Saturation Index (Langelier, etc.)

[medvampire]

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

[chem geek]

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

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

[chem geek]

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.

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.

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.

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?

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.

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.

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

[chem geek]

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

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.

[medvampire]

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

[chem geek]

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

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. 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

[waterbear]

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!

[medvampire]

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.

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

[DaveD]

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?

[medvampire]

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