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)

(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

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!

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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?

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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

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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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

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The TA can be ajusted for CYA and that result used on the watergram so that point is moot.

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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

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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

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

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.

Let me through some things out there, just for the fun of it:eek:

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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

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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?

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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.

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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.

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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.

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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.:)

I am not trying to disprove the chemistry. The chemistry is good on paper. The problem with seeing it as an open or closed system is in an open system there are many asumptions we have to make. In closed system we know what the exact chemistry is due to the fact we put it in there. The pressure issue is gasses are seen at different concretions at different pressures. Take a soda pop for example here. Pop the top and the CO2 in the liquid just dropped. You are looking at other problems with these indexes due to many variables that you to assume when you have liquid to atmosphere interface. We cant really predict the out gassing gases across this interface as a constant due to atmospheric conditions. We can get close but not close enough to be a constant.
Well past my bed time. Have a good day and don't work to hard, zzzzz
Steve

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Originally Posted by medvampire

In closed system we know what the exact chemistry is due to the fact we put it in there. The pressure issue is gasses are seen at different concretions at different pressures. Take a soda pop for example here. Pop the top and the CO2 in the liquid just dropped. You are looking at other problems with these indexes due to many variables that you to assume when you have liquid to atmosphere interface. We cant really predict the out gassing gases across this interface as a constant due to atmospheric conditions. We can get close but not close enough to be a constant.
Steve

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Steve,

Yes, the pool system is open so there are all kinds of things entering into the system to throw things off away from the exact chemistry of a closed system. And yes, the pool system is way out of equilibrium with respect to the amount of dissolved carbon dioxide in the pool vs. in the air. However, this doesn't mean that you can't look at the chemistry to make predictions with the caveat that such predictions are approximate and subject to change based on factors that are unknown. If one were to ignore the chemistry, then you couldn't even have the rough rules of thumb for figuring out how much base or acid or bicarbonate to add to the system. Yes, Ben and others recommend putting in half the "calculated" amounts and then re-measuring, but no one is saying to completely ignore the calculated amounts and just make a total guess as to what to do with regard to adding chemicals to make adjustments. In fact, the main reason to not use the tables as is, is that they have assumptions built into them. If you measure all the major factors and calculate as is done in my spreadsheet, then it predicts quite well, at least in my pool. I never have to dose twice.

Also, as far as the out-of-equilibrium carbon dioxide part of the system, it is absolutely true that the factors are too complex (surface tension, wind, etc.) to calculate the precise rates of outgassing, but that does not mean one cannot look at the chemistry to understand that lowering the pH increases this rate dramatically and that increasing the alkalinity also increases this rate though not as dramatically. The fact that the outgassing of CO2 has the pH rise without a change in alkalinity also comes from the known chemistry. The "unknown" factor is the rate of aeration, but with a somewhat constant amount of aeration [EDIT]as an "average" over a period of time and a slow process in most pools[END-EDIT], then the pH and alkalinity effects on CO2 outgassing are known chemistry. In fact, Ben depends on such known chemistry (even if he did it with his gut feel, or experimentation, or knowing the chemistry itself) in creating the successful alkalinity lowering procedure. The chart I created showing the relationship of outgassing of CO2 to alkalinity and pH uses a relative outgassing scale since I have no way of knowing this rate accurately. The only assumption made in this scale is that the rate of outgassing is proportional to the difference in concentration between the actual dissolved CO2 gas in the water vs. its equilibrium concentration (based on the actual CO2 in air). Even if Henry's Law [EDIT](for equilibrium, and other laws for rate kinetics)[END-EDIT] doesn't apply, there would still be a concentration dependence though it may not be linear, but for a "relative" scale that isn't that important. [EDIT]Also, one does not need to know the rates to know that any decline in alkalinity (after adding acid to restore pH) is due to outgassing of CO2 and one can calculate such amounts as accurately as the measurements are made. This assumes that there are no other sources or sinks of CO2, such as algae in the pool, but that's a good assumption for a "clean" pool.[END-EDIT]

