Swimming Pools And The Langelier Saturation Index (LSI)



Balance is key when you are striving for pool water perfection.

While there’s the common practice of balancing the foundational elements of pool water (pH, alkalinity, and calcium), if you want to get really in-depth, you should learn how to balance your water using the Langelier Saturation Index.

The LSI can give you a greater overall picture of what’s going on with your pool water, making it easier to maintain over the long haul.

What Is Pool LSI?

The Langelier Saturation Index (LSI) is an index that measures mineral saturation in pool water, and more specifically, the measure of calcium carbonate. Developed in 1936 by Dr. Wilfred Langelier, understanding the pool’s LSI can help you predict and control the pool water chemistry.

When it comes to calcium carbonate, if there’s too much of it in the water, it falls out of solution. Too little, and the water will seek out calcium from wherever it can find it, in an attempt to self-balance.

With the LSI Index, the ideal range you want your water to be at is 0.00. This reading means the water is properly balanced. A reading of +0.3 means the pool is oversaturated with calcium, and a -0.3 reading identifies the pool as undersaturated.

Similar to LSI is the Calcium Saturation Index, or CSI.

LSI is the simplified version of the two indexes. While the LSI is more commonly used, if you want a more precise reading for your calcium carbonate levels, using the CSI is the recommended method.

How Do You Calculate LSI?

The calculation of LSI can be confusing. Thankfully, there are apps and online calculators that can do all the work for you. These are a godsend for those that aren’t mathematically inclined. 

All you have to do is enter in the respective numbers and the app will magically output the ideal levels you should have to balance the various chemicals in the pool.

The LSI calculator from Orenda Technologies makes it easy to figure out the totals you need based on your pool size, and is our preferred calculator.

However, if you are so inclined to do it yourself, the formula looks like this:

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

What Factors Influence LSI In Your Pool?

The LSI reading takes into account a few factors when coming up with the overall LSI number. All of these have to be balanced to each other to achieve a low LSI reading.

  • Calcium Hardness: Calcium hardness results from an oversaturation of calcium in the water. In pools, this can be exacerbated if using a high-calcium chlorine such as calcium hypochlorite. This will cause a high reading on the LSI index.
  • Water Temperature: The temperature of your water can change it’s saturation limit. For example, if you dissolved salt in room temperature water, you’ll eventually get to a point where the salt will stop dissolving, and just sit at the bottom of the glass no matter how much you stir it. But if you boil the water, the salt dissolves. Warmer pool water also allows for a higher concentration of calcium carbonate in the pool.
  • Water pH Level: The pH level of your pool must always be balanced in the range of 7.2 to 7.6. When the balance goes off-kilter, depending on which way it goes, it will lead to either high, or low levels of calcium carbonate in the water.
  • Total Alkalinity: Or, more accurately, carbonate alkalinity which removes cyanurate alkalinity. While alkalinity is usually kept between 80 and 120 ppm, if it increases, so does the LSI reading. For this reason, some pool owners will keep their water under 80 ppm, ensuring the LSI is balanced.
  • Pool Stabilizer: Pool stabilizer is cyanuric acid, which works to protect chlorine from being destroyed by the sun’s UV rays. However, the presence of too much of it can contribute to an out of balance pH level, and as a result, throw off the LSI.
  • Total Dissolved Solids (TDS): The level of total dissolved solids is a measure of everything that has ever dissolved in a pool’s water. This undoubtedly affects the LSI level, as there are minerals, organic and inorganic matter, and the chemicals you’ve used to keep your pool balanced all floating in the water at any given time.

What Happens If LSI Is Too High?

While LSI levels can become too high, they are usually manageable, with water scale being the biggest headache due to an oversaturation of calcium carbonate.

Scale forms at the warmest parts of the pool first, with the waterline being the most common place you’ll find it. But it can also build up on spillways, shallow areas, the heater, and salt generator.

Removing scale may take some time but can usually be done using a chemical agent and some elbow grease. Keeping it under control can be a bit tricker, but testing regularly and keeping your pH level balanced go a long way in this battle.

What Happens If LSI Is Too Low?

If your LSI level gets too low, the water will start to become aggressive, and will start etching.

This is because water is always seeking to balance itself out. When it recognizes there isn’t enough calcium in the water, it will begin to look for it anywhere it can get it.

Unfortunately for pool owners, this means it will corrode cement, plaster and aggregate finishes, as well as the grout from pool tiling. Additionally, it can eat away at your vinyl liner leaving it permanently faded. In some cases it can even damage your pipes or other pool equipment.

Langelier Saturation Index: Simplified!

Using an online calculator, the Langelier Saturation Index will help keep your pool water at safe, balanced levels, and add a level of understanding of when and why the water gets out of sorts.

While it isn’t a “must” when it comes to pool maintenance, it certainly doesn’t hurt to know more about what makes your pool tick, especially at times when things have gone off the rails with your water and you’re not sure why.

Categories: Pool Care, Pool Chemistry