Reading the LSI
Last reviewed August 11, 2026
Which lever to pull
pH moves the LSI more than anything else you can change in one visit, and it's the cheapest to change. Take pH down and the LSI comes down with it.
| Reading | How fast you can change it | Notes |
|---|---|---|
| pH | Same visit | Your main lever. Acid is cheap. |
| Total alkalinity | A few visits | Acid plus aeration. Slow on purpose. |
| Calcium hardness | Only by replacing water | Set by the fill water. Drain or RO. |
| Temperature | Not at all | Seasonal. Plan around it. |
| TDS | Only by replacing water | Barely moves the number anyway. |
Water either deposits calcium, dissolves it, or leaves it alone. The Langelier Saturation Index puts that on one number so you don't have to eyeball five readings and guess.
The pill at the top of your water test screen calculates it live as you type. You'll see the band change before you leave the pad.
The bands
| LSI | Band | What's happening |
|---|---|---|
| +0.5 and up | Corrective | Heavy scaling. Calcium building on tile, equipment, the salt cell. |
| +0.3 to +0.5 | Caution | Light scaling. Cells scale faster. |
| −0.3 to +0.3 | Balanced | Stable. No scale, no etch. |
| −0.3 to −0.5 | Caution | Mild etching. The surface roughens slowly. |
| −0.5 and below | Corrective | Active etching. Plaster fails if it stays here. |
What goes into it
Five readings: pH, water temperature, calcium hardness, total alkalinity, and total dissolved solids. Each gets weighted, then added up. The pill needs all of them, which is why it shows what's missing until you've entered them.
The math uses carbonate alkalinity, not the total. It subtracts about a third of your CYA reading, because stabilizer shows up in TA without doing the same buffering work.
Hard water markets
In Phoenix, Vegas and similar, the fill water arrives at 300–500 ppm calcium, sometimes higher. Add summer water temperatures and the LSI wants to sit at +0.5 to +0.8 all season. You can't fix the calcium without replacing water, so the job is holding pH at 7.4 and TA at 80–100 to pull the number back down.
When calcium passes 600 ppm, or you can't get the LSI under +0.5 with chemistry, it's time to talk to the owner about a drain and refill or an RO service.
Soft water markets have the opposite problem. Fill water under 100 ppm calcium pulls the LSI negative, and you add calcium chloride to protect the plaster.
What scaling looks like
- A white calcium line at the tile
- Salt cell output dropping, and descaling needed more often
- Heater efficiency falling as the exchanger plates up
- Multiport valves sticking, unions crusting
What etching looks like
- Plaster that feels rough or gritty underhand, with the gloss gone
- Grout receding between tiles
- Pebbles working loose, on the steps first
- A pink tint on copper or brass fittings
Etching doesn't reverse. The plaster is gone and chemistry won't bring it back. Catching a negative LSI early is the whole game.
Salt pools
Salt pools carry 3,000–3,500 ppm of TDS just from the salt. The TDS factor is small, so it doesn't move the number much, but older calculators built for non-salt pools will read a salt pool as slightly more scale-forming than it is.
Logging chemistry on every stop? PoolSpartan keeps the readings, works out the LSI as you type them, and sends the customer a service report when you close the visit. Free to start. We only get paid when you do.
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