The calcite saturation index is a tool for estimating the likelihood of plaster corrosion or calcite scaling. The LSI, Langelier Saturation Index, is a very similar but slightly less acurate measure. The CSI uses PH, TA, CH, CYA, temperature, Borate, and Salt levels to estimate the likelihood of problems. A low saturation index means the water is likely to dissolve calcite out of paster, pebble, tile, stone, and concrete surfaces (and perhaps fiberglass) which will eventually cause damage. A high saturation index means the water is likely to deposit calcite scale on the walls of the pool and in the plumbing.
CSI is most sensitive to changes in PH. With a plaster pool, it is best to try and get your CSI a little below zero, so that changes in PH won't shift your pool too far towards corrosion or scaling. With a vinyl pool the CSI can be kept more negative, which makes it very unlikely that PH changes could get the CSI into the range of scaling risk.
This can be found at The Pool Calculator
