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CSI and salt cell longevity
Salt cells are expensive — $400–700 to replace — and the fastest way to kill one early is letting your water drift out of balance. The metric that captures balance is the saturation index (CSI, the calcite saturation index; LSI is the closely related Langelier version). It tells you whether your water wants to deposit scale or dissolve it.
What the index means
The index folds pH, total alkalinity, calcium hardness, water temperature, cyanuric acid, and salinity into one number describing the water's tendency:
| CSI | Water behavior |
|---|---|
| below −0.3 | Corrosive — etches plaster, dissolves grout, attacks metal and the cell |
| −0.3 to +0.3 | Balanced — the target zone |
| above +0.3 | Scaling — deposits calcium scale on surfaces and cell plates |
Why salt cells care so much
The cell is the worst place for scale to land. As the plates generate chlorine, the water right at their surface becomes locally high in pH — which nudges that thin layer toward scaling even when the bulk water looks fine. Calcium scale on the plates insulates them, cuts chlorine output, and forces the unit to work harder and fail sooner. Keep the bulk water slightly below neutral CSI and the plates stay cleaner for longer.
Salt raises TDS, which shifts the index. A saltwater pool runs much higher total dissolved solids than a chlorine pool, and that changes the CSI math. A balance calculation built for a freshwater pool will read your salt pool wrong — use one that accounts for salinity.
The levers that move CSI
You don't chase CSI directly — you adjust the inputs:
- pH — the strongest and fastest lever. Salt pools tend to drift pH upward, so it needs regular attention.
- Total alkalinity — buffers pH and feeds the index; keep it in range (often 60–80 ppm for salt pools).
- Calcium hardness — raise it if it's low (soft water is corrosive); you can't easily lower it except by dilution.
- Temperature — warmer water scales more readily, which is why summer needs a slightly lower pH target.
Lowering pH to kill scale can tip the water corrosive if alkalinity or calcium are already low. CSI exists precisely because these levers interact — change one and re-check the index.
Saline tracks CSI as its hero number
Enter your test readings and Saline computes the salinity-aware CSI, flags whether you're trending corrosive or scaling, and tells you which lever to move — with brand-cited dosing to get there. Free to download.
Sources
- Calcite / Langelier saturation index methodology (water-balance chemistry)
- Trouble Free Pool wiki (CSI for saltwater pools); SWG manufacturer scaling guidance
Saturation-index targets are guidelines — confirm against your surface type (plaster, vinyl, fiberglass) and cell manual.