Now even if the LSI (or my improved CSI) were "correct", that doesn't mean it should be used as regularly as other guidelines such as the NSPI ranges [EDIT](except for chlorine)[END-EDIT] or Ben's CYA/FC table. I absolutely concede that we do not know enough about the specifics of pool chemistry to know how far away from 0 the saturation index must get before, in practical terms, scaling or corrosion becomes a problem. That does not mean, however, that the principles of the index are useless. Everything Ben has said in terms of his practical analysis of the index are [EDIT]consistent with[END-EDIT] the science, even if he didn't start from the science to draw such conclusions. That is, the pH is by far the most important factor, alkalinity is next, and calcium is a very distant third. Nevertheless, if you didn't put any calcium into your plaster/grout pool, you would get corrosion -- if you didn't, then no one, not even Ben, would recommend it.

Sorry to get so riled up about this. I absolutely understand the limitations of science as applied to the real world, but I would rather use the science as a guideline with caveats so long as there is some reasonable fit to real world data. This is why in some posts I am most interested in finding answers when the real-world data isn't fully or mostly explained by the science and if I can't come up with an explanation or modification, then I will strongly caveat that particular part of the science as not applying since it is too incomplete or incorrect. The unexplained extra rise in pH several members are seeing is one such example. In some cases, it is explained by the CO2 outgassing as measured by the slow drop in alkalinity (after pH is restored with acid) over time. In other cases, it is not and we may very well not figure out what's going on, but I don't give up that easily because we may figure out something important for people to know. One person suggested that DE filters may leach base (from the clay) and if that is true, then this would be important for people to know -- not so much to calculate, but to understand the "normality" of what they see, just like people who use Tri-Chlor (most shouldn't except to raise CYA) should understand that its very acidic and that they shouldn't [EDIT]take the tablets out of the floater and put[END-EDIT] them in the skimmer without the pump running and that they can expect to add lots of base to compensate, etc.

I don't believe we are in disagreement. You don't want me to use the science blindly as if it's perfect because there are too many variables in an open system and I don't want you to not use the science at all for those same reasons. I think we can agree on the judicious use of the science with appropriate caveats -- we may disagree on exactly where to draw the line or how vigorously to pursue corrections for real-world situations, but that's more a question of being practical vs. model-centric and yes, I'm more in the latter camp.

Richard

I responded to Steve (medvampire) but just now noticed that CarlD and David (DavidD) had posted, so I'll respond to those posts now. Some of this may be repetitive, but if so I'll try to phrase things differently to make things clearer.

First of all, I believe Carl's summary of how close we are or what we are debating is right-on. So there's hope that I'm being clear about at least a few things as I worry that my jumping back and forth between being very technical and trying to speak non-technically may not be successful.

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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.

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Though the chemistry explains a near-neutral net effect for bleach and a slow net rise from the extra base in liquid chlorine, as well as the net decrease in pH when using di-chlor, the fact is that there are quite a few users out there (see rising ph levels) who are experiencing quite substantial pH drifts upward using only bleach. Some have alkalinity drops (after adding acid to restore pH) and are at least partly explained by outgassing of CO2, but others have no such drop nor explanation. This is a perfect example of where reality does not match the science and yet I'm going to try, with the help of these users, to figure out what's going on if possible. If we fail, then we will not throw out the science of what is explained, but will at least have some parameters for how bad it can fail to predict and in which direction.

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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?

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The use of trichlor is acidic plus the using up of chlorine is acidic so the combination is very acidic. This ends up lowering the pH and alkalinity a lot which is why you constantly have to add some sort of base (soda ash of caustic soda). If you add soda ash, then you are increasing both pH and alkalinity, the latter beyond that which would be increased by pH alone (that is, you are actually adding carbonate to your water). So such users do not see a rise in pH even though they are probably outgassing lots of CO2 because the acidic process (lowering of pH) from trichlor vastly overwhelms the CO2 outgassing effect and its relatively smaller rise in pH.

As has been pointed out, we really have no way of easily and accurately knowing how much outgassing of CO2 is occurring except by witnessing "after the fact" how much the alkalinity drops over time (after adding acid to restore pH). The rate of aeration is not only hard to calculate except from such long-term "after-the-fact" analysis, but it changes in the short-term depending on pool usage (e.g. splashing) and environmental conditions (e.g. wind). The good news is that it doesn't really matter what the aeration rate is, you can still justifiably tell people that if they operate at lower pH then they will have more of a problem than if they operate at higher pH. Now it is certainly possible that they have such a low outgas rate already that the pH change won't show up visibly, but we know that if they go low enough then the outgassing will certainly occur -- otherwise, Ben's method to drop alkalinty wouldn't work reliably.

