This app determines water balance calculations. Once you have measured the various parameters below, you can use this convenient Langelier Saturation Index calculator to determine whether your water is corrosive, scale forming or nicely balanced. If the water is corrosive, the water will try to saturate itself by dissolving the surface composition structure of your pool. If it is scale forming it will try to correct its over saturation by precipitating the hard water salts causing cloudy water and eventually deposit scale on pool components.
It is a port of the calculator at CSGNetwork.com - http://www.csgnetwork.com/langeliersicalc.html