Langelier Saturation Index

Also Known As: LSI, Langelier Stability Index

The Langelier Saturation Index (LSI) is used to assess the pH balance of pool water. It may also be called the Langelier Stability Index. Using a process developed by Wilfred Langelier in 1963, LSI is a number that expresses the difference between the actual pH and the saturation pH. The LSI shows whether the pH is in or out of balance.

To test LSI, find the water’s 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 manufacturer’s 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 manufacturer’s instructions.