The spreadsheet is still accessible at the PoolEquations.zip link found near the end of the first post in this thread. The corrections or enhancements I made to the Langelier saturation index were not huge (to the temperature and TDS portions of his index), but it is just based on the solubility of calcium carbonate. The theory is that having calcium carbonate just "in balance", that is in full saturation, means that the plaster will not have a tendency to dissolve nor will it have a tendency to form additional scale. In practice, one needs to be well away from the "0.0" balance of the index to see problems. Some people don't notice issues until +0.7 while others don't see problems until +1.0 (on the positive side for scaling -- don't know for corrosion or pitting of plaster, but may also be this same order of magnitude).

With regard to copper and other metal pipes, the theory is that having near-saturation calcium carbonate forms a thin film or other properties on the metal that helps to inhibit corrosion. Too much saturation will cause scale and too little prevents the film from forming. However, even without the thin film, corrosion of metal requires other factors including lower pH and dissolved oxygen and conductivity (ionic strength). The pool environment and the above neutral (> 7.0) pH usually mean that metal will not corrode. It is generally only when one uses Tri-Chlor tablets and doesn't carefully montor the pH that one risks getting the pH lowered. Storing chlorine in non-airtight containers in the pump house can also lead to corrosion of pumps and other metal.

Richard