The Water's "Appetite"
Think of water as a hunger creature. It wants to be full of calcium carbonate. The LSI (Langelier Saturation Index) tells you if the water is starving or stuffed.
Hungry Water (Low LSI)
If LSI is below -0.30, the water is Aggressive. It is "starving" for calcium, so it will eat it from wherever it can find it: your plaster, your grout, and the metal warmth of your heater coil (corrosion).
Stuffed Water (High LSI)
If LSI is above +0.30, the water is Overfed. It cannot hold any more calcium, so it "vomits" the excess out. This appears as white crusty scale on your tiles, salt cells, and plumbing.
Temperature is the hidden variable. As water gets cold, LSI drops.
Scenario: You balance your pool perfectly in October (LSI 0.0). By
December, the water is 40°F colder. Your LSI drops to -0.6 (Corrosive). You open the
pool in spring to find pitted plaster.
Tip: In winter, deliberately keep your pH and Calcium higher to offset the
cold.
LSI FAQ
Yes, but only slightly. High TDS (Total Dissolved Solids) from salt lowers LSI by a small amount, but the high pH often caused by salt chlorine generators raises it, making scale the more common issue in salt pools.
pH has the biggest immediate impact on LSI. However, Calcium Hardness is the "ballast" that keeps LSI stable. You can't easily change temperature, but you can maintain a solid Calcium Foundation (300-400 ppm).