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Thread: Possible "shielding" benefit of CYA

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    iam4iam's Avatar
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    Possible "shielding" benefit of CYA

    I was very intrigued to read in earlier posts about the possible "shielding" benefit of CYA. The suggestion with regard to this benefit has been that its significance increases with pool depth. My thought is actually the opposite, since I assume the affect of UV rays on HOCl would be less at greater depths anyway, so the shielding benefit would be more significant for HOCl that is closer to the surface. (Is this assumption incorrect? It is certainly not based on physics beyond the fact that refraction is obviously involved.)

    Speaking of refraction, I'm conducting an experiment today and tomorrow measuring FC loss with returns directed upward to create maximum disturbance in the water surface (maximum refraction) compared to FC loss with the returns turned downward to minimize surface disturbance. My hypothesis is obviously that the disturbance in the water surface could inhibit UV penetration due to increased (or perhaps more "disordered) refraction.

    I've been running the pump basically all day (with returns directed upward) based on this yet to be validated hypothesis, but reading this thread has motivated me to experiment. Fortunately, today is mostly clear skies and tomorrow is supposed to be the same. I'm measuring FC loss from 10:30 am to 4:30 pm both days. In retrospect, since clouds are now in the sky (1:00 pm), I wish I had made today the "low disturbance" day. Tomorrow is scheduled to be clear all day and if I lost more FC on the low disturbance day even with a few clouds in the sky, that would offer more conclusive evidence in support of my hypothesis. I might just measure now since the vast majority of time between 10:30 and now has been clear.

    Based on my retrospective wish in the previous paragraph, I decided alter the experiment. I just took a measurement and pointed the returns down (minimal surface disturbance). The rest of the day looks like it is going to be partly cloudy (lots of fluffy cumulus), so I'll compare FC loss this afternoon (calm pool surface) to tomorrow afternoon (disturbed pool surface) which is scheduled to be clear. I'll also reverse the 10:30-1:15 return orientation (pointed down tomorrow since they were pointed up today), which will give me two sets of comparative data. It is obviously difficult to minimize lurking variables in an experiment like this!
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    Re: Possible "shielding" benefit of CYA

    I doubt you will be able to measure a significant difference. It seems unlikely that you will be able to make enough of a difference in the transmission of UV to be able to measure a difference above the noise level. Another thing to watch out for is that the amount of sunlight actually getting to the water can vary from day to day without any obvious difference in the weather. The human eye isn't good at detecting absolute light levels and small difference in high altitude haze can affect the amount of sunlight reaching the ground without any obvious difference in illumination.

    Regardless, it will be interesting to see what results you get.
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    Re: Possible "shielding" benefit of CYA

    Good point, Jason. You've just added credence to my last statement in my previous post (which I edited as you were posting). For what it's worth, the UV index for both days is the same.
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    Re: Possible "shielding" benefit of CYA

    It turns out there are going to be "bathers" in the pool even thought it's only 74 degrees, so at least the afternoon part of my experiment is shot anyway. I'll compare tomorrow's 10:30-1:15 to today's. Unless the difference is quite significant, the experiment will obviously be inconclusive due to lurking variables. I wish I had a UV lamp so I could perform the experiment with a smaller body of water and more controlled conditions!
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    Re: Possible "shielding" benefit of CYA

    Wouldn't pointing the returns up "mix up" the water surface, thus creating a scattering effect (and in your hypotheysis slowing the loss of FC) but at the same time mix up the water, allowing UV to reach areas where it might not have. I guess it depends on where your intakes are (bottom drain or not)

    Apparently, UV penetration also depends on the angle of incidece of the sun (which makes sense) as on page 24 here http://ies.jrc.ec.europa.eu/uploads/fil ... 217_EN.pdf

    It is an interesting idea, but as was said, I think ahy vaiation is going to be lost in the error bars.

    Two black graduated cylinders, with a grow light over them, may be a better idea (just remember to mix them before you measure) and would be a more scientific and repeatable experiment.

    -dave
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    Re: Possible "shielding" benefit of CYA

    Good thought, Dave. I was thinking that the "scattering effect" would likely decrease the amount of UV that penetrates far below the water surface, but as you point out, since the angle of incidence determines this, unless the sun is directly overhead, some of the UV rays may actually penetrate deeper than they would have with a calm surface. So I should modify my hypothesis to state that the assumption would be that the "surface disturbance" factor would be more significant the closer the sun is to directly overhead.

