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Thread: Saturation Index (Langelier, etc.)

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    Default Saturation Index (Langelier, etc.)

    This thread discusses the Saturation Indices, especially Langelier and some corrections I made to that index.

    I've read the following three threads that Evan (waterbear) referred me to:

    http://www.poolforum.com/pf2/showpos...91&postcount=7
    http://www.poolforum.com/pf2/showpos...3&postcount=13
    http://www.poolforum.com/pf2/showpos...1&postcount=17

    I'm going to start off in a controversial way so please, no one take offense. If I'm wrong about this, I'd like to know. Though I start out sounding defensive, my conclusions are not different then yours in a limited use of the index, but I still think it is of some rough use.

    One argument against using the LSI for pools is that pools are an open system while the LSI was designed for boilers which are a closed system. This is a bogus argument (see, I told you I'd be controversial). It is true that the pool is an open system and is in fact a system that is out of equilibrium, but that does not mean that parts of the system, namely the pool water and its chemical contents, do not follow equilibrium chemistry. The only part of the pool water chemistry system that is significantly out of equilibrium is the amount of dissolved CO2 in the water vs. the amount of CO2 gas in the air. It is true that if this out of equilibrium situation was dynamically changing at a rapid pace on par with the rate of the equilibrium reactions, then you couldn't apply equilibrium equations to the rest of the pool system. However, this is simply not the case -- except for low pH, high alkalinity situations possibly with aeration where a regular rise of pH is being observed and is rapid, it is simply not true that the rest of the pool is not in equilibrium. The chemical reactions for the rest of the cabonate buffer system and calcium carbonate balance (and ion pairs and almost everything else) are absolutely occurring at extremely rapid rates with the exception of the scaling or corrosion of calcium carbonate solid (plaster and grout) itself. In other words, the pool water chemistry behaves as if it were a closed system with the air having a much higher quantity of CO2 gas (i.e. an equilibrium quantity so that no outgassing would occur). The bottom line reason that the pool is more like a closed system than an open system is that the chemical reactions in water are equilibrating extremely rapidly while the out-of-equilibrium state of CO2 is changing very slowly. Obviously, the pool isn't "closed" when you add chemicals to it, but after such chemicals get close to equilibrium (which is typically within a few hours, with decent circulation) then the system behaves as if it were "closed" and close to equilibrium [EDIT] in the short run. Obviously over time the outgassing of CO2 occurs and must be taken into account, but the huge difference in the fast equilibrium inside the pool water compared with the slow outgassing of CO2 means you can look at these two processes separately. [END-EDIT]

    I have seen the reaction rate constants for various parts of the chemical system and the slowest ones are with the chlorinated cyanurates, but are still occurring with half-lifes of 1/4 and 4 seconds. The hypochlorous acid and carbonic acid and cyanuric acid equilibriums are incredibly fast (as expected for simple protonation reactions). Reactions of chlorine with organics can be slow and breakpoint of chlorine (when it occurs) is usually slow (on the order of a 15-minute half-life), but this doesn't enter into the LSI since it is not related to the concentration of either calcium nor carbonate ion.

    The Langelier Saturation Index, or a better version of it (that I developed -- more on that later), is simply based on the solubility of Calcium Carbonate in water. It really has nothing specific in it related solely to boilers except possibly the interpretation of the values of the index itself (more on that later) and maybe the simplifications in the temperature and TDS parts of the index (which I have fixed). The LSI index does take into account the effect of ionic strength (mostly via TDS but done as an approximation), temperature (again, does an approximation), pH and alkalinity (both done correctly). The LSI index does not take into account ion pairs, but my spreadsheet calculations do though this doesn't have much of an effect unless you have a lot of sulfates in the water. I agree with you that the Ryznar Stability Index (RSI) does not apply at all to pools because it is not simply based on the solubility of calcium carbonate, but is an attempt to compare scale vs. corrosion of metal (not plaster) and is really quite simplistic for both. After all, the index is 2(pHs)-pH so that even when the pH is equal to the pHs (which means that there is chemical equilibrium for calcium carbonate), the RSI is equal to pHs which is ridculous as a general solution. It works only for certain pHs ranges as found in the database of water systems he used.

