Saturation index (LSI / CSI)
Also called: LSI, Langelier Saturation Index, CSI, Calcite Saturation Index, water balance.
The saturation index, known as the Langelier Saturation Index (LSI) or its modern refinement the CSI, folds pH, temperature, calcium hardness and carbonate alkalinity into a single value that says whether pool water is corrosive, balanced or scaling. Balanced is −0.3 to +0.3.
Why it matters
Below that range the water is aggressive: it pulls calcium out of plaster, grout and tile. Above it, the water scales, including on your heater and salt cell. You can have every number in range and still be out of balance. pH is the biggest, fastest lever: a 0.2 change moves the index about 0.2. Fix pH first because it's fast, then calcium and alkalinity because they last. Temperature swings it too: the same water can read about 0.5 higher at a 104°F spa temperature than at 55°F. For salt pools, Trouble Free Pool's advice is to keep the index slightly negative.
In numbers
A pool at pH 7.5, alkalinity 60 and calcium 150 has each line looking about where it should be, and the water is still aggressive enough to slowly etch plaster and grout.