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Thread: TDS impact on [C/L]SI

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    TDS impact on [C/L]SI

    Quote Originally Posted by In 'Testing and Balancing Your Water', chem geek
    The Taylor watergram (at least the one I have) also does not account for TDS, that is, salt levels. 3000 ppm salt has a saturation index that is 0.2 lower compared to 500 ppm typical startup TDS, all else equal. I agree with you that one should just using The Pool Calculator to calculate the saturation index and not worry about adjusting any numbers. Be sure to enter in temperature as well since that is also a factor, though in practice doesn't make a lot of difference unless you are comparing a pool to a spa.

    Posted September 19th, 2009, 2:35 pm Stabilizer Adjustment for TA?
    Richard (and any others who may wish to contribute to this thread),

    I would like your opinion of the assertions made by Matthew Griffith in an article I read some months back. When calculating saturation index, I pretty much follow what's printed in the Taylor Testing and Treatment Guide, but I use a 'TDS factor' of 12.2 instead of 12.1. In fact, I've read elsewhere that due to my high Total Dissolved Solids (> 5000 ppm, inclusive of ~3200 salt) that the factor should really be 12.3. If I read your post correctly, you may agree with this. Mr. Griffith, however, seems to feel this is overstated. Here's what he says:


    Quote Originally Posted by In [i
    The Real CSI,[/i] Aquatics International, April 2009, Matthew Griffith]Over the years, the total dissolved solids (TDS) influence has been greatly exaggerated. The constant (12.1) includes the TDS factor for 1,000 parts per million of TDS. This is not an uncommon value in many pools. It is important to know that the CSI has only been shown to work with TDS values up to 1,000 ppm. But, using professor Langelier’s original equation, the next tenth of negative influence would not occur until 10,000 ppm TDS, not the commonly referenced 1,000 ppm.

    Excerpt from http://www.aquaticsintl.com/2009/apr/0904_techtalk.html
    He makes other assertions in the same article, among them...

    Quote Originally Posted by IBID. Matthew Griffith
    One largely unknown flaw is in the published tables. The temperature factors have been miscalculated and ever since, copied in all major pool operation texts.
    I don't think the article makes clear what his sources are. My questions are:

    • 1. Do you agree that the factor for high-TDS water has been overstated?

      2. What are your thoughts on whether temperature, among other factors, as published by equipment manufacturers et al are incorrect?


    Thanks,

    Greg

    [attachment=0:18d17yc6]CSI Factors.jpg[/attachment:18d17yc6]
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    Re: TDS impact on [C/L]SI

    There are a lot of misunderstandings about the saturation index in the pool industry and John A. Wojtowicz tried to clear this up and I wrote about some of this in this thread a while back. I disagree that high TDS is overstated -- if anything, it has been understated. Temperature dependence is also important and is not always done correctly.

    The article you refer to has a few things that aren't quite right. Though it is true that Langleier himself never wanted the index named after him, it isn't exactly a calcium saturation index. What is saturated is calcium carbonate; calcium alone does not get saturated so that is misleading. The solid form of calcium carbonate that is specifically saturated is called calcite (as opposed to aragonite or vaterite). So the index could be properly called the Calcite Saturation Index and is what I call it since it is quite specific about this being calcium carbonate and specifically in terms of the solubility of calcite which is the first form of calcium carbonate that would precipitate as scale and it is also what is in limestone (including marble dust often used as an aggregate in pool plaster) and it is the equilibrium constant (solubility product) used in the saturation index.

    If, as stated in the article, Professor Langelier's original tables only had a one-tenth change going from 1000 to 10,000 ppm TDS, then that is clearly wrong. The TDS adjustment is a very specific adjustment for ionic strength and its effect on the effective concentration (aka "activity) of ions, a chemical concept that has been known for a very long time and that Langelier should have known as well. This sounds more like a mistake, either in original development of the equations or in some transcription of it either by Langelier or in the pool industry. Also, the temperature dependence has become more precise over time as the thermodynamic quantities for the various equations have been more finely determined (or, somewhat equivalently, equilibrium constants at various temperatures).