    As for whether to use the Hamilton index (even more simplistic) or to look at the total alkalinity instead of carbonate alkalinity, the answer is no, not in pools. All that matters is 1) what could possibly percipitate out of the pool water and 2) what could the pool water possibly dissolve. The answer to (2) is calcium carbonate found in plaster and grout. The answer to (1) is also calcium carbonate if you have this at equilibrium levels to prevent (2). The other items that could percipitate do not do so because they are not at saturation levels though it is theoretically possible to precipitate calcium sulfate but you'd need extremely high levels of both such as 1000 ppm calcium with 10,000 ppm sulfate (there is a neutral ion pair CaSO4º that prevents precipitation earlier). Magnesium Carbonate could percipitate if there is more than about 170 ppm of Magnesium (I calculated ppm is as Magnesium, not MgCO3) in the water. Of course, there are various sorts of reactions to precipitate metals, but these concentrations are minor (as far as the carbonate and calcium equilibriums are concerned).

    As for the variability of each of the components, it is true that pH is the biggest factor since it is a direct component while the other factors are logarithmic so that larger changes are required (and that alkalinity is more important than calcium hardness). That means that measuring those other factors doesn't have to be particularly precise while pH should be the most precise (how precise is debatable -- more on that later). However, it is not true that you'd see a big difference with salt pools since the difference in TDS is roughly from the 550-1000 range in a non-salt pool to the 3000-3500 range in a salt pool (remember that some of the initial 550-1000 is already salt so the total TDS number is pretty close to the salt number). The difference in the LSI is only about 0.1 while in my improved index it's about 0.2 which is still pretty small.

    Now the only thing I've tried to demonstrate so far is that the chemistry of equilibrium for calcium carbonate is valid. This does not tell us the meaning of the index in terms of its scale (meaning "size"). Sure, a "0" means theoretical equilibrium, but how far does the scale need to stray before either scaling or corrosion of the plaster/grout occurs? This is where I agree with you guys that the recommendation of staying with "+/- 0.3" and that you run into trouble outside of "+/- 0.5" is something that is probably not correct and that these are not the right numbers for pools. All reaction rates will be slower at the lower temperature of pool water compared to boiler water so at a minimum, any straying from equilibrium will either scale or corrode very, very slowly. Of course, pool surfaces are exposed to the water all year long for many years, but if Ben says he's never seen a problem, then I most certainly believe him. I can't find the link anymore, but somewhere I saw a post or something from Ben giving two extreme examples where he never saw any scaling or corrosion, but those examples produced an LSI (or my improved index) from around -0.15 to +0.55 which only proves that this range for the index doesn't say that problems will occur. Perhaps problems only start to occur when the range gets to +/- 1.0 or +/- 2.0 or perhaps a lopsided range. It is unfortunate that no one seems to have done any research on this.

    Now as to whether it matters to have calcium in your pool at all, I think you all agree that it is needed for plaster/grout pools. When I added sodium carbonate to the pool I noticed it being cloudy until it dissipated and I also found that if I dumped it rapidly in one place I could precipitate calcium carbonate. So on another day I took a sample of pool water and added sodium carbonate to it and saw it first get cloudy and then precipitate (what I believe was) calcium carbonate. Then I added acid (diluted muriatic acid) to this sample and saw the precipitate dissolve and the water become clear. What I didn't do was to see where the "boundaries" were when these processes occurred (i.e. at what LSI levels). Even if I did that, it wouldn't be the same as having a safety range for the long-term exposure of plaster/grout to water.

    (a little more to be continued in next post)
    Last edited by chem geek; 07-24-2006 at 09:28 PM. Reason: Added [EDIT] [END-EDIT] section(s).

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    Default Re: Saturation Index (Langelier, etc.)

    (continued from previous post)


    As for Ben's points about inaccuracies, that is not only true but is also even more reason why it's important to at least try to shoot for the middle (0) area of the index. Because there appears to be lots of leeway on either side, people do not need to get worried if they are a somewhat out of balance, but since we do not know which way all the measurement and other errors add up (though usually they statistically add up to approach a bell curve), it is safest to target the 0 index level even if it has an unknown bias. Obviously, if real-world experience implies a bias, then that should adjust the formula, but without such knowledge targetting the "0" is the safest thing to do.

    As for the point about how the NSPI recommended ranges give a large range of LSI values, that just says that some sort of "combination" table was needed just like Ben did for FC vs. CYA. The same argument that says that independent FC and CYA are not the way to go holds for saying that independent pH, alkalinity and calcium are not the way to go either.

