYA levels at the same FC/CYA ratio lose less absolute amounts of chlorine (see this post later in this thread and see Mark's experiments in this post and this post. However, the primary reason to have higher CYA and Chlorine is to have a sufficient buffer of chlorine to prevent it from dropping to dangerous levels. There is obviously a tradeoff here. Though using no CYA results in the least amount of chlorine loss, the fact is that you simply can't maintain a pool with only 0.05 ppm chlorine everywhere in it -- hence a minimum level is needed as a buffer.



    Salt Water chlorine Generation (SWG) pools seem to require a higher level of CYA, about 70-80 ppm, to operate efficiently. The theory is that the CYA is slow to "store" the chlorine as it is being generated so without enough CYA there is a build-up of chlorine that degrades the performance of the salt cell. I would prefer that the SWG manufacturers offer a larger lower-power (per length) cell that would work efficiently at lower CYA concentrations.
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    Re: FC loss versus CYA level

    Yes, I read that explanation.

    I just wanted to verify, because I thought the story was that adding CYA would also reduce chlorine usage.

    I understand the need of a FC "buffer" so some can be lost without the concentration dropping too low.

    Thanks!
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    Re: FC loss versus CYA level

    I think I've read on this forum that there's no way to measure CYA below something 20-30 ppm.

    So, has the predicted 1/2 life (see chart below) at low CYA been empirically verified?

    I see very low FA depletion in mid-day, full sunlight. 20-25% in four hours. Not a problem!

    I've never added any CYA to the pool and the CYA as measured by the Taylor test kit is below the minimum detectable 30 ppm.

    I'm hesitant to add CYA, since the water is always crystal clear and there's no detectable skin or eye irritation. Chlorine use is minimal, and its no problem for the Intellichem to maintain 1-2 ppm, or 2-4 ppm or .5 - 1.0 ppm using ORP as a proxy for FA (when PH is held constant at 7.4).

    I know the amount of FC I need depends on the pool CYA, but apparently there's no way to know how much you have, other than knowing it's "no more than 30 ppm"?



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    Re: FC loss versus CYA level

    Quote Originally Posted by davethomaspilot View Post
    Thanks for the link.

    So, if I'm reading the chart "Chlorine Use at Different Levels of HOCL", I think its telling me that the higher the CYA the MORE chlorine loss (in absolute terms) there is.

    Is that a correct interpretation?
    Yes. Percentage-wise it will be less, but in absolute quantities, it will be higher. That's why a pool that typically loses 2 ppm/day when maintained at 4 minimum can't just be dosed to 18 and ignored for a week and still have 4 left.
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    Re: FC loss versus CYA level

    It looks like you answered a question to my prior post.

    My latest question was whether the predicted depletion rates at low CYA (< 20 ppm) have been empirically verified.
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    Re: FC loss versus CYA level

    But, on the original point.

    I've seen many posts on the forum where "experts" have said less (absolute) chlorine is used when CYA is higher. It's just the opposite, since so much more FC is needed as CYA goes up.

    And, those false assertions aren't corrected by other experts.

    I bet if you took a poll, a large percentage of respondents would say chlorine use is less when CYA is added.
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    Re: FC loss versus CYA level

    Dave,

    It sounds like you might be the research guy we need. Most of us are happy knowing we can obtain a roughly 2 ppm daily loss using the suggested features on the forum.

    You seem to want more data than that.

    We are hopeful somebody with your interests will take the bull by the horns and set up some empirical tests that will verify more accurately what we have discovered from anecdotal reports.

    If you can set up those experiments in your pool and verify their accuracy, we could take a look at your data and consider it for a Pool School article.
    Dave S.
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    Re: FC loss versus CYA level

    I agree the half-life chart makes more sense in the apparent benefit of CYA in helping minimize FC loss (vs the loss rate chart), and also wondering how the low CYA data were obtained. Not enough to want to run the experiment though!
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    Re: FC loss versus CYA level

    Duraleigh,

    We are hopeful somebody with your interests will take the bull by the horns and set up some empirical tests that will verify more accurately what we have discovered from anecdotal reports.
    I've been trying to do that. The very low loss of FC I'm seeing in full daylight with little or no CYA is what prompted me to ask if the predicted loss had been empirically verified. Please see this thread:

    http://www.troublefreepool.com/threa...ay-as-expected

    I did the "experiment" three times ans saw similar results. Only 25% loss in four hours, when FC is in the 1 to 2 ppm range.

