Mineral scale CaCO3 formation on a water supply pipe (C) Daniel Friedman illustrating Langlier Index Scale FormationLangelier Saturation Index
Definition of the LSI or Langlier Index & LSI action levels

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The Langlier index or more properly the Langlier Saturation Index or LSI is defined and its use in evaluating the corrosivity of a water supply is explained.

This article series describes effects of low pH, acidic or corrosive water on building piping, leaks, dissolved copper, health hazards, and the plumbing system in general. We describe how to detect corrosive or aggressive water and what should be done about it.

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Water Corrosivity: Definition of the Langelier Saturation Index - LSI & How it is Used

Deposits and stains on faucet strainer (C) D FriedmanArticle Contents

Definition of the Langlier Index (LI)

Langelier Index (LI). The LI is calculated using pH, temperature, total dissolved solids, alkalinity, and total hardness. The LI is a measure of the balance between pH and calcium carbonate (CaCO3).

As the LI value becomes more negative, the water is increasingly under-saturated with CaCO3 and therefore has increased corrosion potential.

As the LI value becomes more positive, the water is increasingly oversaturated with CaCO3

Over-saturation results in CaCO3 precipitation which can coat and protect pipes from corrosion but can cause scaling in pipes, hot water heaters, and fixtures. - Sigler & Bauder ret. 2016 cited at REFERENCES


On acidic well water and water corrosivity - using the Lange Saturation Index

It's true that the lower the pH (more acidic) the higher the level of corrosivity of the water supply.

But because pH is only one of several factors that will determine how corrosive the water is, while we should look at acidity or pH, if you want to know how worried to be about your water supply corroding your pipes, you want to perform a corrosivity test on your water.

But looking at building water supply properties and pipes and fixtures for evidence of what contaminants or water chemistry problems may be present can be confusing.

Our faucet outlet (photo at left) shows both thick white mineral deposits (suggesting high calcium and high pH) and blue/green stains and deposits that might to some suggest acidic water and low pH.

While there is no doubt your water is acidic, there are other factors affecting corrosion of piping. Let's start by looking at the LSI.

Experts us the Langelier Saturation Index (LSI) to estimate the corrosivity of a water supply. The Langelier Saturation Index or LIS is a number calculated from several factors and intended to tell you the chances that minerals, principally calcium are likely to precipitate out of the water.

Langelier, who developed this index, realized that the acidity or pH of water determines how much calcium carbonate CACO3 the water can hold.

So a combination of the water's actual pH and the actual level of calcium in the water, along with other factors we list below, allow us to predict the chances that the water will leave scale in the plumbing system or components by precipitating out the calcium. We think of it as a more complete picture of water chemistry, with regards to both hardness (mineral content) and acidity or pH.

The Langelier calculation factors in the main components of corrosivity of a water supply including:

Water Corrosivity Action Levels in the Langelier (LSI) Index

Mineral scale CaCO3 formation on a water supply pipe (C) Daniel Friedman illustrating Langlier Index Scale Formation The Langelier Index (LI), when calculated ranges from +4 through 0 to -5. Water with an LI above 0.5 (tending to form scale) or with an LI below -0.5 (tending to corrode metal pipes, tanks, etc), probably needs treatment.

But you don't have to calculate anything. Just ask your water test company to perform this test for you, OR ask the water test company to test your water for hardness (you can do this free or cheap at MEASURE WATER HARDNESS ) and for corrosivity.

+4 LSI = scale producing water: At +4 the water is very likely to form scale or typically calcium carbonate (CaCO3) and magnesium deposits in the water piping, especially hot water piping or in a water heater.

Readers whose water is hard and/or has a high positive Langelier index should see WATER SOFTENERS & CONDITIONERS.

More generally, a Langelier saturation index or LSI greater than zero means that the water is super saturated with minerals and therefore will tend to precipitate mineral deposits or form scale in building piping, fixture, and the water heater or geyser or water cylinder.

Zero LSI = just right: like the baby bear who found one of the porridges "just right", at an LSI of zero the water supply is "neutral" - it is not likely to precipitate calcium into the piping nor is it acidic enough to dissolve more calcium.

With respect to the corrosive effects of acidic water on copper water piping, an LSI of zero also means the water is close to ideal - it is not corrosive to the piping.

More generally, LSI=0 means that water is saturated but not super-saturated with minerals. The water is "neutral" and will neither deposit scale nor dissolve solid minerals such as CaCO3.

At -5 LSI = very corrosive water: at -5 LSI the water is extremely corrosive. Langelier index values of +0.5 to -0.5 are basically OK in that you wouldn't treat water at those levels.

More generally, at an LSI less than zero water is "under-saturated" with minerals and thus is likely to dissolve CaCO3 as well as possibly metal piping or other materials.

This is an "aggressive" or acidic water supply.

If you decide to treat the water based on acidity alone or based on a Langelier index reading of -3 or lower (or optionally anywhere below -0.5) there are various systems that inject something basic like calcite (a calcite neutralizer tank or calcite neutralizer filter) or soda ash (a soda ash feeder) to treat the water and described above.

Other Indexes of Water Condition: Ryznar Stability Index, Puckorius Scaling Index, Stiff-Davis Index, Oddo-Tomson Index

Other indices of water condition include the Ryznar Stability index that predicts water chemistry and how it will affect piping and appliances by examining the thickness of scale formation in municipal water systems, the Puckorius scaling index, the Larson-Skold index, the Stiff-Davis Index, and the Oddo-Tomson Index.[4]

The Ryznar Stability Index (RSI) was developed from actual observations of steel water main pipe corrosion and scale deposition.

RSI = 2 pHs - pH

where the pH is measured. An RSI between 6 and 7 is at "saturation equilibrium" and thus is similarly neutral as a 0 LSI index reading. At RSI greater than 8 water tends to be corrosive or to dissolve CaCO3. For an RSI below 6,5 water tends to deposit CaCO3 or to form scale.

Article Series Contents


Continue reading at LEAD IN DRINKING WATER, HOW to REDUCE if your building is supplied by lead water mains or pipes;

To continue this article series go to LEAK INSPECTION of WATER SYSTEMS, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




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LANGLIER SATURATION INDEX LSI at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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