    My final comment is regarding pool heaters since Ben mentions that in his post. The LSI shifts by about 0.3 with a 30ºF change, but my more accurate index only changes by 0.2. The index I developed matches the Taylor watergram almost everywhere except at extremely high temperatures while the LSI does not track the Taylor watergram very well. The fact that the Taylor watergram didn't match LSI was what got me started on the spreadsheet and improving the index (which I did through chemistry theory, not through trying to match the Taylor watergram). Anyway, as for the pool heater, I'm not sure that the increase in temperature at the pipe surface contacting the water is really 30ºF except in poorly designed inefficient heaters. I have a 200,000 BTU output heater that raises the pool temperature by about 1.5ºF per hour and my turnover is around 3-4 hours so this implies that the temperature of the water passing through the pool heater is perhaps 6ºF higher. Of course a poorly designed heater won't mix the water in the pipes very well so the water running along the pipe surface might not carry away heat towards the middle of the pipe fast enough. Anyway, even Ben's 30ºF number just says to err a little on the side of corrosion at normal pool temperatures.

    Anyway, that's what I think. I just didn't want people to think that the LSI is junk when it comes to pools. It may be overblown, but that's not the same thing as saying it has very little value.

    Richard

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    Default Re: Saturation Index (Langelier, etc.)

    The LSI as it is currently applied as 'gospel' along with TDS measurements in most pool stores are bogus measurements. While in theory it holds some value it is a most abused measurement today. It's main purpose seems to be to allow pool stores to sell unnecessary chemicals to customers. The store I work in has disabled the LSI function in our software and we rarely take a TDS measurement (We have a MyronL TDS meter...pretty much industry standard). This measurement is so abused in real world situations it is not funny. Taylor's watergram is useful as a rough guide but it's very construction does not allow for accurate results. It is really a 'ballpark' device. In the real world the temp. of a pool will vary by several degrees over the course of a season or year. My pool is open year round and my temperatue will vary from a low of about 40 deg. to a high of 90 deg. Does that mean I have to change my chemical balalce with every temperature change? Or should I shoot for a 'happy medium' that will keep my pool 'balanced' for the majority of the year? There are no easy answers to these questions. In real world applications things don't occur as they do under controlled conditions. I beleive the goal is to find measurements that will produce good results under the varous conditons that occur that are easy for the average pool owner to implement. The LSI has not proven itself to be one in real world conditions!
    Retired pool store and commercial pool maintenance guy.

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    Default Re: Saturation Index (Langelier, etc.)

    Quote Originally Posted by waterbear
    My pool is open year round and my temperatue will vary from a low of about 40 deg. to a high of 90 deg. Does that mean I have to change my chemical balalce with every temperature change? Or should I shoot for a 'happy medium' that will keep my pool 'balanced' for the majority of the year?
    I believe we are in agreement. I just wanted to address some of the reasons given for why the LSI wasn't applicable as being incorrect. It's a complicated formula (even more so when done correctly as I have done with more accurate TDS and temperature adjustments) and in practical terms there is a lot of leeway until you really "get into trouble" so it's best for most people to avoid using or thinking about the formula. I'm not proposing otherwise, though I did bring it up in the forum and probably shouldn't have. I'll probably continue to use it for myself, but that's because I'm a "chem geek".

    It's interesting you bring up the point about the variation with temperature. It turns out that pH itself varies with temperature, and does so in such a way as to partially mitigate the change in LSI with temperature. Specifically, if your pool (with standard NSPI "middle" settings) at 90ºF is at a CSI (my Calcite Saturation Index) of 0.09 with a pH of 7.5, then if the temperature drops to 40ºF your pool's pH will rise to 7.8 for a CSI of -0.06 so there isn't even a hint of an out-of-balance situation here. I have witnessed this pH change with temperature through two temp down/up cycles in our pool (and with our opaque pool cover that is mostly on, we normally get no pH drift at all when the water temperature doesn't vary much). Our pool is typically at 7.4-7.5 in the summer and 7.7-7.8 in the winter and I don't add any chemicals to make this happen! If the pH had not changed with temperataure, the CSI would have been -0.35 which is still not any sort of problem -- the two extreme's from Ben's data indicated that a range on the order of 0.50 wasn't a problem.

    As for your comment about the Taylor Watergram's very construction not allowing for accurate results, I don't understand that. It looks like I can easily get accuracy of 0.1 LSI. They make an assumption for CYA level since the watergram uses TA instead of Carbonate Alkalinity, but that's a minor error since a change of 30 in CYA results (at typical 7.5 pH) in about 10 TA change which results in a roughly 0.05 LSI change. Of course, Ben made the point about all the errors in the individual measurements introducing error in the LSI, but that's true whether the watergram is used or not.

