Pool care and the Langelier Saturation Index (LSI)

You don’t need a degree in chemistry to keep your pool clean but understanding the basics of pool water balance can go a long way towards avoiding some of the most frustrating pool problems, from excessive scaling on the walls to that unseemly green hue.
Let’s go over how water balance in a pool is determined, and how each of the chemicals we use as additives play an important role in ensuring that a pool is both safe and enjoyable.

What’s in a pool?

Pool water is typically more than just pure H2O. Most homeowners fill their pools with municipal water, which often also contains magnesium, manganese, copper, calcium, potassium, sodium, iron, phosphorus, and zinc. Some municipalities have more of these minerals than others, and your pool may also be filled with other inorganic compounds depending on the age and state of your piping.

Wind and rain introduce many other elements into a pool, including algae spores, fungi, decaying leaves, and more inorganic matter from nearby soil. Most of these elements are almost imperceptible at first, and they don’t really make their presence known until enough of them accumulate to leave a stain wherever they settle or discolor the water. The exception to the rule here is algae, which multiply and thrive in unattended pool waters, turning them deep green and brown over time.

To help mitigate the growth of organic matter and help isolate and remove inorganic matter, pool owners are encouraged to do three things:

  1. Pump and filter pool water
  2. Utilize specific chemicals to kill organic matter
  3. Balance your pool’s mineral and pH levels

To that extent, most pools are treated with detergents (the most common is pool chlorine), calcium carbonate, metal sequestrants, and acids or bases. There are other pool chemicals, such as cyanuric acid, which is used to reduce the rate at which chlorine is broken down by the sun’s radiation – but the four types mentioned above represent the majority of what goes into keeping a pool clean and safe.

They are also in constant flux with one another, and each chemical level supports the other to maintain an equilibrium that allows it to be safe for humans while discouraging algae growth. While everyone knows that a pool should have a certain amount of free chlorine in it, and should be at a steady pH level close to neutral, most people aren’t aware of the role of calcium and alkalinity in pool water, and how too much or too little can massively impact the linings of your pool, and the quality and safety of your water.

Water balance and the LSI

The Langelier saturation index (LSI) is a measure of the potential for calcium to scale in your pool. What this means is that it allows pool specialists testing your water to determine whether you’re above or below a limit of calcium carbonate in your pool.

Too little can lead to fluctuating pH levels, corrosion in your pool cement, and water that is generally too soft. Too much, on the other hand, creates calcium scaling: the appearance of white calcium deposits on your pool walls.

The “saturation” in LSI refers to calcium carbonate saturation, specifically. The ideal LSI is expressed as 0.0, which is water in perfect equilibrium in reference to calcium carbonate and pH. This means that both the water’s pH and calcium levels (measured in parts-per-million) are in a state that allows for the pH to remain as stable as possible for as long as possible, without the water being too hard or too soft. LSI in the negatives or positives indicates too little or too much calcium carbonate.

The data needed to know your pool’s LSI includes:

  • Your pool pH.
  • Your pool’s cyanuric acid level
  • Your pool’s temperature
  • Your pool’s alkalinity (parts per million or mg/l of calcium carbonate).
  • Your pool’s calcium hardness (parts per million or mg/l of ionized or free calcium).
  • And your total dissolved solids (parts per million or mg/l of all minerals in your pool water).

LSI is calculated as pool pH (potency of hydrogen, or the measure of a pool’s acidity or basicity) less pH at saturation in calcium carbonate. Getting the latter requires you to apply a specific formula, which may be found here. The result will determine the water’s potential to scale, i.e. to cause calcium buildup.

Negative LSI is the easiest metric to address because it means that your pool water will readily dissolve calcium carbonate and put it to good use. Positive LSI generally means having to “reset” your pool water by draining a portion of it, filling it back up with fresh water, and redoing your chemical balance.

Nearly anything can affect the LSI. Water evaporating due to increased temperatures, more or less chlorine, or the involuntary addition of organic and inorganic materials introduced by rain and weather can affect your LSI.

This isn’t really an index you should have to pay too much attention to, especially because it can be difficult to calculate. But understanding it can help you make sense of what role each chemical plays in bringing your pool water closer to the ideal balance between pH, chlorine levels, and alkalinity.

How a little chemistry can save you a fortune

Information like pool alkalinity, calcium hardness, and LSI can help pool water experts determine how far or close your pool is to a chemical equilibrium, and help you take the steps needed to maintain that equilibrium. If any of these levels are off the charts, you will find yourself spending a small fortune on different chemicals to compensate for a metric that is too high or too low.

As with anything else in nature, your pool is not a closed system, and it is impossible to maintain water balance for long. But through consistent testing and conservative chemical use, you can save your wallet and your skin.

Not sure what to do next?

No worries! If you want to know how your pool water is doing, it’s often best to just bring a sample to your local pool expert and get it tested there. Pool care specialists have everything they need to analyze water samples and understand what their potential customers might need to do.

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