The Saturation Index: Also called the Langelier Index, this chemical
equation or formula is used to diagnose the water balance in the pool. The formula is SI =
pH+TF+CF+AF-12.1. To calculate the Saturation Index, test the 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 in fact, it is not.
Regardless, the SI can be used to pinpoint potential water balance
on a chemical parameter to learn more: pH |
Alkalinity | Calcium Hardness
| Saturation Index