    Richard

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    Default Re: Saturation Index (Langelier, etc.)

    The one problem with using Langelier index is that it is based on a closed system. You not only have to look at temp but also pressure in these systems. You increase pressure along with temp can changes in saturation rates. You also have to take in to mechanical forces on the materials themselves causing stripping or scaling of the materials
    Steve

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    Default Re: Saturation Index (Langelier, etc.)

    Quote Originally Posted by chem geek

    As for your comment about the Taylor Watergram's very construction not allowing for accurate results, I don't understand that. It looks like I can easily get accuracy of 0.1 LSI. They make an assumption for CYA level since the watergram uses TA instead of Carbonate Alkalinity, but that's a minor error since a change of 30 in CYA results (at typical 7.5 pH) in about 10 TA change which results in a roughly 0.05 LSI change. Of course, Ben made the point about all the errors in the individual measurements introducing error in the LSI, but that's true whether the watergram is used or not.

    Richard
    The TA can be ajusted for CYA and that result used on the watergram so that point is moot.
    Retired pool store and commercial pool maintenance guy.

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    Default Re: Saturation Index (Langelier, etc.)

    Quote Originally Posted by medvampire
    The one problem with using Langelier index is that it is based on a closed system. You not only have to look at temp but also pressure in these systems. You increase pressure along with temp can changes in saturation rates. You also have to take in to mechanical forces on the materials themselves causing stripping or scaling of the materials
    Steve
    Yes, but the Langelier index is based simply on the equilibrium equation:

    Ksp of CaCO3(s) = aCa(2+) * aCO3(2-)

    and then the hard work is calculating activities from the concentrations using activity constants that depend on ionic strength and determining the concentrations using equilibrium equations that are functions of temperature. I did all of that (it's in my spreadsheet you can download from the link near the end of my "Pool Water Chemistry" thread in The China Shop) and saw that the LSI does the same thing except that the formula parts for temperature and TDS dependence are over-simplifications and not accurate (especially at higher temperatures).

    Are you saying that the pressure from 6 feet of water is going to play a significant role in the equilibrium? If so, then this would mean that the index was still reasonable at shallow depths, but not perhaps at the floor of the deep end of the pool. Or are you saying that the LSI has some cooling tower or closed pipe pressure aspects in it? If so, I didn't see it.

    Richard

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    Default Re: Saturation Index (Langelier, etc.)

    The original formula was created for closed boiler systems. In hot water systems the pressure as well as temp does effect the saturation of water. The conversation of water to steam will cause an increase of TDS in the water and a drop TDS with in the steam. The primary reason that anticorrosion chemicals were added to antifreeze in automotive cooling systems. We used saturation indexes when setting up cooling systems for large earth moving trucks when I worked as a mechanic years ago.
    IMHO
    In a normal pool setting these indexes are not as relevant as in closed boiler settings due to high temps and pressure.
    Steve

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    Default Re: Saturation Index (Langelier, etc.)

    Steve, Evan, Richard:

    I see you all talking at cross-purposes. For example: Steve, Richard addressed the open system vs closed boiler system at the beginning of his post and argued that it wasn't relevant. I don't know enough chemistry to know if his argument holds water or not (pun intended), but you cannot simply dismiss it and ignore his arguments--you have to address them if you want your argument to be effective.

    Evan and Richard: I think you are both a lot closer than you realize. Evan's argument that in the real world, the LSI measures you get from the pool store are bogus is not at all negated by Richard's argument. What Richard has done is establish a way that the LSI can be used effectively--but in the hands of trained, knowledgable professionals, not pool store clerks.

    Clearly, for those of us who do not have a heater, having a pool store clerk check the "LSI" is about as useful as an electric fork. For those who do have heaters, it's no more useful because they don't know what they are doing, and even if they did, they aren't running the test meaningfully.

    What I'm getting out of all this, is the following practical application: IF a properly designed and applied LSI is used, it can help fine-tune the pool's chemistry to reduce scaling effects on heaters and SWGs.

    But, unfortunately, until Richard's indices can be applied correctly, and analyzed correctly, for the vast majority of us, the "LSI" at the pool store is and remains a bogus measure.

