The Langelier Saturation Index is an overall measure of the water’s scale-forming or corrosive tendencies. It takes into account the water temperature, the Total Alkalinity, pH, Calcium Hardness, and Total Dissolved Solids (TDS). Each of these agents, in varying degrees, contributes to scale formation or corrosiveness in pool water. If the LSI is between -0.3 and +0.3, it is likely that scaling or corrosion will not occur, although this also depends on the balance of the individual factors. Above +0.3, scale forms, causing cloudy water and rough surfaces. Below -0.3, scale dissolves, allowing the water to attack and corrode surfaces and equipment.
Although all are interrelated, some agents affect the LSI much more than others. The amount of Total Alkalinity, Calcium Hardness and TDS have less effect on water balance than do pH and water temperature. A slight change in pH makes a major change in scale-forming tendencies and therefore in the LSI reading. Temperature also greatly affects the LSI. Lower temperatures make the water more corrosive, and therefore in need of higher balancing levels. Higher levels of TDS make it more difficult for Calcium to stay in solution. The TDS is figured into the 12.1 constant because it only changes 0.1 for every 1000 ppm of TDS.
The formula for calculating the LSI for In Season analyses is as follows:
LSI = pH + Fta + Ft + Fca – 12.1
where Fta = total alkalinity factor, Ft = temperature factor, and Fca = calcium factor. The following tables lists the temperature factor for several water temperatures, and the total alkalinity and calcium factors for various levels of TA and CH.
Temperature Factor (Ft)
| Temperature, °C | 8 | 12 | 16 | 20 | 24 | 29 |
| Ft | 0.2 | 0.3 | 0.4 | 0.5 | 0.6 | 0.7 |
Total Alkalinity Factor (Fta)
| Total Alkalinity, ppm | 50 | 75 | 100 | 150 | 200 | 300 |
| Fta | 1.7 | 1.9 | 2.0 | 2.2 | 2.3 | 2.5 |
Calcium Factor (Fca)
| Calcium Hardness, ppm | 50 | 75 | 100 | 150 | 200 | 300 |
| Fca | 1.3 | 1.5 | 1.6 | 1.8 | 1.9 | 2.1 |
The water analysis program balances based on the LSI. First, the Total Alkalinity is adjusted to within acceptable range, then a final adjustment is made to the pH. At this point, the Langelier Saturation Index is recalculated based on these figures, and the Calcium Hardness is adjusted to bring the LSI within range, if possible. If the TDS is too high, a partial drain is recommended.
For spas using Perfect Balance, the LSI is not used for balancing. The calcium is removed and replaced with a phosphate buffer, so balancing is done by range rather than by LSI.
