Basics of Pool Water Saturation Index

Owning and maintaining a swimming pool is a lot more complex than just dumping hose water into a hole and calling it a day. The best pools have an excellent water balance to prevent damage to the pool structure.

Water with the perfect balance will also be the most pleasant to swim in. It’s soft and crystal clear, which is what every pool owner wants. The water saturation index, otherwise known as LSI, is the way to calculate your water balance and find out how to fix it. Let’s dive into what that means and how it affects your pool maintenance routine.

What is Pool Water Saturation Index?

Whether it’s called a water saturation index or LSI (Langelier Saturation Index), this test measures how saturated your pool water is with metals and calcium. Here’s a diagram I drew that explains everything.

If this number falls into the negatives, you’re dealing with corrosion (figure #1 in the chart on the left side). If this number falls into the positives, you’re dealing with scaling (figure #0).

The water saturation index scale ranges between -1 and +1. Ideally, your pool’s water saturation will fall between -0.3 and +0.3. However, the goal is to get a perfectly balanced 0 (known as equilibrium, figure #2 in the diagram) and keep that number to avoid dealing with corrosion and scaling at all.

Why is Pool Water Saturation Index Important?

Keeping track of where your pool’s LSI is is purely to defend against corrosion and scaling. Neither of these effects is good for your pool equipment or the people swimming in the pool.

Corrosion will begin to break down the pool equipment and the walls of the pool itself. Any fixtures, pipes, vinyl liners, or plaster will start to break down and become unusable. If the water is corrosive enough to begin affecting the pool equipment, you don’t want it on your skin.

Scaling will begin causing buildup and clogging in your pool’s ventilation systems and pipes. These calcium deposits are tough to remove, so preventing them is a better alternative to fixing them. The water in the pool will also become cloudy and hard to see through.

The water in your pool wants to be in equilibrium, and scaling and corrosion are ways of trying to fix the balance. If it doesn’t have enough calcium and minerals, it starts eating away at what it can to get it. If it has too much, it will start depositing in places like pipes or on the walls of the pool.

Factors that Affect LSI

You have to measure these factors to figure out your pool’s balance. There are five main factors to look for.

Temperature

Temperature affects how efficiently your pool chemicals work and the speed of specific reactions in the water.

Keeping your pool too hot makes it much easier for calcium deposits to begin forming throughout the pool. Keeping your pool too cold makes it much easier for the water to start corroding your equipment. Read my full article on the perfect pool temperature for more.

pH Levels

pH is how close to an acid or a base a particular liquid is. In the case of your pool, an out-of-whack pH balance will throw off all the other chemicals in the water.

The ideal range for the pH in your pool is between 7.4 and 7.6. Anything more or less increases the risk of corrosion and scaling dramatically.

Alkalinity

Alkalinity is purely used as a tool to assist in keeping your pH levels steady and avoid any fluctuating. The ideal pool alkalinity is between 80 and 120 ppm.

You should test your alkalinity before your pH because of its ability to assist. High alkalinity levels are needed to adequately resist changes to the pH levels.

Calcium Hardness

Calcium hardness directly indicates how much calcium is lurking in your water. A high amount doesn’t necessarily mean that scaling will occur, as it depends on pH levels.

However, you still want to shoot for a low amount for softer water. The ideal calcium hardness should be close to 150 ppm, or you risk having harsher water and more significant potential for scaling.

TDS (Total Dissolved Solids)

Total dissolved solids track almost anything that isn’t calcium in the water. You’ll find the number of metals, salts, minerals, and random organic materials dissolved in the water.

This will usually tell you when to replace your pool water. Algae blooms, scaling, and pool stains indicate that your TDS is probably too high. TDS scales pretty well with LSI, so if your TDS is high, then your LSI will likely be high as well.

How to Calculate LSI

To calculate your LSI, you will have to do a bit of math. You will first need to figure out your pool’s temperature, pH, alkalinity, calcium hardness, cyanuric acid, and total dissolved solids.

This is the equation you need to use:

(pH) + (temperature in Fahrenheit) + (calcium hardness) + [(total alkalinity) – (CYA correction factor at the current pH)] – (TDS) = LSI.

This link will provide a table to help you find variable values and determine other factors with the raw data.

Of course, not everyone wants to go through all the trouble of cross-referencing a chart while doing math. While you’ll still have to measure all of the data, you can plug it into my saturation index calculator to find your LSI automatically.

Remember that your pool water is doing well as long as your final LSI number is between -0.3 and +0.3. Minor corrections in chemicals and keeping up with your temperature are crucial to making changes if necessary.

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Wrapping Up

This should have answered everything you need to know to figure out your pool’s saturation level and how to fix it if necessary. Remember that any tweaks you make to the pool’s chemistry must be in small doses, so you don’t risk over-correcting.

The easiest way to maintain your pool is to consistently stay on top of it and not let problems blow up into calcified pipes or corroded equipment. Testing a couple of times a week will quickly show if you need to take corrective action or not.

Still have questions? Let me know! Always happy to help.

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