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Saturation Index

Last updated: December 4, 2017

What Does Saturation Index Mean?

Saturation index (Si) relates to the alkalinity and calcium in a given pool of water. It can also be defined as the indicator to the water balance. A well-balanced pool of water has an Si of 0 ±0.2. Below -0.2, the pool of water is corrosive and can damage metallic material in the water. When the Si is above +0.2, there will be an occurrence of scaling and staining.

Temperature controls the appetite of water for most water balance products. Water can be returned to a stable condition when it’s treated with balancing products, thus its aggressiveness is removed. Absence of these balancing products makes the water aggressively destructive.


Corrosionpedia Explains Saturation Index

The Langelier method is very important in testing water to determine whether it is balanced. It is comprised of a formula with slight variations existing to accommodate geographic and climatic conditions. This can be calculated using a formula, although many websites have come up with a formula that automatically generates the Langelier Saturation Index (LSI).

The LSI ascertains either the aggressiveness or scaling condition of water without considering the suppression of other present compounds of the water balance scale. This calls for care in altering the chemistry of water in the pool environment. An increase in temperature sees a decrease in the solubility of calcium carbonate. Low pH or high pH can cause drastic chemical balance swings in concentrated applications.

The LSI may be generated for a pool or spa when certain instructions are followed:

  • The correct temperature in degrees Fahrenheit must be determined. The preferred temperature is one at which the pool is regularly maintained.
  • Test strips are used to determine alkalinity, pH, cyanuric acid, total dissolved solid and total hardness.
  • After applying the formula and using the obtained data, one can determine whether the water is corrosive, scale forming or balanced.

A positive value is considered favorable to a negative one. Some mineral deposits, such as stains and scales can be removed when the value is slightly negative.


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