Langelier Saturation Index
Also Known As: LSI, Langelier Stability Index
To test LSI, find the waters total alkalinity, calcium hardness, cyanuric acid, temperature and measured pH. LSI calculators are available online or as smart phone apps. Plug in the measurements and indicate whether or not the pool uses salt water. The LSI will be calculated automatically.
If LSI is between -0.3 and 0.3, the pool water is balanced. If LSI is less than -0.3, the water is corrosive, which can cause etching, pitting and staining of plaster walls, grouting and plumbing. To counter this phenomenon, add sodium bicarbonate or soda ash, following the manufacturers instructions. If LSI is greater than 3.0, the water may deposit minerals on the pool surfaces and equipment. This is called scaling, which can reduce the effectiveness of the filter and damage equipment. A high LSI may be caused by increases in temperature, pH, alkalinity or calcium hardness. To lower LSI, add muriatic acid, following the manufacturers instructions.