I will preface this by stating that I think the pool calculator is fantastic and have been using it since I found this site last year (thanks Jason Lion!!). I have been logging my test results in a spreadsheet and have been working on a log that will graphically show if my results are in the acceptable range. I wanted to make something so that I wouldn't have to jump back and forth between the calculator and my spreadsheet. I found JesseWV's graphical pool testing log (also very excellent) the other day but due to the way it is laid out (observations listed under the dashboard) it is very difficult to use on the laptop that I use for logging my results. I have been trying to figure out how to calcuate the CSI, and unfortunately ChemGeek's thread on the subject (http://www.troublefreepool.com/threa...ater-Chemistry) is way over my head. I've never taken any chemistry, and even if I did I don't think I's understand it ). I found a simpler formula in many places online which isCalcium Saturation Index (CSI) = pH + TF + CF + AF - 12.1. I've also seen this listed as the Langelier Index. Are those one in the same? I guess my question is if that formula is close enough for my purposes. I notice my results are slightly different between that formula and the pool calculator's formula. The factors that follow were in tables on the websites that listed the CSI formula. Using a pH of 7.5, temp of 55 ( factor = 0.4), CH of 140 (factor = 1.8), alkalinity of 110 (factor = 2.1), TDS factor = 12.1 I get a result of -3.0. The pool calculator's result is -0.46. Am I ok to use the simpler formula, and if not can someone list and explain the way the pool calculator's formula works?