If you’re a pool owner and if you’ve ever gotten a professional to test your water, you’ve probably heard of this term and you just kept nodding your head pretending to know what he was talking about. While it is a pretty complex thing by itself, we’re going to simplify it and break it down to its most basic level. Ready? Here we go.

Your LSI is the overall state or balance of your water.

That’s it. Just kidding, that’s the basic gist of LSI and let’s break it down even further. The LSI or Langelier Saturation index tells you the state of your water. Water will always try to go back to its normal state. This means that if water is oversaturated (in our case, calcium) or has a high positive LSI it will do everything to can to release the excess calcium so that it will reach equilibrium, or Zero LSI. What happens then? The calcium has to go somewhere right? The effect of this is pool scales. Since the calcium is being ejected from the water molecules, they end up on your pool surfaces as scale.

On the other side of the spectrum, if your water has a negative LSI or is undersaturated, it will do everything in its power to fill in that gap and reach Zero LSI. Where does it get the calcium to fill the void? Well, since water can’t go to the shed and grab some calcium chloride, it does the next best thing, it finds the nearest source of calcium and eats it. And the nearest source of calcium? Your pool surfaces! Cement surfaces contain calcium and it’s there all around your pool water. This is the number one reason why you get corrosion on your pool surfaces.

Sadly, there is no single test that can determine your pool’s LSI index. It’s a combination of a variety of factors, but don’t worry, if you’re religious about checking your pool chemical levels then you should already be familiar with the following readings. So what are you waiting for? Break out your test kits and let’s start testing!

• pH – This reading determines the alkalinity or acidity of your pool. This is one of the most important factors affecting LSI as the pH value is used as is when computing the LSI index and it has the most impact on the outcome.
• Total Alkalinity – This is measured in parts per million (ppm) and determines the total alkalinity of your pool.
• Calcium Hardness – This measures the amount of calcium in your pool. While the main issue with LSI is with calcium deposits and corrosion, it’s not the only factor that matters. We’ve seen pools with high calcium levels but with no scaling because the other factors balanced it out.
• Temperature – There’s nothing you can really do to adjust water temperature unless you have a heat pump or pool heater, but nonetheless it is a factor in determining your pool’s LSI index. The rule of thumb is, the higher the temperature, the faster calcium will come out of suspension (which explains why water scale usually shows itself during summer and at hot/warm places like your heater or salt cells). The lower the temperature, the faster corrosive reactions will occur.
• TDS or total dissolved solids – This reading isn’t typically used in a home pool, but if you’re testing for TDS then it’s also a helpful number to have a more accurate reading. If you’re not testing for TDS then that’s fine, we can always assume that it’s under 1000 ppm which is the norm for home pools.  

Now comes the fun part! The math. The first thing we should do is convert the values we have into their saturation index values or equivalent factors.

Now that we’ve converted them into their values, time to add them all up. The formula to get your LSI index is:

pH + Temperature Index + Calcium Index + Alkalinity Index – TDS index.

The resulting number you’ll get is your Pool LSI. The number you should aim for is Zero, which means that your water is perfectly balanced, you won’t get any scaling or corrosion problems if your LSI is zero. For home pool owners, it’s actually acceptable to have a level of 0.3 all the way down to -0.3 since the effects of corrosion or scaling would be very minimal and barely noticeable, but again, if you can adjust to get it to zero, then better.

Any value outside of 0.3 to -0.3 should be considered a danger and you should correct the values immediately. For water that is below -0.3, this is potentially corrosive and could eat away at your pool surfaces, plaster, or grout. Now depending on what’s low, you can either adjust the calcium hardness levels or raise the alkalinity to increase your LSI to bring it closer to neutral.

On the other hand, if your LSI value is over 0.3 you’ll have a scale problem. This will become apparent when scale forms all over your pool water lines, plumbing, and even equipment. Scale, while it looks harmless and is just considered a visual nuisance by a lot of people, can actually be damaging. It can restrict water flow and reduce the quality of filtration by your system, and if you have a salt water chlorinator, decrease the life of your salt cells. The usual culprit for this is usually a high pH or alkalinity so reduce the levels as required to bring your LSI index closer to zero.

If your eyes have glossed over from the wall of text, don’t worry, here are some concrete examples of how to determine the LSI index and what they mean.

Perfect / Near Perfect LSI

pH: 7.3
Temperature: 84F or 0.7 Temperature index
Calcium Hardness: 200 or 1.9 Calcium index
Total Alkalinity: 100 or 2.0 Alkalinity index
TDS Level: < 1000 or TDS index of 12.1

Use the formula pH + Temperature Index + Calcium Index + Alkalinity Index – TDS index.

We’ll get: 7.3 + 0.7 + 1.9 + 2.0 – 12.1 = -0.2 which falls within acceptable levels when we’re talking about home pool LSI index. We shouldn’t be expecting any corrosion or scaling to happen to our pool.

Here’s another example of something that’s out of bounds.

pH: 7.3
Temperature: 84F or 0.7 Temperature index
Calcium Hardness: 100 or 1.6 Calcium index
Total Alkalinity: 100 or 2.0 Alkalinity index
TDS Level: < 1000 or TDS index of 12.1

We’ll get: 7.3 + 0.7 + 1.6 + 2.0 – 12.1 = -0.5 which falls outside of our acceptable levels. Since it’s a negative value we can expect the water to be corrosive and it will eat away at your pool surfaces if you don’t do anything about it. Luckily we this is easy to fix. As we can see, our calcium hardness levels are below recommended levels anyway so we can raise that. We can simply raise our calcium hardness levels by 200ppm and that should add 0.5 points to the LSI levels (300ppm is 2.1 calcium index vs 100ppm 1.6 calcium index). Once we’ve adjusted that, the new LSI level should be 0 which is our perfect LSI level.

One you’ve familiarized yourself with the basic concept and the formula to get your LSI, determining it and adjusting the LSI shouldn’t be a problem at all. Do you really NEED to know how to determine your LSI levels? The answer to that is subjective.

But what we’ll say is this. Knowing how to get your LSI levels will give you a broader understanding of your pool chemical balance beyond just the normal pH and chlorine levels. The broader your knowledge is, the easier you can diagnose your pool problems and treat them before they become worse.

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Happy swimming :)