    By the way, I can't afford a grow light right now (great idea though), which is why I'm so concerned with maximizing my FC retention, or should I say, minimizing my chlorine cost.
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    Re: Possible "shielding" benefit of CYA

    I'm also recalling data from Janet (username Aylad) referred to by Richard early in his Pool Water Chemistry topic:

    Some users, most notably Janet (user name Aylad), report that in their non-SWG pools using high levels of CYA shows dramatic improvement in chlorine's staying power. In Janet's case, with a CYA below 60 she found that the FC would go from 7-8 to 2-3 in one day (5 ppm FC per day) while with a CYA of 80-90 the FC would go from 8-9 and take 3 days to go below 5 (about 1.2 ppm FC per day). That is a huge improvement that is wholly inconsistent with the graph.
    This is in part what led Richard to the discussion of the significance of the shielding benefit to begin with. Janet's report caught my eye in particular because I have been gradually raising my CYA level and observing my own FC loss relative to CYA level. The observations haven't been "scientific" enough to report at a quantitatively with any specificity, but I have noticed a slight reduction in FC loss as I have progressed from CYA = 20-30 to my current level of 70 ppm. I'm still losing 3-4 ppm of FC on a clear day, which is dissatisfying, as bleach costs me about $0.50/ppm for my 13,800 gal. pool. I'm going to give it some time before I intentionally raise CYA above 70, but reading what Janet reported makes me very curious.
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    Re: Possible "shielding" benefit of CYA

    See this post for more detail about UV absorption and the extinction coefficient. Unfortunately, that post contains special characters that have been fouled up since the forum upgrade and I'm not able to fix them (I can make changes that look OK in Preview, but get fouled up from a Submit). Anyway, also look at this post where Mark from this forum did experiments that showed how a CYA of 70 ppm protected chlorine from breakdown compared to 40 ppm by more than would be explained by a proportional mechanism. That is, there must be some additional UV absorption effect and since we know that CYA itself can absorb UV, that's the likely reason.

    Water absorbs infrared much more than UV so any UV-shielding effect from CYA will be important. So while most of the chlorine protection comes from it being bound to CYA, nevertheless some of it comes from direct CYA absorption of UV shielding lower depths. Note that this is absorption so is partial shielding -- don't think of it as a complete cut-off.
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    Re: Possible "shielding" benefit of CYA

    Quote Originally Posted by chem geek
    this post . . . Unfortunately, that post contains special characters that have been fouled up since the forum upgrade and I'm not able to fix them (I can make changes that look OK in Preview, but get fouled up from a Submit).
    I think I have fixed it so posts with multi-byte characters can again be edited without problems. At least I was able to fix that particular post.

    If you have other posts that you are having problems with, send me some links and I will see if I can clean them up as well.

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    Re: Possible "shielding" benefit of CYA

    Thanks Jason. So how can I create posts with these sorts of characters like micron µ and bullets • and epsilon ? ? It looks like new posts are OK (at least this one was) so is it only older posts that are a problem that need special editing capability (that I don't have -- or did you just fix this so now I should be able to edit older posts to fix them as well)?
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    Re: Possible "shielding" benefit of CYA

    I've been trying out various fixes for weeks. The current one, from the day before yesterday, appears to finally be working. I've had others that worked for a while and then stopped working, so I have my fingers crossed, but I'm hopeful I finally found the issue.

    Posts from before the server upgrade are still messed up, and so far I haven't found any way to fix all of them globally. I had been going through and searching for a few of the most common cases and fixing them here and there manually.
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    Re: Possible "shielding" benefit of CYA

    Quote Originally Posted by chem geek
    See this post Water absorbs infrared much more than UV so any UV-shielding effect from CYA will be important. So while most of the chlorine protection comes from it being bound to CYA, nevertheless some of it comes from direct CYA absorption of UV shielding lower depths. Note that this is absorption so is partial shielding -- don't think of it as a complete cut-off.

    Interesting. That would make me think, that pumps should be run at night. That way the water can stratify during the day. The CL in the upper layers will be degraded, and the CL in the lower layers will remain protected by the CYA in the water column. Of course this is all predicated by the assumption that nobody actualy USES the pool during the daylight hours.

