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Unlocking Water Balance: A Comprehensive Guide to Langelier Saturation Index (LSI)

Updated: Nov 26, 2023

The Langelier Saturation Index (LSI) stands as a crucial tool in the pool industry, a silent guardian ensuring water balance and thwarting potential issues like scaling or rust formation. In this guide, we'll unravel the mystery of LSI, exploring its significance, how to calculate it, and why it's the key to maintaining your pool's utopian state.


Understanding LSI: The Crucial Buffer


Imagine LSI as a unbiased vigilant guardian, monitoring your pool's overall water state. It provides a simple verdict—whether your water is corrosive (negative LSI) or scale-forming (positive LSI). Ideally, a perfect LSI is 0.0, but achieving it is rare. Hence, the acceptable range is from -0.3 to 0.3. Think about this though, should the same acceptable range for pH,Calcium hardness and alkalinity apply in January that also apply in July? I think not. While the methods may differ the ultimate goal remains consistent.


Asking the question, "what does water care about?"


Understanding Water: Always Seeking Equilibrium

The answer is that water cares about one thing- balance. When water teeters on being unbalanced it will stop at nothing to find equilibrium. But, water cannot over-saturate itself. (it can never dissolve more calcium than it needs) That is typically caused by rising pH, water temp, etc.


When the LSI is low (less than (-0.3)) Water will dissolve calcium in the most available sources first ( corrosion of plaster, equipment and possibly plumbing if left unchecked for too long)

That is water trying to balance itself.


If the LSI is high (above 0.3) then it must precipitate calcium to get itself back under then threshold ( below 0.3). The goal is to maintain a LSI-balanced water year-round so neither of those things happen.




Decoding LSI: The Numbers Behind the Balance

For those unfamiliar with LSI, the formula might seem like a complex incantation. Fear not; here's a simplified version:

Where:

  • pH: Pool water pH

  • TF: Temperature in Fahrenheit

  • CF: Calcium Hardness

  • AF: Total Alkalinity

  • TDF: Total Dissolved Solids


LSI=pH+TF+CF+AF−TDF


Let's put this into practice. But First, Lets look at the conversions:

​Temp in Fahrenheit = TF

Calcium Hardness = CF

Total Alkalinity - AF

​32° = 0.0

​5 = 0.3

5 = 0.7

37° = 0.1

25 = 1.0

25 = 1.4

46° = 0.2

50 = 1.3

50 = 1.7

53° = 0.3

75 = 1.5

75 = 1.9

60° = 0.4

100 = 1.6

100 = 2.0

66° = 0.5

150 = 1.8

150 = 2.2

76° = 0.6

200 = 1.9

200 = 2.3

84° = 0.7

300 = 2.1

300 = 2.5

94° = 0.8

400 = 2.2

400 = 2.6

105° = 0.9

800 = 2.5

800 = 2.9

Now there is also a TDS table.

Total Dissolved Solids (TDS)

TDS Factor

1000>ppm

12.10

1000 ppm

12.19

2000 ppm

12.29

3000 ppm

12.35

4000 ppm

12.41



Example Scenario: Calculating LSI

Assume your pool water temperature is 76°F and TDS is less than 1000ppm. Using the LSI formula, let's evaluate three scenarios:

Calculation 1:

  • pH=7.4

  • TF=76° (0.6 in the LSI table)

  • CF=110(1.6 in the LSI table)

  • AF=100 (2.0 in the LSI table)

  • TDF= 900 (12.1 in LSI table)

LSI=7.4+0.6+1.6+2.0−12.1=−0.5

Result: -0.5 falls below the acceptable range, indicating corrosive water.


Calculation 2:

  • pH=7.6

  • TF=76° (0.6)

  • CF=110 (1.6)

  • AF=100 (2.0)

  • TDF= 900 (12.1 in LSI table)

LSI=7.6+0.6+1.6+2.0−12.1=−0.3

Result: -0.3 is within the acceptable range, but there's an underlying issue with low calcium hardness.


Calculation 3:

  • pH=7.2

  • TF=76° (0.6)

  • CF=110(1.6)

  • AF=100(2.0)

  • TDF= 900 (12.1 in LSI table)

LSI=7.2+0.6+1.6+2.0−12.1=−0.7


Result: -0.7 falls outside the acceptable range, indicating corrosive water.


In this scenario, even though Calculation 2 is technically acceptable, the low calcium hardness is a brewing issue. Understanding each component's significance is crucial for effective pool maintenance.


Conclusion: Mastering LSI for a Pristine Pool

Maintaining an ideal LSI is your passport to a utopian pool—clean, sparkling, and ready for your enjoyment. While LSI is vital, the individual components within the formula hold equal importance.


For a more in-depth dive into each element, explore our other articles HERE or let us handle it with our Water Management Program. At Utopia Custom Pools & Spas Inc, we're here to ensure your pool remains a shimmering haven, year-round. Trust us to keep your utopia pristine.

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