    BTW, Richard's analyses of various reactions has been fascinating. There has long been the comment from bleach users that pH does NOT seem to rise for most of us, despite bleach's pH of 11. Richard successfully explained that the breakdown of the chlorine lowers pH in nearly the exact amount that bleach raises it, creating a balance and sum of a neutral effect. Combined with the proper buffering of Tot Alk, we don't see pH move from adding bleach.

    This further explains why I have long noted that di-chlor powder drops pH despite being near-neutral (I believe it's 6.9 in pH). The chlorine breakdown lowers pH and the di-chlor isn't alkaline enough to offset it.

    Again, I'm no chemist so I can barely follow the science, but I think I'm gleaning the practical applications from it.
    Carl

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    Default Re: Saturation Index (Langelier, etc.)

    Let me through some things out there, just for the fun of it
    Quote Originally Posted by Richard
    except for low pH, high alkalinity situations possibly with aeration where a regular rise of pH is being observed and is rapid
    Sounds like about half (completely speculative) of the pools out there using trichlor to chlorinate and soda ash to adjust PH. Most people like the "ripple effect" of their returns so there is the aeration right? Also, diving, jumping and splashing around also is considered aeration is it not? Would these all not cause a regular rise of PH?
    Quote Originally Posted by Richard
    In other words, the pool water chemistry behaves as if it were a closed system with the air having a much higher quantity of CO2 gas (i.e. an equilibrium quantity so that no outgassing would occur). The bottom line reason that the pool is more like a closed system than an open system is that the chemical reactions in water are equilibrating extremely rapidly while the out-of-equilibrium state of CO2 is changing very slowly. Obviously, the pool isn't "closed" when you add chemicals to it, but after such chemicals get close to equilibrium (which is typically within a few hours, with decent circulation) then the system behaves as if it were "closed" and close to equilibrium.
    Hard to argue, especially since I am obviously waaay out of my knowledge base, but it seems to me that you are forgetting what effect swimmers might have on this closed system. Especially when you consider the contaminants they bring in which will in turn cause the chlorine to began to react. I also believe that debris (pollen, leaves dust) will also play a role.
    Quote Originally Posted by Richard
    My final comment is regarding pool heaters since Ben mentions that in his post. The LSI shifts by about 0.3 with a 30ºF change, but my more accurate index only changes by 0.2. The index I developed matches the Taylor watergram almost everywhere except at extremely high temperatures while the LSI does not track the Taylor watergram very well. The fact that the Taylor watergram didn't match LSI was what got me started on the spreadsheet and improving the index (which I did through chemistry theory, not through trying to match the Taylor watergram). Anyway, as for the pool heater, I'm not sure that the increase in temperature at the pipe surface contacting the water is really 30ºF except in poorly designed inefficient heaters. I have a 200,000 BTU output heater that raises the pool temperature by about 1.5ºF per hour and my turnover is around 3-4 hours so this implies that the temperature of the water passing through the pool heater is perhaps 6ºF higher. Of course a poorly designed heater won't mix the water in the pipes very well so the water running along the pipe surface might not carry away heat towards the middle of the pipe fast enough. Anyway, even Ben's 30ºF number just says to err a little on the side of corrosion at normal pool temperatures.
    I believe you are saying that if you shoot for "0", you can play the ranges. Makes sense to me. I did however believe the 30ºF was a little more accurate. I have a 100,000 BTU Heat Pump and I thought that there was definitely more than a 6ºF difference in the pool water compared to the water at the heat exchanger. So, just as a quick test I started the heat pump this morning and let it run for only 15 minutes so I'm not sure how accuarte my results are. Pool temperature was 82ºF (left the pump running all night with waterfall, spa jets and returns pointed up to cool the pool). I opened the lid of my inline chlorinator with the pump running (It has been empty for a month or so), with the valve set wide open, and stuck a digital thermometer in there and it read 94ºF, then 95ºF and slowly rising. The outside air temperature was 74ºF, the chlorinator is 16" from the heat exchanger and was out of the sun. Seams that 30ºF is in fact probably a safe number but wouldn't a gas or propane heater be much much hotter?
    I guess my doubt about the effectiveness of LSI is based on the apparent observation that soo much depends on "laboratory" parameters and a TYPICAL (especially not a poolforum subscriber) swimming pool has anything but such parametrs.

    Dave

    P.S.-I sure am enjoying your astute analysis and you have already brought so many things to light and to the forum so far. Please keep it up.

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