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-awful-trip-to-the-pool-store-t1825-20.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).