    Since them, I've turned the IntelliChem back on to regulate FC via ORP to between 1 and 2 ppm. Loss is around .5 ppm per day.

    JoyfulNoise suggested I must have non-zero CYA for FC loss to be so low, but, I don't have a way to know what my CYA level really is. So, I don't have a way to get the FC loss versus CYA at very low CYA levels. Since it's so difficult to get that data, I asked if it had ever been empirically verified.

    I'd be happy to get more data before I add stabilizer. We're not forecast for lots of sun until Wednesday, so I could get more of the same sort of data later this week.

    But, the difficulty is how to measure CYA below 20 ppm.

    Thanks!
    17,000 gallon gunnite pool and spa, PebbleTech finish
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    Re: FC loss versus CYA level

    Quote Originally Posted by davethomaspilot View Post
    JoyfulNoise suggested I must have non-zero CYA for FC loss to be so low, but, I don't have a way to know what my CYA level really is. So, I don't have a way to get the FC loss versus CYA at very low CYA levels. Since it's so difficult to get that data, I asked if it had ever been empirically verified.
    Dave,

    I was mistaken because you said you were adding "shock" weekly to your pool in the beginning and I thought it was dichlor shock but it turns out that you were adding cal-hypo. So my assumption that you must have had a low level of CYA was wrong.

    Also, let's keep in mind that your pool is very different than most TFP pools. You run a copper ionizer system with a low level FC. The original method of sanitation you were told to use was to have the ionizer on generating copper all the time and then to just "shock" weekly to get some FC into the pool. But, you surmised that that didn't seem like a reliable method so you added an IntelliChem system to your pool which doses the FC based on ORP feedback (and also actively controls pH). This way you maintain a more consistent FC level rather than the "shock weekly" method. That's certainly beneficial, but not bullet-proof.

    First, to address the question you had specifically - no, there is no way to measure CYA less than 20ppm because the test simply cannot read lower than that due to the inability of the melamine-cyanurate complex to form precipitates when the concentration of CYA is less than 20ppm. Even at 20ppm it is very difficult to get a good reading (although members do have some success with it). There are no other chemical methods for testing CYA at low levels so you would have to use a spectrophotometer method (probably something like a LaMOtte ColorQ) to get lower readings although I have no idea if that system is reliable at low levels.

    The data is those charts by chem geek are all, from what I know, theoretically derived. There is no actual experimental data there. You are, of course, welcome to e-mail him (his external e-mail is linked to his profile) to get the exact details of how he derived most of those charts. However, even though the data is derived from the chemistry, it has been reliably matched up to experience.

    As for your original post, I don't disagree with what you are seeing. In fact, I think you are confirming the low loss rates at the low end of the chart. It is true that when you increase CYA you increase loss rate, but try to understand why - it is because when you use CYA you need to have higher FC in order to maintain a hypochlorous acid level above the threshold that will kill pathogens. The first chart you sighted is nothing more than the loss rate one would expect to have when you include the UV loss rates of hypochlorous acid (HOCl), hypochlorite anion (OCl-) and chlorinated cyanurates (HCY-Cl). So if your FC is higher, then your concentration of all of those species is higher and your loss rate is greater (those are the isolines in the graph, essentially lines of constant FC/CYA ratio). The point of the graph is this - if you jump from one isoline to another, say increase your FC/CYA ratio, then your loss rate goes up. So essentially it tells folks this - it's best to keep your pool at an FC/CYA ratio of 5% than it is at 10%. Some people think, "Wow, if I just jack up my chlorine really high, then I can just sit back and not worry about it for a week or so...." But what that chart is telling us is that idea is false - if you increase your FC to really high levels without increasing your CYA (and thus jumping from a lower isoline to a higher isoline), you will make your chlorine loss rates worse (faster). The chart is designed to do away with the notion that "a little is good therefore a lot is better!"