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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.

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There is no question that the external environmental effects will change the system and therefore the results. However, I believe it is better to understand the baseline "closed" system model as a starting point and then you can adjust that model, even qualitatively, to account for the factors that you mention. If this is either too hard or varies too much, then you can just throw up your hands on some issues, but I believe it's still worth trying to look at the pool and understand it as much as possible first and then only give up on those items that are too hard to figure out. I am also spoiled in my own pool situation because I have an electric (so it's easy to use and therefore covered most of the time) opaque (so UV doesn't get in) pool cover (that does not let air circulate underneath it) so my pool is much closer to being a closed system than most.

However, as far as the chlorine reacting with contaminants, be it ammonia or organics, it turns out that so long as you get to breakpoint with the ammonia and you oxidize the organics (i.e. you get to CC=0), then the chemistry that is involved turns out to have identical acidity as the breakdown of chlorine from sunlight and the breakdown of chlorine via temperature. Because all of these processes have an identical acidity effect, it doesn't really matter which one of them occurs or in what combination. If the chlorine all gets used up and you stay at CC=0, then the acidity of the process is predictable so the net effect with bleach is close to a net zero change in pH. Interestingly, a salt pool could start to see a chlorine outgas effect, especially at very low CYA or with no CYA, and this effect is very basic (i.e. causes a large increase in pH) and this effect would also be made more rapid by aeration and somewhat increased by lower pH, but mostly increased through very low to zero CYA and higher chlorine (FC) and salt (TDS). This is theory at this point as I have not heard of this effect in real pools yet (but then I have not heard of a salt pool with no CYA in it).

Now, of course, the junk that gets put into the pool may have a direct effect on the pH due to the pH of the junk that is put in and that is pretty much a total unknown. I believe that Evan (waterbear) mentioned on one thread about acid rain having an effect. Diligent users could actually make measurements of their rain water and actually predict what the change in their pool pH would be with various amounts (inches) of rain. I don't expect anyone to do this, of course. I'm just saying that there are things about the chemistry that can be understood even without going into the details.

As for the heater temperature, the difference in temperature that is relevant is the difference between the incoming pool water temperature of 82ºF and the outgoing temperature of 95ºF+ that you measured. The outside temperature of 74ºF is not relevent since the saturation index you would use would be based on pool temperature, not outside temperature. Anyway, you are finding a larger increase than is found in my pool, but I could only measure at the point of jets in my pool and there may be some (though I don't think that much) heat loss from the heater to my pool. I think the temperature difference is just a function of heater output and GPM and I probably have an overdesigned system with too high a GPM (though I have a solar system so maybe this is "reserve" power for when the solar panels are used -- I should probably have a 2-speed pump, but that's a whole other discussion).

Richard

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Originally Posted by waterbear

The TA can be ajusted for CYA and that result used on the watergram so that point is moot.

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You are right. I forgot about that. It's been a while since I've used the watergram since I've got my spreadsheet instead. I took a quick look at it in responding to your post and saw the "Total Alkalinity" scale, but forgot they had a CYA adjustment in their instructions so that the watergram scale is really "Carbonate Alkalinity" -- too bad they didn't mark it that way to begin with.

By the way, I'm going on vacation later this week through the next and won't have Internet access so if you don't hear from me, that's why.

Richard
First of all let me commend you on your calculations and models. I had though about doing something along the same lines but just don’t have the time. I do agree that looking at the models of what could be happening can be of great importance. I don’t want to discourage you in any way of seeking the truth behind what is occurring in our pools, after all science is about the truth. The only caution I have is due to assuming constants that cannot be assumed but that are very dynamic. I have followed your post with great interest and have learned from your models. The models you have presented here are very informative but in the end it’s a Schrödinger’s cat situation.
Steve

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Originally Posted by medvampire

Richard
First of all let me commend you on your calculations and models.

