Owning and maintaining a 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 any damage to the pool structure itself.
Water with the perfect balance is also going to 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.
What is Pool Water Saturation Index?
Whether it’s called a water saturation index or LSI (Langelier Saturation Index), this is a test that measures just how saturated your pool water is with metals and calcium.
If this number falls into the negatives, then you’re dealing with corrosion. If this number falls into the positives, then you’re dealing with scaling.
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) 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 either your pool equipment or for the people swimming in the pool.
Corrosion will begin to break down the pool equipment as well as the walls of the pool itself. Any fixtures, pipes, vinyl liners, or plaster will begin to break down and become unusable. If the water is corrosive enough to start 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 incredibly difficult 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 its way 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
These factors are essentially what you have to measure to figure out your pool’s balance. There are five main factors to look for.
Temperature affects how efficiently your pool chemicals work as well as the speed of certain 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 begin corroding your equipment. Read my full article on the perfect pool temperature for more.
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 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.
Calcium hardness directly indicates how much calcium is lurking in your water. Having a high amount doesn’t necessarily mean that scaling will occur, as it’s very dependent 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 greater potential for scaling.
TDS (Total Dissolved Solids)
Total dissolved solids track pretty much anything that isn’t calcium in the water. You’ll find the number of metals, salts, minerals, and random organic material that has dissolved in the water.
This will usually tell you when to replace your pool water. Algae blooms, scaling, and pool stains all 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 to all the trouble of cross-referencing a chart while doing math. While you’ll still have to measure all of the data yourself, you can plug it all into this calculator to find your LSI automatically.
Just remember that as long as your final LSI number is between -0.3 and +0.3, your pool water is doing well. Small corrections in chemicals and keeping up with your temperature are key to making changes if necessary.
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This should have answered everything you need to know to figure out your pool’s saturation level and how to fix it if necessary. Just remember that any tweaks you make to the pool’s chemistry need to be in small doses, so you don’t risk over-correcting.
The easiest way to maintain your pool is to stay on top of it consistently 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.