Understanding LSI: The Langelier Saturation Index

November 16, 2016

 The LSI is one of the cornerstones of the Orenda program. In fact, LSI Balance and Calcium Management is our first of our Four Pillars of Proactive Pool Care. It's all about calcium carbonate saturation, which determines if your water is aggressive/corrosive (low LSI), balanced, or scale-forming (high LSI).

The Langelier Saturation Index (LSI) is a formula developed from studies conducted by Dr. Wilfred Langelier in the early 20th century. The LSI is the basis for water balance and saturation, and this article will try to explain how it works in a simplified way. This is complex science, but very helpful to know as a pool owner or operator.
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Think of the LSI as a scale with a fulcrum

langelier saturation index, LSI scale, LSI, water LSIA perfect score on the LSI is zero (0.00). Zero is perfectly balanced water; saturated with the perfect amount of calcium carbonate, and has a stable pH. Being the universal solvent, if water is out of balance, it will naturally try to find its own balance and equilibrium, because it wants to be at 0.00 LSI. For instance, if there is not enough calcium, water will dissolve and extract it from the most readily available source. Usually in pools, that means the cement in the plaster or pebble finish.

LSI GRADIENT, orenda LSI, langelier saturation indexThe LSI is basically a way to determine if water is corrosive (negative LSI) or scale-forming (positive LSI). LSI between -0.30 and +0.30 is the widely accepted range, while 0.00 is perfect equilibrium. We at Orenda have designed our LSI Calculator to encourage users to keep their LSI between 0.00 and +0.30, because if you're going to err one way or the other, we prefer to err high. It gives more of a cushion against potential etching problems. Scale is not damaging, but etching is.

Water wants to be in equilibrium, and will find a way to get there. Under-saturation is corrosive, and over-saturation is scale-forming. Water can only hold so much calcium in solution. If water is in LSI equilibrium, neither etching nor scaling will happen. As pool professionals, our goal is to balance water up front (and maintain it) to LSI standards so that neither etching nor scaling occur. 

Low LSI does not just etch plaster, it can corrode pool equipment too and plastics too. Yes, that includes vinyl liners and fiberglass pools. Water will stop at nothing to find equilibrium...so when it's hungry for calcium, it will aggressively look for it. When the water does not have a readily available source of calcium, corrosion and degradation can occur anywhere in the pool (or equipment). Another important thing to remember: water cannot over-saturate itself. It will take only what it can hold, and nothing more.

Analogy: Sugar in a Drink

When you add sugar to your drink and stir it, it will dissolve. Add more sugar, and it will dissolve too. But at some point, when you add too much sugar, what happens to that sugar? It just swirls around at the bottom of the glass, unable to dissolve. That's because you have exceeded the drink's saturation limit; it can no longer hold any more sugar.

If you insist upon dissolving more sugar, there are a couple of things you can do. First, you can make it a much larger drink, which changes the volume, and reduces the saturation. You can also increase the temperature, say, to a boil. If you have ever made desserts, you know that boiling water can hold a LOT more sugar than cold water, because its saturation properties have changed.

Now replace the word "sugar" with "calcium". The LSI tells us how saturated the water is with calcium. The properties can change with six factors, not just water temperature. And it's worth noting that unlike sugar, cold water can hold more calcium in solution, and that's why cold water is more aggressive than warm.

How to calculate the LSI

There are six variables you need to test for calculate the LSI:

*We only test for Total Alkalinity, but the LSI calls for carbonate alkalinity. There is a correction factor to remove cyanurate alkalinity, which is why Cyanuric Acid is taken into account. Total Alkalinity minus Cyanurate Alkalinity gives us Carbonate Alkalinity.

These variables are given numerical equivalent “factors”, as assigned in The Langelier numerical equivalents table:

langelier saturation index equivalents table

The LSI Equation

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

Let’s use an example.

You manage a pool that has the following chemistry:

  • 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)

Let’s calculate.

