Understanding the LSI: Langelier Saturation Index
LSI is the unbiased measurement of water balance
In every water treatment industry (including drinking water, irrigation, waste water, cooling towers and other industrial applications), the objective standard of water balance is the Langelier Saturation Index (LSI). Dr. Wilfred Langelier came up with it in the early 20th century as a means of understanding corrosive tendencies in water. It falls on a spectrum of corrosive water to scale-forming water; the LSI essentially tells you how saturated the water is with minerals.
In swimming pools, we're primarily talking about calcium hardness.
Water craves equilibrium. It will stop at nothing to get there. Part of equilibrium for swimming pool water is 200-400 parts-per-million of calcium hardness, depending on factors such as temperature and alkalinity. In fact, the LSI is based on six factors.
Play around with the Orenda Calculator App to learn how these factors affect the LSI. The goal is to have your water balanced as close to 0.00 as possible. Low, negative scores on the LSI means the water is more corrosive; it will etch plaster, wear down vinyl liners, and fade fiberglass in search of calcium or alkalinity. High, positive scores on the LSI means the water is over-saturated and will tend to form scale. The further you are from 0.00, the more severe your water is out of balance. The acceptable range is -0.3 to +0.3.
You can correct for high or low factors by using other factors that you can control. For example, when winterizing pools, you cannot control how cold the water temperature will be (and it will lower the LSI and corrode if you don't account for temperature!). You can correct that and maintain LSI balance by raising the pH, alkalinity or calcium. Again, use our calculator for more.