Saturation Index

The Langelier Saturation Index is a calculation used to determine the adequacy of the pool or spa water chemistry, as it relates to corrosion and scale-formation. The adjustment of the pH, total alkalinity or calcium hardness can be used to adjust the Langelier Index, so that it falls within the ideal range. Corrosion is a major concern in swimming pools and is caused by acidic conditions and/or the absence of proper water balance. The factors that contribute to corrosive conditions include: low pH readings, low total alkalinity and low calcium hardness (soft water). Corrosive water conditions are more likely to cause irritation and sensitivity to chlorine. Scale-forming conditions, associated with cloudy water conditions, are caused by alkaline water conditions and/or the absence of proper water chemistry. The factors that contribute to scale-formation include: high pH readings, high total alkalinity and high calcium hardness (hard water). Scale-forming tendencies are more likely to be associated with cloudy water problems and a loss of chlorine efficiency.

The ideal range for the Langelier or Saturation Index is -0.5 to +0.5. Test values more negative than -0.5 (i.e., -0.6) are considered corrosive and steps should be taken to adjust the pH, total alkalinity or calcium hardness, in order to avoid the effects of corrosion. Test values higher than +0.5 (i.e., +0.6) are indicative of scale-forming tendencies and steps should be taken to adjust the pH, total alkalinity or calcium hardness, in order to avoid scale formation and cloudiness.

Low Langelier Indexes can be raised, to within 0.5 to +0.5, by adjusting the pH to 7.2 to 7.6, the total alkalinity to 80 to 120 PPM and the calcium hardness to 150 to 200 PPM for a masonry pool or spa or 80 to 200 PPM for a vinyl or fiberglass unit. A low Langelier Index can result in corrosion, bather irritation and discomfort.

High Langelier Indexes can be lowered, to within 0.5 to +0.5, by adjusting the pH to 7.2 to 7.6, the total alkalinity to 80 to 120 PPM and the calcium hardness to 150 to 200 PPM. While it is always possible to lower the pH, it is not as simple with the total alkalinity or calcium hardness. Lowering the total alkalinity usually lowers the pH as well. Lowering the calcium hardness is not always possible, given the fact that some pools are filled with hard water. In those situations, where the calcium level is high, attention should be paid to lowering the pH and/or total alkalinity as a means of improving the Langelier Index. A high Langelier Index can lead to scale formation, cloudy water, filtration problems, heater problems, loss of chlorine efficiency and bather discomfort.