The models you have presented here are very informative but in the end it’s a Schrödinger’s cat situation.
Steve

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Steve,

Well, I certainly hope that my looking at the pool and taking measurements is not causing a quantum state reduction that affects my results.:eek: Seriously, I thank you for the compliment and encouragement and I hope that if and when I stray too far to extending a model when it simply won't fit or sounding too pompous or absolute about a result, that you chime in with a word of reason and caution. I'll try to be more careful with my posts in the newbie sections since my attempts at brevity can sometimes lead to sounding more authoritative than is justified.

Richard

Richard,
I think your theoretical discussions of pool chemistry are extremely important! They provide a good basic model that we can then apply to real world instances and in doing so discover what other factors are playing a role in the outcome.....keep up the good work!

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Richard
Well, I certainly hope that my looking at the pool and taking measurements is not causing a quantum state reduction that affects my results.:eek:

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Richard
If I find a worm hole in my back yard its your fault.:D
Steve

Saturation or scaling indices - puckorius, Ryzner or Langlier are used extensively in the water treatment industry to prevent scaling on heat exchangers (heaters) and we also use them on the industrial pad type humidifers (Munters). I am no expert on them though I do know that using them will prevent excessive scale (calcium) build up. While everyone dumps on pool stores that use them improperly, they can help in a situation where scaling is an issue. An example is my fathers hot tub and his unsoftened well water with a Calcium level of 1000 from the tap. He has scale rings foring in the fiberglass tub and is concerned about his heater scaling. So who knows how to use properly and can tell me the right combination of Alkalinity, pH, etc. to stop his scaling?

The previous discussion from months ago did not totally dismiss the saturation indexes but did explore the non predictability of a pool environment. Even after my discussion with Richard (chemgeek) I do feel there are merits to predicting the saturation index using his calcations. I can’t find his spreadsheets he created readily but he did some great work in prediction scaling and I would recommend using his calculations. Down and dirty answer. Lower pH and ALK may prevent the precipitation of calcium but may damage the heater if the heater core is copper.
Steve

The spreadsheet is still accessible at the PoolEquations.zip link found near the end of the first post in this thread. The corrections or enhancements I made to the Langelier saturation index were not huge (to the temperature and TDS portions of his index), but it is just based on the solubility of calcium carbonate. The theory is that having calcium carbonate just "in balance", that is in full saturation, means that the plaster will not have a tendency to dissolve nor will it have a tendency to form additional scale. In practice, one needs to be well away from the "0.0" balance of the index to see problems. Some people don't notice issues until +0.7 while others don't see problems until +1.0 (on the positive side for scaling -- don't know for corrosion or pitting of plaster, but may also be this same order of magnitude).

With regard to copper and other metal pipes, the theory is that having near-saturation calcium carbonate forms a thin film or other properties on the metal that helps to inhibit corrosion. Too much saturation will cause scale and too little prevents the film from forming. However, even without the thin film, corrosion of metal requires other factors including lower pH and dissolved oxygen and conductivity (ionic strength). The pool environment and the above neutral (> 7.0) pH usually mean that metal will not corrode. It is generally only when one uses Tri-Chlor tablets and doesn't carefully montor the pH that one risks getting the pH lowered. Storing chlorine in non-airtight containers in the pump house can also lead to corrosion of pumps and other metal.

Richard

To clarify, my Dad's issue is with a fiberglass hot tub. He has white rings forming just above the water line and is concerned about the heater getting fouled. His tap water is 1000 Calcium (from a well). When he called the spa manufacturer, they recommended a sequestering (also know as chelating?) agent such as Leisure Time Spa Defender. Am requesting MSDS sheets for it. They also suggested the ultimate solution would be to use 50% salt softener water with 50% well. My suggestion to him was to lower his pH from 8 to 7.4 and his alkalinity from 80 to 60. He uses bromine. Will look at the calculator you mention for langlier. Doesn't temp have a major influence on the scaling indices? Since the "surface temp" of a heater element is much higher than the surrounding water, doe this effect the optimum point to prevent scaling?

My other suggestion to him was that he may be better off just getting the water to where it won't form the white rings and not worry about the heater. It is supposedly S.S. Suggested he find out how difficult it is to remove and clean it since S.S. will easily tolerate acid cleaning.

