Langelier Saturation Index (LSI) is a method of measuring how “balanced” the water in a pool is. This determines if the pool water is going to have any effect on equipment, and what that effect might be. While it’s helpful for swimmer comfort, it’s primary use is for maintaining the life of the equipment. A scale is used to determine if the water is going to be corrosive to the equipment, or scaling.
Seen above, it’s a simple scale that determines what the water is going to do to pool equipment. If it’s kept between -0.3 and +0.3, the effect is negligible. The water is balanced. The pool equipment is safe. Too far one way or the other and the water is corrosive or will cause buildup (scaling) on anything it touches.
This means that if you’re water is consistently maintained at -0.5 or beyond, it will slowly eat away at plaster surfaces, heat exchangers, chlorine generator cells, and much more. And if you have +0.5 or beyond, all surfaces will slowly build up with scale. Neither are desirable.
Determining LSI is a simple equation. Gather the following results: pH, Temp, Calcium Hardness, Total Alkalinity, and if it’s not a saltwater pool, check TDS. This is the equation:
pH+ Temperature Factor+Calcium Factor+Alkalinity Factor-Total Dissolved Solids Factor=LSI
Or written with acronyms to make it tidier:
The pH is added to the equation as tested, meaning you don’t do anything else to it, and the rest has a chart you can check. This chart should be posted someone in the pump room or wherever the water is tested.
Using the chart above, match up a number for each test result. Round to the nearest factor or split the difference. That means, if the temp of the water is 80, then you can either go to 0.6, 0.7, or more accurately say 0.65. Here’s an example:
- pH: 7.4
- Temp: 80 = 0.65
- Calcium Hardness: 240 = 1.9
- Total Alkalinity: 100 = 2
- Saltwater Pool: YES
The final part of the equation has to do with Total Dissolved Solids. You will either be using 12.1 or 12.2 If you have a saltwater pool, it will always be 12.2 If your pool is not saltwater, test for TDS and if it’s less than 1000 ppm, then use 12.1.
Therefore, using the numbers from our example: pH (7.4) +
pH (7.4)+ TF (0.65)+CF (1.9)+AF (2)-12.2=LSI
7.4+0.65+1.9+2-12.2= 0.25 LSI
This is an example with balanced water. If you get results that show the pool is NOT balanced, steps should be taken to correct that. In some cases, adjusting your calcium hardness or total alkalinity may bring the LSI in line.
Water quality is paramount not only for the swimmer, but for the life of the equipment. Maintaining consistent water quality is the key. There will times when the water is not balanced. As long as that’s caught and adjusted, the equipment will be fine. Consistently poor water quality will destroy pool equipment. It’s not a question of if, it’s a question of when it will destroy equipment. Check your LSI at least once a week and adjust accordingly.
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