Langelier Saturation Index

Published: November 30, 2017 | Last updated: July 5, 2023

What Does Langelier Saturation Index Mean?

The Langelier Saturation Index is a water quality parameter used to determine the tendency of water to form calcium carbonate scaling. It measures the degree of saturation and is determined by calculating the difference between the actual and the theoretical pH of the water.

LSI = pH – pHs


LSI = Langelier Saturation Index

pH = Actual pH of the water

pHs = Calculated theoretical pH of the water based on chemical analysis

The Langelier Saturation Index is often used during trenchless rehabilitation and construction of waterlines in order to choose the best materials for piping in accordance with the type of water the pipes will be transporting. The index is also a useful maintenance tool that helps contractors estimate when waterlines will have to be repaired or replaced.


Trenchlesspedia Explains Langelier Saturation Index

The Langelier Saturation Index is dependent on several water quality factors including:

  • pH
  • Alkalinity
  • Total hardness
  • Conductivity
  • Total dissolved solids and,
  • Water temperature

The Langelier Saturation Index can be interpreted as follows:

  • If the LSI is equal to 0, then the water can be described as balanced and is less likely to cause scale formation.
  • If the LSI is positive, then the water is oversaturated and will have a high tendency of causing calcium carbonate scale formations
  • If the LSI is negative, then the water is undersaturated with respect to calcium carbonate equilibrium and is more likely to have a corrosive effect on piping systems.

The Langelier Index, though an approximation, is a useful tool for water operators as it gives guidance for stabilizing measures to mitigate scale formation and internal corrosion.


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