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Thread: CSI formula

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    CSI formula

    Can anyone provide the formula for CSI? I'm not looking for anything ultra-technical. I just wonder if there is a CSI formula (even a simplified version) that is similar to the LSI formula Si = pH + Ti + Ci + Ai - 12.1. Ti, Ci, and Ai come from the LSI supporting tables. I know that CSI is more detailed and brings CYA and salt into the equation, possibly along with other things. I know that TFP prefers the CSI over the LSI presumably b/c it's more accurate and is based more on the total chemistry of your pool. I have incorporated the LSI calculation into my pool test log spreadsheet, and I'd like to do the same with the CSI if possible.

    Here's an excerpt from my pool log. I enter the test results from a test session, and the LSI is automatically calculated. But, I'd prefer to use the CSI if possible.

    Date FC TC pH Ca TA CYA Tmp LSI Notes
    4/25 7.0 7.0 7.7 180 90 55 71 0.00

    Thanks.
    25k gal IG freeform pool, gunite/plaster, built in 2006; Aqua Logic AQL-PS-8 control w/ T-CELL-15 SWG; Sta-Rite S8M150 cartridge filter; Sta-Rite SR400NA gas heater; Polaris 280

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    JasonLion's Avatar
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    Re: CSI formula

    Chem Geek explains it here.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    Re: CSI formula

    Cool, thanks. I'm going to copy chem geek's explanation below for convenience.

    1) Probably meaningless, but why does the first formula say "LSI" instead of "CSI"? I thought we were trying to draw a distinction b/w the two, so why mix up the acronyms here?

    2) If I've never used borates, is it safe to plug in 0 for that part of the equation?

    3) My TF-100 test kit doesn't measure TDS. Without a TDS measurement, can I use the alternate "extra NaCl" formula (extra NaCl ppm = Salt - 1.168*(ppm CH)), and use the reading off of my AquaLogic for the "Salt" number in ppm?


    From chem geek at http://www.troublefreepool.com/my-aw...20.html#p14664:

    A very close approximation to the accurate Calcite Saturation Index uses the following formulas (I do refer to these in that post, but in the spreadsheet itself, not in the text of the post):

    LSI = pH - 6.9395 + log(ppm CH) + log(ppm CarbAlk) - 2.56*sqrt(I)/(1+1.65*sqrt(I)) - 1412.5/(oC+273.15)

    I = (1.5*(ppm CH)+(ppm TA))/50045 + (extra NaCl ppm)/58440

    extra NaCl ppm = TDS - ( 1.109*(ppm CH) + 1.679*(ppm CarbAlk) + (ppm CYA) ), but if <0 then use 0

    ppm CarbAlk = (ppm TA) - 0.38772*(ppm CYA)/(1+10^(6.83-pH)) - 4.63*(ppm Borates)/(1+10^(9.11-pH))

    [EDIT2]
    If one uses a salt measurement (which measures chloride and reports as ppm sodium chloride) instead of actual TDS, then the formula for extra NaCl ppm is as follows:

    extra NaCl ppm = Salt - 1.168*(ppm CH), but if <0 then use 0
    [END-EDIT2]

    The "log" are logarithms to the base 10 (i.e. they aren't "ln"). So start with the bottom formula and work your way up. So first calculate CarbAlk using TA and CYA. Then calculate the extra salt via the TDS, CH, CarbAlk just calculated and CYA. Then calculate the "I" which is the ionic strength using the CH, TA and extra NaCl (salt) that was calculated. Finally, the LSI is computed.

    Obviously, the above is best done online, or in a spreadsheet, or in a program. Michael Smith can add this to BleachCalc and you can add this to your online calculations. It will be close to, but not the same as, the Langelier Saturation Index (LSI), but will actually be more accurate. It seems to be virtually the same as the Taylor Watergram wheel except at very high temperatures above 120F and even then the error is small. You'll need to convert the temperature from Fahrenheit to Celsius but that's just oC = 5*(oF - 32)/9. The LSI as used by the pool industry is actually incorrect and uses logarithms for the temperature and TDS portions of the computation when they should have used the formulas I showed above, but then they were trying to simplify things though the temperature formula isn't complicated so I'm not sure why they went with logarithms for that except to "appear" consistent with all factors except for pH (which is already a logarithm).
    25k gal IG freeform pool, gunite/plaster, built in 2006; Aqua Logic AQL-PS-8 control w/ T-CELL-15 SWG; Sta-Rite S8M150 cartridge filter; Sta-Rite SR400NA gas heater; Polaris 280

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    Re: CSI formula

    This thread would be better continued in the Deep End.
    Dave S. - Forum owner
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    Re: CSI formula

    I don't think I'm qualified to visit the Deep End , but feel free to move the thread there if you like.

    ph
    25k gal IG freeform pool, gunite/plaster, built in 2006; Aqua Logic AQL-PS-8 control w/ T-CELL-15 SWG; Sta-Rite S8M150 cartridge filter; Sta-Rite SR400NA gas heater; Polaris 280

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    Re: CSI formula

    1) The name CSI was developed later.
    2) Correct, use zero for borates unless you have intentionally added borates (even if you occasionally use borax to raise PH).
    3) Right. And if you don't know either TDS or Salt then set "extra NaCl" to zero.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    Re: CSI formula

    Great, thanks Jason. I'll be curious to see if I can get the formulas working in my spreadsheet to match the Pool Calculator results.
    25k gal IG freeform pool, gunite/plaster, built in 2006; Aqua Logic AQL-PS-8 control w/ T-CELL-15 SWG; Sta-Rite S8M150 cartridge filter; Sta-Rite SR400NA gas heater; Polaris 280

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