    I let my pool sit unused (17,000 gal ABG with almost NO shade, 27 ft round, so whatever pi r^2 is, is the surface area) yesterday, during what was a semi-sunny day, to see what would happen (and to get an idea of what my CL consumption is). I did run the filter for 7 hours, but nobody was in the pool. I had my return pointed slightly up of horizontal. It would create some chop, but not a whole lot of vertical mixing.

    My CYA is about 50

    7:50 AM - CL 6.5 ppm (CC = 0)
    8:10 PM - CL 6.0 ppm (CC = 0)
    6:30 AM - CL 5.5 ppm (CC = 0)

    In reality, the 8:10 PM reading was one of those where it was *barely* pink at 5.5 and I added another drop and the 6:30 AM was one where it was even less *barely* pink (or maybe my eyes were blurry) at 5.5 so I didn't add another drop. Could call the 8:10 PM reading 5.75 and the 6:30 AM reading 5.70 and I would believe it.

    If I can, this weekend I will bring it back up to 6.5 and try it with the return pointed down. I don't expect to see much difference, but it gives me something to do while it is too cold to swim.

    -dave
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    Re: Possible "shielding" benefit of CYA

    Quote Originally Posted by phonedave
    Interesting. That would make me think, that pumps should be run at night. That way the water can stratify during the day. The CL in the upper layers will be degraded, and the CL in the lower layers will remain protected by the CYA in the water column. Of course this is all predicated by the assumption that nobody actualy USES the pool during the daylight hours.
    It doesn't work out that way, even though it seems like it ought to. This test has actually been done carefully with lab equipment and there is no significant difference in chlorine usage based on when the pump is run.
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    Re: Possible "shielding" benefit of CYA

    Quote Originally Posted by JasonLion
    Quote Originally Posted by phonedave
    Interesting. That would make me think, that pumps should be run at night. That way the water can stratify during the day. The CL in the upper layers will be degraded, and the CL in the lower layers will remain protected by the CYA in the water column. Of course this is all predicated by the assumption that nobody actualy USES the pool during the daylight hours.
    It doesn't work out that way, even though it seems like it ought to. This test has actually been done carefully with lab equipment and there is no significant difference in chlorine usage based on when the pump is run.

    Thats unexpected. Mixing of the water column has no impact.

    Hmmmm. Maybe I am overthinking this. It is *just* a pool. 4' to maybe 12'. It's been a while since I had waste water and hydro classes, but we were modeling much larger free bodies, and much greater depths where stratification, seasonal inversions, and thermoclines come in to play when determining the effects of polutants. Heck, the "sunlight zone" of the ocean goes down to about 200 meters. 4 meters is just a drop in the bucket (or pool)

    -dave
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    Re: Possible "shielding" benefit of CYA

    The CYA doesn't stratify. It's mixed and stays mixed and sunlight doesn't break it down to a measurable degree (it just breaks off the chlorine from some of it; much less so than unbound chlorine). In your scenario with the pump off what happens is that the unbound chlorine gets broken down more near the surface, but all that does is remove some of its own self-shielding effect (which it has) and more sunlight just goes lower. Some sun goes to the deepest part of the pool anyway (the UV in particular), but the net effect is that the chlorine level is lowest near the surface (it's not zero because there's plenty of chlorine that was attached to CYA to replenish it, but it should be measurably lower later in the day). The CYA aspect of this doesn't change.

    So what you might see with careful measurement is that during the day in still water the surface measures a lower chlorine level than deeper parts of the pool. This happens a little bit even at night from a small amount of chlorine outgassing, but the effect should be much stronger during the day. Remember, though, that there is still diffusion even in still water and if the pool is exposed to any wind then it can still get some minor circulation that way.

