The Langelier or, as it is sometimes referred to, the Saturation Index is a formula used to diagnose the water balance in the pool. The formula is:- Langelier Index = pH+TF+CF+AF-12.1. To calculate the Index, test the pool water for pH, temperature, calcium hardness and total alkalinity. Refer to a chart for assigned values for your temperature, hardness and alkalinity readings and add these to your pH value. Subtract 12.1, which is the constant value assigned to Total Dissolved Solids, and a resultant number will be produced. A result between -0.3 and +0.5 is said to indicate balanced water. Results outside of these parameters require adjustment to one or more chemical components to achieve balance. This formula is not foolproof, however. Some readings for pH, calcium and alkalinity which, taken individually would be considered to be well beyond recommendations, can combine within the formula to produce "balanced water", even when it isn't. Nevertheless, the Langlier index can still be a useful tool to help detect potential water balance problems.
