Pool Water Saturation Index

Pool Water Saturation Index is not the simplest topic to write an article about; there is a lot of numbers involved. Langelier Saturation Index is a scientific formula that leads to calculating water balance and saturation. The Canadian Municipal Affairs and Environment website identifies the Langelier Saturation Index as “an approximate indicator of the degree of saturation of calcium carbonate in water.”.

LSI was invented by chemist Wilfred F. Langelier in the 30s to make sure that the water stored in large boilers wouldn’t damage the equipment. Calcium scale represented a difficult challenge in municipal water storage back then. Today, the formula is used in pools because the scaling challenge can affect and damage some pool structures.

The Importance of Water Saturation Index

We have already established the importance of balanced water because corrosive water will damage the equipment on the one hand, and result in clogging and cloudy water on the other.

I can hear you thinking “why do I need LSI when I can test my pool water with a testing kit?”. That’s enough but LSI gives you an extra advantage. LSI is used to reach a perfect chemistry situation when it gets challenging to keep water in balance. According to an article by Orenda Technologies, “A perfect score on the LSI is zero (0.0). Zero is perfectly balanced water; saturated with the perfect amount of calcium and dissolved solids, and has a stable pH.”

Balanced water is necessary for the sake of your swimmers and preserving a safe and healthy pool.

The Langelier Saturation Index Factors

To calculate LSI, we use the same factors we measure to balance pool chemistry with the addition of the water temperature. This is why these factors are important:

  • pH: it indicates acidity in the pool. Any change in pH will result in changes in all the other factors and therefore affect the water balance. This is why pH is considered as the most important element to keep an eye on in any water chemistry situation.
  • Temperature: it’s usually measured to ensure the swimmers comfort in the pool but the temperature has a very important role in the pool chemistry. When the water gets too hot, the pH level increases and that results in faster chemical reactions. The ideal temperature should always remain at 76-104°F.
  • Total Alkalinity: it’s interrelated with pH. The higher it gets, it easier it resists pH changes.
  • Calcium Hardness: It informs you how much calcium the water contains and to which extent the water is hard or soft. Calcium Hardness is affected by pH so you can’t count on it alone to determine LSI.

How to Calculate LSI

This is the formula you need to use in order to calculate LSI:

(pH) + (Temperature ºF) + (Calcium Hardness) + [(Total Alkalinity) – (CYA correction factor @ current pH)] – (TDS factor) = LSI

OrendaTech used this example in their article so let’s check it up:

ph: (7.4)

temperature: 84ºF (0.7)

calcium hardness: 300 (2.1)

alkalinity: 100 (2.0)

cyanuric acid: 100 (pH 7.4 = 0.31)

total dissolved solids: < 1000 (12.1)

Based on the numbers above, this is how the calculation should be:

[(7.4) + (0.7) + (2.1) + [(2.0)-(0.31)] – (12.1) = X LSI

[(10.2) + (1.69)] – (12.1) = X LSI

[11.89] – (12.1) = -0.21 LSI

The water used in this example is not totally balanced but not in an unacceptable range.

If you want to avoid all the calculation and numbers involved, you can always find an online LSI Calculator to help you.