Langelier Saturation Index (LSI)

Wilfred Langelier published his index in 1936 to answer one question every plant operator still asks: will this water lay down calcium carbonate, or will it eat my pipe? The index is nothing more than the distance between the pH you measure and the pH at which the water would be exactly saturated with CaCO₃. A positive LSI means the water is supersaturated and will deposit scale; a negative LSI means it is hungry and will dissolve any protective carbonate film it finds, exposing bare metal. Zero is theoretical equilibrium. A cooling tower running pH 7.8 against a saturation pH of 7.31 sits at LSI = 7.8 − 7.31 = +0.49, comfortably scaling and in need of acid or a scale inhibitor.

The trap is treating LSI as a rate. It is a direction only — a thermodynamic yes/no, not a mils-per-year number. Two waters can both read +0.5 and behave completely differently because one carries 400 mg/L of calcium and the other 40. That is why field practice pairs LSI with the Ryznar index and, in cooling systems, with a calcium-times-alkalinity limit. Also remember that LSI is a bulk-water number: heat exchanger skin temperatures run 10–20 °C above the bulk, and a water that is −0.2 in the basin can be +0.6 on the tube wall.