What is the LSI?

The Langelier Saturation Index (LSI) tells us how balanced the water is, and is calculated using several chemistry factors.

In short, the Langelier Saturation Index (LSI) is the objective measure of water balance. It tells us the saturation equilibrium of calcium carbonate (CaCO3). Water always seeks equilibrium, and we measure that equilibrium using the LSI. The LSI is the aggregate of six  factors, or seven if you have borates in your water:

  1. Water temperature
  2. pH
  3. Carbonate Alkalinity
  4. Calcium Hardness
  5. Cyanuric Acid (CYA)
  6. Total Dissolved Solids (TDS), including salt
  7. Borate (only if applicable)

LSI Balance

When calculated, the LSI is a number value on a spectrum where 0.00 is perfectly balanced.

LSI gradient and sugar water, langelier saturation index explained, LSI definition

We have color-coded the LSI to match colors on The Orenda Calculator™.

  • +0.31 and above (purple) = oversaturation of CaCO3.
    • Scale formation, carbonate clouding, calcium dust, etc.. Calcium carbonate precipitates out of solution.
  • 0.00 to +0.30 (green) = ideal balance of CaCO3.
    • Water is happy.
  • -0.30 to -0.01 (yellow) = acceptable balance of CaCO3.
    • Water is close enough to happy, though slightly undersaturated.
  • -0.31 and below (red) = undersaturation of CaCO3.
    • Water is starving for calcium carbonate. Water will seek and dissolve it wherever it can be found, usually within cement finishes and tile grout (etching).

What the LSI teaches us

Water itself only wants one thing: equilibrium. That equilibrium is measured on the LSI. Either you balance the water according to the LSI, or the water will seek to balance itself.  If it has too much CaCO3 in its current conditions, water will release some to get back down to equilibrium (scale, dust, cloudiness, etc.). If it has too little CaCO3 in its current conditions, water will find it and steal it by dissolving CaCO3 into solution from cement or tile grout. If no calcium carbonate is readily available, the water will destroy just about anything looking for it (fading vinyl liners, degrading gelcoats on fiberglass pools, etc.).

Maintain LSI balance year-round (Orenda Pillar 1)

Our first pillar of proactive pool care is about maintaining LSI balance all year long. If you do this, the water will not have the ability to destroy surfaces or precipitate scale or dust. It makes pool care much easier because the water is happy.

To maintain LSI balance, use the Orenda Calculator™ and adjust your factors as necessary. We recommend not only adjusting the LSI today, but also predicting where it will be the next time you treat the pool. When you're able to treat the pool and know it will still be LSI-balanced when you come back next week, you're doing things right.

Here's an easy way to do it, shown in the graphic below:contain pH with Orenda Calculator