    In your case, you have no CYA in your pool and you keep your FC at extraordinarily low levels. Therefore, your loss rate is lower simply because there is less chlorine around for you to lose. Now your ORP probe will add chlorine on-deamnd which is great and it likely keeps your levels stable. But the main problem that I see with your setup is that your levels are so low, and it is a fact that water circulation in all residential pools is terrible ineffective, that it would be easy for your ORP sensor to think that your water is fine when, in fact, there are probably dead spots in the chlorine. Despite what we might think of our pools, there can easily be stagnant areas of the water where circulation is almost non-existent. Light niches are a great example of this. But, because you have copper ions in your water, you are less likely to get an algae bloom and can therefore maintain that lower level of FC....until the day you do get an algae outbreak (it can happen even to the best of us) and then the situation becomes much more complicated.

    For those of us that use chlorine alone for sanitation, it would be an impossible task to maintain a pool with no CYA. Yes, one would use less FC, but there would simply be no may to keep the FC level consistent enough in the water volume to be at the proper level for disinfection and the water would need a constant dosing system (like your setup). So, the trade off is to use CYA which allows the pool owner to maintain a higher chlorine residual (so as to never go below the disinfection limit) and to help protect the chlorine from UV photolysis. So in a clean, well managed pool with TFP recommended levels of CYA and FC, TFP users will find that they can drive their FC loss down to 2ppm/day or less (mine is actually less right now and I live in a pretty extreme UV/heat environment) and that makes the water completely manageable for the average pool owner who may only be able to dose their pool once per day with chlorinating liquid.

    And finally, if you read that pool water chemistry section carefully, you'll see that chem geek did qualify the loss rate and half-life charts with the idea that the protective effects of CYA have a component to them which seems to not be dependent on chlorine loss. In other words, based on the anecdotal data we have from users and real world pools, higher CYA levels seem to lower the loss rates more than what would be predicted from the basic chlorine chemistry alone. One theory is that, in deeper waters, CYA has more of a chance to absorb UV directly and re-radiate it at a lower wavelength (either as IR radiation or heat). If that's the case, then most of those charts would show a loss rate higher than what is typically seen when using higher levels of CYA.

    Should you add CYA to your pool? Nope. Your pool is different from what we recommend, and because you use an ORP probe, I don't think you should add CYA as that would cause you to have recalibrate it (CYA fouls the ORP probe membranes). I think you should stay-the-course with your pool and see what happens.
    Matt
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    Re: FC loss versus CYA level

    Wow, thanks Matt for the reply!

    I'm still keeping the ionizer off. Copper has dropped to somewhere between .1 and .2 ppm from .3 - .4 ppm when I was running the ionizer continuously.

    So the copper PPM is decaying, but not nearly so fast as when I wasn't using chlorine except for weekly shocking. Yeah, I know it's not supposed to matter (FC level for copper depletion) just reporting what I'm seeing. We have had a lot of rain, but not nearly enough to account for that much depletion. I figure it would take an entire pool full of water to cut the copper level in half.

    Along the same lines as why my setup might differ from most---

    Intellichem--

    - Dosage rate not limited by an SWG cell's ability to create FC. It can dose whatever's needed based on the ORP reading. Limited only be mixing rate of the pool.
    - CYA can be an FC "buffer". So can a 4 gallon tank of bleach!

    I'm still brushing the pool once weekly, since that's what the PB said to do. But, I'm not sure I'm really doing anything--I never see anything getting brushed off.

    Filling the acid tank is still the biggest hassle (by far). I'm still going through 3-4 gallons of muriatic acid weekly. I've checked the Intellichem Ph reading against the Taylor test kit--spot on. I target TA at 80 and add baking soda at TA=40.

    I'm hoping the acid depletion rate will start coming down soon. It's been over 6 months since the pool was filled., so it seems higher than what it should be.

    Addressing the possibility of "dead" spots-- I think I'll target higher FC levels, until I notice something undesirable. But,I'd think even the nightly temperature change (convection) should be enough to keep the water mixed.