    I uploaded some spreadsheets I have used in the past for these sorts of calculations -- Water-Absorption.xls that shows the absorption of light in water by wavelength and calculates total absorption of sunlight vs. depth of water just for water's contribution only (i.e. no chlorine -- see also Water Absorption and Heating from Sunlight) and HOCl-Extinction.xls which shows the self-absorption of hypochlorous acid and hypochorite ion affecting depth as well as how the two breakdown at different rates (so having a strong pH dependence). There are some guesses at CYA absorption (there are no sources for UV absorption showing what happens above 255 nm), but I never added the breakdown of bound chlorine to CYA and now understand that mechanism better as to how it doesn't break down CYA itself. I believe it goes something like the following:

    HClCY- + h? ---> Cl• + HCY-
    HCY-• + H2O ---> H2CY- + OH•
    --------------------------------------
    HClCY-+ H2O + h? ---> H2CY- + OH• + Cl•

    which is the same net effect as the breakdown of hypochlorous acid itself (and hypochlorite ion) though the above is much slower (less frequent -- lower quantum yield) than the following (which has a quantum yield near 1 meaning any light that is absorbed results in the breakdown of the molecule -- see this paper):

    HOCl + h? ---> OH• + Cl•

    OCl- + h? ---> O-• + Cl•
    O-• + H2O ---> OH• + OH-
    ----------------------------------
    OCl- + H2O + h? ---> OH- + OH• + Cl•
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    Re: Possible "shielding" benefit of CYA

    Quote Originally Posted by phonedave
    . . . with almost NO shade. . . during what was a semi-sunny day . . .

    My CYA is about 50

    7:50 AM - CL 6.5 ppm (CC = 0)
    8:10 PM - CL 6.0 ppm (CC = 0)
    6:30 AM - CL 5.5 ppm (CC = 0)
    I'd be ecstatic to have FC depletion that slow! My CYA is 70 and I went from ~6.7 ppm (barely pink after 13 drops) to 5.5 ppm between 10:30 am and 1:15 pm (choppy surface as part of my very uncontrolled experiment).
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    Re: Possible "shielding" benefit of CYA

    As has been mentioned before, my "experiment" is very uncontrolled, having numerous lurking variables, but here are my results:

    6/7 (returns pointed up for choppy surface)
    10:30 am--FC = just over 6.5
    1:15 pm--FC = 5.5
    FC loss > 1 in 2.75 hrs
    (I was planning on measuring loss for 6 hours, but cut short for 2 reasons: sky was becoming cloudy and FC was approaching min. for CYA level, so I added bleach at 1:15.)

    6/8 (returns pointed down for smooth surface)
    10:30 am--FC = just over 7
    1:15 pm--FC = 7
    FC loss < 0.5 in 2.75 hours!

    Ignoring uncontrolled variables, this indicates that smooth water surface actually reduces FC loss, which is the opposite of my original hypothesis being tested! (Dave, is this what you were suggesting?)

    All I know is that I won't be intentionally pointing my jets upward anymore because it clearly doesn't reduce HOCl dissipation and it does increase the rate of pH increase, which I have successfully reduced over the last few weeks by getting my TA down to 100 with acid and intentional aeration via returns pointed upward. Now I just need to experiment to find what return orientation most effectively moves small surface objects toward the skimmer! I'm also going to evenly split pump run time between day and night with shorter on/off intervals.
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    Re: Possible "shielding" benefit of CYA

    Neat. If you repeat that test once or twice more and get the same result that will rule out most of the uncontrolled variability (i.e. solar intensity) having anything to do with it.
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    Re: Possible "shielding" benefit of CYA

    Quote Originally Posted by iam4iam
    Ignoring uncontrolled variables, this indicates that smooth water surface actually reduces FC loss, which is the opposite of my original hypothesis being tested! (Dave, is this what you were suggesting?)

    I wasn't sure what I wsa suggesting. Just sort of spitballing there.

    -dave
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    Re: Possible "shielding" benefit of CYA

    Quote Originally Posted by JasonLion
    Quote Originally Posted by phonedave
    Interesting. That would make me think, that pumps should be run at night. That way the water can stratify during the day. The CL in the upper layers will be degraded, and the CL in the lower layers will remain protected by the CYA in the water column. Of course this is all predicated by the assumption that nobody actualy USES the pool during the daylight hours.
    It doesn't work out that way, even though it seems like it ought to. This test has actually been done carefully with lab equipment and there is no significant difference in chlorine usage based on when the pump is run.
    Assuming the test referred to above was testing UV dissipation, was the water surface still smooth when the pump was on? This is not at all intuitive or even necessarily logical from what I can see, so I may be going off the deep end in the deep end, but if the pump was disturbing the surface of the water, could it be that surface disturbance and subsurface movement are two separate variables that canceled each other out? I was actually of thinking of testing (non-laboratory, of course, and therefore only moderately controlled) pump on vs pump off with returns pointed down so water surface is smooth in both cases.
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