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