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    Re: FC loss versus CYA level

    For pH stability you probably need to drop your TA target from 80 to 60ppm. Since you have no CYA in your water or borates, your TA is almost entirely from dissolved inorganic carbon (CO2, bicarbonate & carbonate) with the bicarbonate anion (HCO3-) being the largest contributor to TA. pH rise in pools is almost entirely dominated by the outgassing of CO2 from water. As the CO2 outgasses, the bicarbonate alkalinity consumes a proton to form more aqueous CO2. This is a constant equilibrium reaction that occurs in all bodies of water and results in a net pH rise. I target a TA of 60ppm (and I've gone as low as 50ppm); my pH stabilizes at 7.8 for very long periods of time and, when I drop my pH to 7.6 it can take 10-14 days for it to rise up back to 7.8. This is a consequence of me keeping my TA low and using 50ppm borates as an additional pH buffer.

    Brushing is important for many reasons but one, in particular linked to what we are talking about - stagnant water. It is a fact of fluid dynamics that the flow rate of water at the pool's wall surface is exactly zero and so this tends to create a stagnant boundary layer at the wall surface. This is one area where you find water with lower FC levels than bulk. Also, I am not a hydraulics expert (there are others on TFP that can answer this better than me) but I do not believe there is any significant effect from convection in the pool water that would cause any reasonable level of mixing. So circulation is quite critical in pools. People who have in-floor cleaning systems do have an advantage if their equipment is setup so that they can introduce chemicals and heated water using the in-floor jets as opposed to wall returns since the in-floor jets allow for a lot more mixing of the deep water.

    Finally, the concept of CYA as a buffer is meant in a chemical sense of the word buffer. CYA greatly moderates the amount of active chlorine (hypochlorous acid/hypochlorite) in the water by forming a chlorinated cyanurate when chlorine is added to water. If you look at the follow-on charts that chem geek posted in the Pool Water Chemistry thread, you can see that once CYA is added to water, over 95% of the chlorine is bound to the CYA where it is held in reserve as a chlorinated cyanurate (HCY-Cl) and protected form UV loss. The rest of the chlorine breaks up into their equilibrium amounts of HOCl and OCl- (at a pH of 7.5, the equilibrium distribution is 1:1 for HOCl:OCl-). This is why I can have 4-6ppm of FC in my pool water and not have my eye's stinging or my clothes bleached out, the HOCl is moderated all the way down to ~ 0.10ppm HOCl which is more than enough to kill algae and other pathogens. As the HOCl & OCl- get used up (either through disinfection, oxidation or UV photolysis), the HCY-Cl releases the bound chlorine to make up for the loss and the equilibrium balance is restored.

    One thing you should track is your CC levels. You have quite a high level of active chlorine in your water (because the CYA is zero) and, if you water remains free of algae, your chlorine will spend most of it's time just oxidizing things. CCs are the oxidation by-products of chlorine neutralizing bather waste so it would be interesting to see how you CC levels track over time as well. At low FC levels, the oxidation chemical reactions are slower (again, because there's less chlorine around) and not always complete (monochloramine, dichloramine and nitrogen trichloride all have different oxidation and outgassing rates), so you may see measurable levels CCs hang around in your pool water for longer periods of time.
    Matt
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    Re: FC loss versus CYA level

    I'll start measuring CC again. I quit, because it's been zero every time I tested for it.

    I'll also try using TA=60 as a target and see if that reduces the amount of acid I'm using.

    Regarding pool homogeneity it's hard to imagine there could be significant chemical gradients (differing concentrations of chemicals at different places in the pool). I understand the boundary layer at the wall and floor surfaces, but still...

    Water is pulled from the bottom and returned around the perimeter of the pool, a couple of feet below the surface level. Also, a spillway from the spa runs once an hour for five minutes, which should also help mixing.

    The skimmers aren't suction--they work via return water and venturi action. Skimmer returns are about a 1' below the surface of the water.

    There's also a cleaner that stirs up water on the bottom while the pump is running. That mixes the water even more.

    So, water is pulled from the bottom of the pool, and returned to the (generally) top side of the pool.

    Too bad there isn't something like the dye I use for algae control in our pond, except that it would go colorless after a few hours. That way, one could easily see how well (and how fast) chemicals disperse. You could spot "dead areas" and perhaps "tune" the return nozzles to minimize them.