To clarify futher - a pool heater is often gas fired into a heat exchanger. Differences in the tube or exchanger to water temp would most likely be in the 30F area as Ben suggests depending on the water flow rate over the heat exchanger. In an electric water heater, the heater is essentially a calrod like an electric stove and some have a sheath over the element made of S.S. Looking at a watlow info sheet on one of their heaters, the surface temp difference to the fluid can be as high as 100-200F. A very minor build up on the element sheath (same with a gas fired unit) can reduce transfer efficiency and cause higher gas or electric usage. For electric heaters, the buildup can be so severe as to cause the element to overheat and burn out. Since a pool heater exchanger material can be materials other than S.S. that do not lend themselves to being isolated and chemically cleaned with acid, it would seem to me to be important to keep the exchanger or elements clean - energy will only get more expensive!

Below is a question about just such an issue:
"Thank you for your replies. I am in New Zealand and investigating why we cannot get domestic water heater elements that last.We can get 304 s/s and Incoly sheathed elements (at a price) but as they are machine made the element leg diameter is rather small to give a good watts/sq.in.ratio. Corrosion of the sheath and/or resistance wire failure occures prematurely because of lime scale buildup which raises the surface and internal temp.of the leg.I also believe that current element design of two or more element legs in parallel causes a hot spot between the legs which encourages the deposit of lime in this area. It soon builds up into a solid mass and
increases corrosion in this area.I am looking at a design for possible manufacture using a single tube of a material which will at least help solve the corrosion problem."

Here is another one that is out there - Hamilton Index/:

http://www.southshoregunitepools.com..._balancing.htm

The Hamilton Index™ was developed by a pool technician for the pool and spa industry. This index was tested by field service and research over a period of 11 years. The index is so accurate, that in 20 years of field application, it has not needed to be modified.

Today, there are an estimated 400,000 pool and spas being run on the Hamilton Index™. Interestingly, in 1991, the National Pool and Spa Institute lowered the recommended total alkalinity towards the guide lines that the Hamilton Index™ has used for 25 years. But, of course, they are still calling it the Langelier Index.

Besides being very accurate, our 3-Step System in extremely easy.

Step 1: Test total hardess (not CH). That tells you where to put your total alkalinity - exactly!

Step 2: Put your total alkalinity where the chart tells you.

Step 3: Keep your pH at 7.8 to 8.2.



The chart is on the link I attached.

More info from the NAtional Pool and Spa Institute - whats the feeling on the general content of this document?

http://www.theapsp.org/NR/rdonlyres/.../chapter_8.pdf

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Originally Posted by DaveD

To clarify, my Dad's issue is with a fiberglass hot tub.
:
Since the "surface temp" of a heater element is much higher than the surrounding water, doe this effect the optimum point to prevent scaling?

My other suggestion to him was that he may be better off just getting the water to where it won't form the white rings and not worry about the heater. It is supposedly S.S. Suggested he find out how difficult it is to remove and clean it since S.S. will easily tolerate acid cleaning.

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So with the fiberglass hot tub you don't need to worry about corrosion or pitting of plaster/gunite surfaces (unless you've got tile grout around a top edge or something like that). So you can tend to run your water in a "corrosive" state with regard to the normal indices. Just don't have it be too low in pH. And yes, having a SS heater means you are probably even less likely to corrode than copper (iron would be worse, of course).

The problem, of course, is that your father's source water is so high in hardness to begin with. We've talked about this issue of high hardness source water in other threads and pretty much come to the conclusion that the most effective way to deal with this is to use a water softener (the type where water flows through an ion exchange resin or similar device) which will essentially exchange the calcium ions with sodium (or potassium).

So assuming the source water is high in calcium hardness but isn't very high in TA (otherwise it would be very cloudy and percipitating calcium carbonate unless it's pH was also high), you could just run the hot tub water on the low side of TA (about 50) to compensate for the high CH. The main downside with the lower TA will be that the pH will be harder to control. Also, even with a TA of 50 and a CH of 1000, at hot tub temperatures you will still be in a scaling state though much less so. You could use some Borax (compensated with acid) for a borate buffer (see this thread for more info on this), but such a buffer tends to help prevent a rise in pH and isn't as good in preventing a drop in pH. All in all, I think getting the CH lowered from the source water is the way to go and using the water softener will let you do that.

