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Langelier Saturation Index

Is your water safe for use?

What is LSI?

In the 1930s, Dr. Wilfred Langelier developed his index to know when water becomes corrosive or scale-forming. 

In short, the Langelier Saturation Index (a.k.a. LSI) tells us how saturated our water is with calcium carbonate (CaCO3).

The LSI is the unbiased measurement of water balance, as defined by calcium carbonate saturation.

It determines if our water is -

1. Aggressive/corrosive (low LSI),

2. Balanced, or

3. Scale-forming (high LSI).

What is LSI?

Significance of LSI

Significance of LSI

Perfect saturation is 0.00 LSI, and the acceptable range is between -0.30 to +0.30 LSI. 

Being the universal solvent, water finds its own balance and equilibrium because it wants to be at 0.00 LSI. But, water is satisfied within the acceptable range (-0.30 to +0.30). 

So, there are 2 situations:

A. If the LSI is -0.31 or below, water is aggressive because it is under-saturated with calcium carbonate. The water hungry for more calcium, and will do anything to find it.

B. Above +0.31, the water has too much dissolved CaCO3, so it begins to precipitate CaCO3 out. The result could be carbonate scale, plaster dust, or other forms of CaCO3.

How to Calculate LSI?

How to calculate LSI

There are six variables which are required to calculate the LSI for water sources as well water storage facilities:

1. pH of Water

2. Water Temperature (in ºF)

3. Calcium Hardness (in ppm)

4. Carbonate Alkalinity (in ppm)

5. Cyanuric Acid/Stabilizer (in ppm)

6. Total Dissolved Solids i.e. TDS (in ppm)

Now, each parameter has a factor assigned to it i.e. a numerical value that plugs into the formula, which is ....

LSI Formula:

LSI = pH - pHs

Where:

a. pH is the measured water pH.

b. pHs is the pH at saturation in calcite or calcium carbonate.

This pHs is defined as:

pHs = (9.3 + A + B) - (C + D)

Where:

A = (Log10 [TDS] - 1) / 10

B = -13.12 x Log10 (oC + 273) + 34.55

C = Log10 [Ca2+ as CaCO3] - 0.4

D = Log10 [alkalinity as CaCO3]

Summary - What LSI tells us?

What LSI tells Us?

The LSI teaches us the importance of calcium hardness. It is a consistent foundation for our water chemistry strategy. Parameters like pH and alkalinity fluctuate, but calcium does not. Also, calcium hardness itself is not the leading driver of scale formation. Just because scale is calcium carbonate does not mean your calcium hardness is too high. Scale occurs when the LSI is too high.

Cold water needs more calcium hardness, but hot water does not necessarily need less. You could instead use less alkalinity, as long as the LSI stays balanced.

Finally, the LSI offers a different way of thinking about water chemistry. Instead of focusing on maintaining chemistry ranges, (which can be physically impossible sometimes), focus instead on maintaining the LSI.

Just Remember...

LSI<0 : Water is undersaturated with respect to calcium carbonate. Undersaturated water has a tendency to remove existing calcium carbonate protective coatings in pipelines and equipment.

LSI=0 : Water is considered to be neutral. Neither scale-forming nor scale removing.

LSI>0 : Water is supersaturated with respect to calcium carbonate (CaCO3) and scale forming may occur.

Reference: https://blog.orendatech.com/langelier-saturation-index

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