    Near the bottom of my spreadsheet here you can see the derivation of the CSI and can note that there is absolutely nothing about this derivation that has to do with open vs. closed systems. The outgassing of carbon dioxide is relatively slow so for practical purposes the pool water is a closed system as far as the CSI is concerned. There is absolutely nothing about being open to the air that invalidates chemical equilibrium when the changes from that openness are slow relative to the reaction rates (and circulation rates) of the chemical equations involved in the CSI. Also, the TDS dependence comes from ionic strength while the temperature dependence comes from the thermodynamics (temperature dependence of the equilibrium constants) of the various equations.

    As for a qualitative reason as to why water that is higher in TDS (specifically, ionic strength) has a lower saturation index, the primary affected equilibrium equation is that of calcium carbonate solid going to calcium ions and carbonate ions. Essentially, when there are more ions (charged substances) in the water, they somewhat shield each other behaving as if they are lower in concentration. Ions that are doubly charged, such as calcium and carbonate ions, are affected more by a factor of 4 compared to singly charged ions. So because the effective concentration (aka "activity") of calcium and carbonate ions is reduced in saltier water, this means it takes higher concentrations to saturate the water or put another way higher TDS has the saturation index be more negative for the same water parameters.

    There are other equations affected by ionic strength including the carbonate buffer system equations for dissolution of carbonic acid. The net effect is that the most important ions that are affected by the ionic strength are calcium and bicarbonate and that the second dissociation (the one from bicarbonate ion to carbonate ion) partly counteracts what I described above for calcium and carbonate. All of these detailed calculations are in my spreadsheet computations as well as in the derivation near the end of the spreadsheet. The spreadsheet closely matches the Taylor watergram for non-salt pools. It is also consistent with what Wojtowicz has done except he used newer equilibrium constants from other sources and I have that as an option in my spreadsheet though it is not the default. Using Wojtowicz numbers, the saturation index would be around 0.12 to 0.14 higher in the most common conditions.

    Technically, TDS is total dissolved solids and doesn't necessarily mean charged substances, but in practice in high TDS pools the main component of TDS is sodium chloride (i.e. salt).

    It turns out that my first foray into pool water chemistry was initiated by my noticing how the Taylor watergram didn't match the saturation index calculated by the "standard" tables (though there was more than one such standard on the Internet). That's when I created my initial spreadsheet. I had Cyanuric Acid equations, but didn't have chlorine combined with CYA (i.e. the chlorinated isocyanurates) and it was in that search, including conversations with Ben Powell with reference to Wojtowicz, when I eventually found the O'Brien paper that definitively determined the chlorine/CYA relationship.

    Richard
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    Re: TDS impact on [C/L]SI

    Quote Originally Posted by chem geek
    I disagree that high TDS is overstated -- if anything, it has been understated.
    So, would you say that for water that is TDS>5000 that the constant I use to calculate (C)SI should be greater than 12.1 ... 12.2 ... some other number?
    (My test results are somewhat imprecise--and may be incorrect--they are based on AquaChek test strips. Awaiting delivery of Taylor drop tests for TDS and sodium chloride, ordered last week.)

    Quote Originally Posted by chem geek
    The spreadsheet closely matches the Taylor watergram for non-salt pools. It is also consistent with what Wojtowicz has done except he used newer equilibrium constants from other sources and I have that as an option in my spreadsheet though it is not the default. Using Wojtowicz numbers, the saturation index would be around 0.12 to 0.14 higher in the most common conditions.
    I have looked over your spreadsheet and have been meaning to condense my questions about its operation and send to you, but I'm intrigued with your statement that the SI would be .12-.14 higher [if applied to salt-water pools.] Is that correct? If the SI typically ranged from -.12 to -.14, I could conclude that my (3200 ppm salt) water SI was more or less 'in balance' (0)?