As for temperature, yes you are right that the scaling is a function of temperature so when a heater is used you want to take that into account. In practice, the heater isn't on all of the time so while some tendency towards scaling might occur while heating, it may have a tendency to dissolve such scale when not heating. Ben tends to run his pools slightly on the corrosive side for this reason (and possibly others) but in your specific case you can safely run more on the corrosive side without harm because the hot tub is fiberglass and has no plaster/gunite/grout.

Your advice of running at lower pH (still above 7.0, however) and lower TA is good. It's just that with a hot tub it is much harder to control pH to begin with so with the lower TA and lower pH operating conditions it's trickier to prevent going too low. Fortunately, the bubbling action in a hot tub would tend to outgass CO2 and make the pH rise (though less so at lower pH and lower TA), but any chemical additions (including bromine) could affect the pH. You'll have to experiment to find that optimum balance of non-scaling, stability and control.

Richard

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Originally Posted by DaveD

Since a pool heater exchanger material can be materials other than S.S. that do not lend themselves to being isolated and chemically cleaned with acid, it would seem to me to be important to keep the exchanger or elements clean - energy will only get more expensive!

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I agree, but the recommendations near a saturation level of calcium carbonate don't generally lead to scaling. The difference in index for a 30 degree rise in temperature is about 0.24 whereas scaling is usually seen only for much higher indices around 0.7 to 1.0 and higher (though Taylor and others talk about +/- 0.3 being a target range and others say +/-0.5). So when using a heater, yes it makes sense to keep the water a little more on the corrosive size of the "ideal" balance. That way, any scaling that occurs during heating could dissolve when cool. I believe the net feeling on this forum is that way too much attention and worry is paid to the indices -- not that they are useless, but rather that you have to be way out of whack before you get into trouble. Unfortunately, your father's situation had an index over +1.0 (pH 8.0, TA 80, CH 1000, Temp 104+) and that is into the much more clear area of scaling. Your lowering of the pH to 7.4 and the TA to 60 gets the index down to around +0.3 which still has a tendency to scale, but only somewhat. It probably won't add much to existing scale, but it won't dissolve such scale either.

Richard

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Originally Posted by DaveD

Here is another one that is out there - Hamilton Index/:

http://www.southshoregunitepools.com..._balancing.htm

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Well, I'm probably going to be showing my biases in what I am about to say and others can whack me over the head to give some balance. Let me begin by quoting something that is recommended on the Hamilton index page:

"To Lower Alkalinity - Add Acid in a 2-3 ft Circle Away From Return Lines And Skimmer"

As lots of people (including Ben) will tell you on this forum, this simply does not work and is not the effective way to lower alkalinity. The ONLY way to lower carbonate alkalinity (since other components of alkalinity such as that from CYA aren't the target for getting lowered) is to remove the carbonate, CO3(2-), and bicarbonate, HCO3(-), from the water and the way to do that is to enhance the outgassing of carbon dioxide, CO2, using Ben's Lowering Your Alkalinity procedure. It is the combination of low pH and aeration that leads to the faster outgassing of CO2 and the subsequent lowering of TA when you add acid to keep the pH down. Adding acid alone will simply lower BOTH the pH and the TA -- not just the TA alone.

The Hamilton recommendation is to never run the pH below 7.6 (with 7.8 to 8.2 as ideal). Running at higher pH does have some advantages and Ben talks about this at this thread where Ben tells us even more information about Jock Hamilton, the creator of the index. It mostly reduces the tendency for the pH to rise in pools due to the outgassing of CO2 since the pool (plaster/gunite ones, anyway) is intentionally out-of-equilibrium with the air to saturate the pool with calcium carbonate. At higher pH, less carbonate is needed so pH tends to be more stable. This reduces chemical (acid) additions and has other benefits that Ben mentions.