    If there's negligible convection in the pool, there would be have to be water temperature differences. That's easy to test! Dead spots should be at a different temperature when the temperature at the probe is changing (around dusk and dawn around here). I might rig something up and go see if I can measure any thermal gradients.

    For those of us that use chlorine alone for sanitation, it would be an impossible task to maintain a pool with no CYA. Yes, one would use less FC, but there would simply be no may to keep the FC level consistent enough in the water volume to be at the proper level for disinfection and the water would need a constant dosing system (like your setup).
    Yes, but with using something like IntelliChem, I don't see a problem. The FC simply doesn't decay that quickly. And, it takes so little FC with zero CYA, it's hard to imagine there wouldn't be enough everywhere in the pool for adequate disinfection. There would have to be some pretty big "chemical gradients" for there to be too little FC for disinfection when the bulk of the pool water is over 1 ppm.

    But, I guess time will tell. Eventually the copper will be near zero and I'll be relying on the FC for all the disinfection (and oxidation).

    Thanks!
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    Re: FC loss versus CYA level

    Quote Originally Posted by davethomaspilot View Post
    I've seen many posts on the forum where "experts" have said less (absolute) chlorine is used when CYA is higher. It's just the opposite, since so much more FC is needed as CYA goes up.

    And, those false assertions aren't corrected by other experts.
    I have actually measured this affect. See this post: CYA Testing Update

    Some of the newer "Experts" may not have seen this since I did this so long ago which is probably why you see varying opinions.




    Too bad there isn't something like the dye I use for algae control in our pond, except that it would go colorless after a few hours. That way, one could easily see how well (and how fast) chemicals disperse. You could spot "dead areas" and perhaps "tune" the return nozzles to minimize them.
    Do you mean like this: Circulation Dye Test
    Mark
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    Re: FC loss versus CYA level

    Do you mean like this: Circulation Dye Test
    Exactly!

    Party Potion at Walmart, eh? I'm going to get some and give it a try!

    In the thread you mentioned that you thought the color dissipated due to the a relatively high FC level? Do you think dissipation rate is a function of FC, independent of CYA. Or, does it also depend on the CYA level?
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  19. Back To Top    #19
    mas985's Avatar
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    Re: FC loss versus CYA level

    Quote Originally Posted by davethomaspilot View Post
    In the thread you mentioned that you thought the color dissipated due to the a relatively high FC level? Do you think dissipation rate is a function of FC, independent of CYA. Or, does it also depend on the CYA level?
    That's a good question. I'm not sure if I have the answer to that because I only did it once and at the same CYA level. However, if you notice, with the pump on low speed, the color took more than 2x longer to change which could also be explained by dilution. So I am not sure which it was although the manufacture mentioned bleaching of the color with high FC levels. A bucket test could confirm one way or the other and you could do it with several CYA levels.

    But I would encourage you to read the other thread on CYA testing which showed that higher CYA actually does save absolute chlorine.
    Mark
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    Re: FC loss versus CYA level

    But I would encourage you to read the other thread on CYA testing which showed that higher CYA actually does save absolute chlorine.
    I did read that. That's the opposite of what theory would predict. The higher the CYA, the MORE chlorine should be used (in absolute terms).

    I'm more interested in what happens at low CYA, since I'm seeing very low chlorine loss at what must be near zero CYA.

    Since chlorine depletion is a non-issue, the remaining reason to consider CYA (in my specific case) would be whether the FC concentration is close enough to uniform to provide adequate sanitation. I figure seeing the dye disperse would at least give some qualitative data on how uniformly dispersed an additive will be.

    So, if FC homogeneity is really an issue, then I'd expect to see places whether the dye color is different (after the initial dispersion). Higher FC concentration areas should be turning greener than the lower FC concentration areas.

    My "gut" tells me it will all be the same color after initial dispersion (with pump running).
    17,000 gallon gunnite pool and spa, PebbleTech finish
    Pentair Gas Heater, AquaCal Icebreaker heat pump, two Intelliflow pumps
    Intellichem with two canister mounted pumps, EasyTouch 8
    Superior Aqua "Health Care" Copper Ionizer and ozone injector
    Pool Miser water level controller (filled from well water)

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