    Quote Originally Posted by you
    It turns out that my first foray into pool water chemistry was initiated by [...]
    Thanks for sharing the history. I fumbled upon a brief paper (1942) from the Journal of The American Water Works Assocation on the Calcium Carbonate Index and a longer one from O'Brien (Ann Arbor, 1974) that I have tried to decipher, but it sounds like you've done all the homework here. I will review the thread you provided the link to and try to acquire more understanding from your prior discussion on this; which may lead to... more questions for you (?!)

    Thanks very much for the response. Greg
    14,555 gal in-ground 16'x29' white plaster Pool w/spa (2007); Goldline Aqua Logic AQL-PS-8 control w/Aqua Cell 15 Salt Water Chlorination (SWCG); Hayward TriStar 1HP (1.85 SF) main / 1.5HP (1.60 SF) spa pumps; Hayward Swimclear cart filter C4025, ColorLogic LED lights; Tankless SP-18-4 electric heater; Polaris 280 cleaner.
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    Re: TDS impact on [C/L]SI

    Greg,

    I wouldn't use the standard formulas at all since some of the factors are not correct. The Langelier Saturation Index used by the pool industry is based on the following formula which is a small modification to the Langelier formula described here:

    LSI = pH + TF + AF + CF - 12.1
    where TF = 13.12*log10(ºK) - (34.55 - 2.6)
    ..... ºK = (ºF - 32)*(5/9) + 273.15
    AF = log10(TA)
    CF = log10(CH) - 0.4
    TDS factor (12.1 in the above) = (9.3 + 2.6) + (log10(TDS) - 1) / 10

    The Alkalinity Factor (AF) and Calcium Factor (CF) are correct and are just based on base 10 logarithm where a doubling in ppm results in the factor increasing by log10(2) = 0.301. The temperature factor is incorrect where the correct formula would be more closely based on an inverse of absolute temperature, -1412.5/ºK, and not a logarithm (the temperature dependence comes from the thermodynamics which has a factor based on the inverse of absolute temperature, ignoring the more minor temperature dependence on enthalpy and entropy themselves). The TDS factor is also incorrect and would be based on the square root of ionic strength in a complex formula. The more accurate Calcite Saturation Index is more fully described here and is what is used in The Pool Calculator. The formula used in my spreadsheet is even very slightly more accurate since it accounts for ion pairs, but that usually only results in a CSI that is 0.02 lower.

    The 0.12 to 0.14 I referred to was for an additional adjustment proposed by Wojtowicz and it not related to TDS. The difference going from a TDS of 525 ppm in a freshly filled non-salt plaster pool to a 3200 ppm in an SWG pool is around -0.2. The bottom line is that you should just use The Pool Calculator to do your saturation index calculations.

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    Re: TDS impact on [C/L]SI

    Quote Originally Posted by chem geek
    The bottom line is that you should just use The Pool Calculator to do your saturation index calculations.
    Richard, you're quite right.

    But you must know from reading my posts that my constitution prevents me from following this sage prescription. I just gotta figure it out for myself. Do you mind terribly if I continue in my hypocrisy by commending to others what I resist so stubbornly?

    (Surely there must be a Homer Simpson smilie I can insert here? If there was, he would be standing over a pickle barrel vigorously mixing chlorine, trichlor and dry acid -- just to see what it would do...)
    Quote Originally Posted by Homer, before passing,
    DOH !!
    Thanks for more stuff to chew on. Let me digest and get back to you. - Greg
    14,555 gal in-ground 16'x29' white plaster Pool w/spa (2007); Goldline Aqua Logic AQL-PS-8 control w/Aqua Cell 15 Salt Water Chlorination (SWCG); Hayward TriStar 1HP (1.85 SF) main / 1.5HP (1.60 SF) spa pumps; Hayward Swimclear cart filter C4025, ColorLogic LED lights; Tankless SP-18-4 electric heater; Polaris 280 cleaner.
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