The Hamilton index uses Total Hardness that includes Magnesium, and not just Calcium Hardness. However, it is calcium carbonate that percipitates and is at saturation, not magnesium carbonate, so using Total Hardness will sometimes work and sometimes not depending on the ratio of Calcium to Magneisum in the water. Typical water has 4 times as much calcium (by weight) as magneisum, but this varies and the S.F. city and peninsula (i.e. Hetch Hetchy water) has a ratio of about 2.6 instead of 4. With the 4 to 1 ratio (by weight) the conversion calculation is to divide Total Hardness by 1.412 to get Calcium Hardness.

The Hamilton index would tend to produce scaling at the higher Total Hardness levels. At the extreme of 2000 ppm Total Hardness, if we assume this is about 1400 ppm Calcium Hardness, then the recommended 40 ppm TA with a pH of 8.0 yields a saturation index of +0.6 which might start to scale. In practice, most pools probably had far lower Total Hardness and it is possible that at the much higher Total Hardness levels that less of it is from calcium and more is from magneisum. In an extreme situation with a lot of magnesium relative to calcium, you could percipitate magnesium carbonate first, but that is not at all typical.

The bottom line is whether there was a problem with people using the Langelier index and finding that they were scaling or corroding their pools. Unless that was the case, then both indices in the typical ranges for pool owners produce similar results especially given the great leeway in the indices before problems are actually seen. What I did find is that the Langelier Saturation Index used by pool stores does not vary correctly by temperature or by TDS for reasons that I cannot figure out except that the index was "simplified" by someone at some point to use logarithms for everything including the temperature and TDS portions when technically those do not vary by logarithm (while calcium hardness and carbonate alkalinity DO). The full calculation for calcium carbonate saturation that is in my spreadsheet does track the Taylor watergram almost exactly except for the very highest temperatures (140F) where it is off by about 0.1 (assuming that Taylor is correct and I have no reason to believe that they are not).

Richard

Quote:

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Originally Posted by DaveD

More info from the NAtional Pool and Spa Institute - whats the feeling on the general content of this document?

http://www.theapsp.org/NR/rdonlyres/.../chapter_8.pdf

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There is a heck of a lot of info in this document and you can find this forum's general feelings on all of these topics by reading through the stickies in each topic area.

The NPSI does not vary their Free Chlorine (FC) recommendations vs. the amount of CYA in pools as per Ben's Best Guess CYA chart and they are wrong not to do so.

The suggestion of lowering high TA water by adding acid is only partially correct since doing so efficiently also requires additional aeration (as I discussed in a post earlier referring to Ben's alkalinity reduction procedure).

As for their discussion of calcium hardness, they overemphasize its importance when you don't have plaster/gunite/grout exposed to the water. As I mentioned, the calcium carbonate is supposed to inhibit corrosion on metal surfaces, but keeping the pH above 7.0 is the best way to avoid metal corrosion. In theory, salt water pools should have more corrosion problems than non-salt water pools, but I haven't seen any reports of that on this forum. The problems may only come to play when the TDS gets exceptionaly high as with sea water where the conductivity increases quite a lot.

I already discussed the problems with the traditional Langelier Saturation Index, but remember that such problems are relatively small (a difference of about 0.1 with LSI too high at 120F). Their discussion of corrosion factors is reasonable and includes additional factors I didn't mention (like flow rates which causes erosion corrosion) and high halogen levels. By the way, remember that the disinfecting forms of both chlorine and bromine are oxidants and therefore can corrode metals by themselves (chlorine more so than bromine). This is more of an issue when CYA is not used (indoor or shaded hot tub, for example) since effective disinfecting chlorine concentrations are higher, but there has been no reporting on this forum about corrosion of metal surfaces except from the obvious problems of very low pH (including my own personal experience of using Tri-Chlor in a feeder that stayed too close to metal bars in our pool).

The discussion of pH and TA for spas is generally accurate and consistent with what we talk about on this forum regarding CO2 outgassing and how this raises pH and then adding acid ends up restoring pH but with lower TA so less buffering capacity.

Users of this forum tend to avoid the use of the "extra" products mentioned toward the end of the document except for borates (Borax). This is mostly to save money and simplify pool maintenance since most of these products are not necessary. The one primary exception is PolyQuat for prevention of algae especially when it is anticipated that sanitizer levels may drop too low (i.e. when on vacation). Generally, though, users only add a sanitizer and then just maintain pH and TA and CH.

Richard