[(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

This is somewhat balanced water (within the acceptable range), but errs to the side of corrosive. Ideally, this water should be balanced by increasing the factors in the equation by at least 0.21. For example, an operator can increase the calcium hardness, or alkalinity. Play with the numbers on your own to find the balance that works for you and your water.

Overview of the six LSI factors

pH: This variable is the most likely to shift up and down, as pool operators already know. In the LSI calculation, the pH value itself (example: 7.2) is the factor. There is no multiplier. The lower the pH, the more acidic, and the higher the pH, the more alkaline. pH is arguably the most important of the factors because it has such as substantial impact on the LSI.

calcium hardness on salt cell, scale formation, salt cell, scaled salt generator, LSI violation, calcium buildup, salt poolWater Temperature (ºF): Temperature affects the speed of chemical reactions in water. The lower the temperature, the easier it is for corrosive reactions to occur. The higher the temperature, the easier it is for calcium to come out of suspension. This partially explains why salt cells often have calcium carbonate scale on them (heat). Heaters tend to have scale in them too for the same reason.

Calcium Hardness: This is a measurement of how much calcium is dissolved in the water. Water that is over-saturated with calcium is likely to be more scale forming, but only if the pH and total alkalinity allow for it to come out of solution. Calcium hardness is a very stable factor in water chemistry, and helps bring consistency to the LSI. We view calcium hardness as the most underrated, valuable and yet misunderstood components in water chemistry.

Alkalinity: In the original Langelier Saturation Index formula, Dr. Langelier used total alkalinity. Over time, however, it became evident that swimming pool chemistry is different from other types of water; therefore it is more accurate to use carbonate alkalinity. One reason for this change is because many pools use cyanuric acid as a stabilizer. We must correct the alkalinity factor by removing cyanurate alkalinity.

Alkalinity requires a mathematical correction when CYA is involved. You can find this in the chart above, but don't worry, the Orenda App's LSI Calculator has it built in.

Cyanuric Acid (stabilizer): If CYA is present, adjust total alkalinity with the cyanurate correction factor in the chart above to find the carbonate alkalinity factor. The CYA correction factor is about 1/3, if you are rounding. To find it exactly, you need to know the pH of the water and follow the chart.

Total Dissolved Solids (TDS): TDS is a measurement of everything that is dissolved in the water, floating around in solution. It is measured in parts-per-million, and can include anything from salt, to calcium, to metals and other solids. Most often, LSI calculations assume either less than 1000ppm or more. But if you want to be exact, use the Orenda App's LSI calculator.

What the LSI teaches us

If you look at the schedule of values, you will see that calcium hardness and alkalinity have a similar impact on LSI. This is important, because it indicates that calcium is not necessarily the driving factor behind scale. If your pool has scale in it, the conventional wisdom is to assume it's because of high calcium hardness. Not so. Carbonate scale occurs because the LSI gets too high. We know of many pools with over 800ppm calcium hardness that do not have scale problems, because they balance the LSI.

Another thing we have learned is how remarkably stable the pool chemistry becomes when the LSI stays balanced. And what we have learned is the easiest way to stabilize the LSI is with higher levels of calcium hardness (because calcium levels hardly change). With elevated levels of calcium, we have customers who have noticed dramatic decreases in acid and sodium bicarb consumption. Acid and bicarb are pH and alkalinity adjustment chemicals...which tells us pH and alkalinity are not changing as much as they normally do. The common theme here is that LSI balanced water is happy. 

Related: The Most Misunderstood Chemistry in the Pool Business

Additionally, the LSI offers a different way of looking at water chemistry. Understanding how saturated the water is with calcium gives us a strong predictor of damage to gunite/plaster before it gets bad. And on the other hand, if the LSI is positive, the water is likely to form scale, even if not seen. Proactive pool care, anyone?

So as owners and operators, the LSI offers a broader view of water chemistry beyond just pH and chlorine levels.

Free LSI calculator available on the Orenda App.

App download card 2019


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