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The Langelier Saturation Index predicts whether water will deposit or dissolve calcium carbonate. It is simply LSI = pH − pHs, where pHs is the pH at which your water would be exactly in equilibrium with calcium carbonate, calculated from temperature, calcium hardness, total alkalinity and total dissolved solids. A positive LSI means the water is supersaturated and can scale; a negative LSI means it is undersaturated and will tend to dissolve calcium carbonate, which usually means it is corrosive. Crucially, LSI is a direction, not a rate — and it is blind to chloride, sulphate, silica and biology.
Calcium carbonate is unusual. Most solids dissolve more readily as water gets hotter; CaCO₃ does the opposite — it becomes less soluble as temperature rises. That single fact is why cooling water scale forms preferentially on the hottest surface in your plant, which is invariably the surface whose job is heat transfer.
The Langelier Saturation Index asks one narrow question: is this water, at this temperature and chemistry, above or below saturation with respect to calcium carbonate? Above saturation and CaCO₃ can come out of solution. Below it, the water is hungry and will take CaCO₃ into solution where it can find it — including out of any protective carbonate film on your pipe wall.
Everything else people attribute to LSI is inference on top of that one question.
LSI is the gap between your actual pH and the saturation pH:
LSI = pH − pHₛ
And pHs is built from four measured properties:
pHₛ = (9.3 + A + B) − (C + D)
| Term | Comes from | Formula |
|---|---|---|
| A | Total dissolved solids | (log₁₀[TDS] − 1) ÷ 10 |
| B | Temperature | −13.12 × log₁₀[°C + 273] + 34.55 |
| C | Calcium hardness as CaCO₃ | log₁₀[Ca] − 0.4 |
| D | Total alkalinity as CaCO₃ | log₁₀[alkalinity] |
Notice that every term is a logarithm. That is not decoration — it has a practical consequence people consistently miss. Doubling your calcium hardness moves C by only log₁₀(2) ≈ 0.30. Doubling alkalinity moves D by the same 0.30. But pH enters the index directly, one for one. So a pH swing of 0.6 does as much to your LSI as doubling your calcium hardness and doubling your total alkalinity, both at once — 0.600 against 0.602. A drift from pH 8.2 to 8.8, which a tower can do quietly over a few weeks, is the chemical equivalent of a change in your water you would have spotted instantly on a lab report.
pH is the dominant variable in LSI. If you are trying to control scale and you are not controlling pH, you are adjusting the small terms and hoping. This is also why a tower that drifts alkaline gets a scale problem and a chlorine problem at the same time, from the same cause — see effects of pH on various biocides.
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pHₛ = (9.3 + A + B) − (C + D)
| Term | Formula | Value |
|---|---|---|
| A | (log₁₀[TDS] − 1) ÷ 10 | — |
| B | −13.12 × log₁₀[°C + 273] + 34.55 | — |
| C | log₁₀[Ca as CaCO₃] − 0.4 | — |
| D | log₁₀[alkalinity as CaCO₃] | — |
LSI = pH − pHₛ. A positive result means the water is supersaturated and can deposit calcium carbonate; a negative result means it is undersaturated and will tend to dissolve it.
LSI is a directional index, not a rate. It tells you which way the water wants to go, not how fast it will get there, and it says nothing about chloride, sulphate, silica or biological fouling. Treat it as one input to a programme, not a verdict on a system.
| LSI | What it means | What it does not mean |
|---|---|---|
| Below −2 | Aggressively undersaturated; will dissolve CaCO₃ | That your metal is safe — the opposite |
| −2 to −0.5 | Corrosive tendency | That corrosion is certain, or fast |
| −0.5 to +0.5 | Near equilibrium; the usual target band | That the system is healthy |
| +0.5 to +2 | Scale-forming tendency; manageable with inhibitor | That scale will definitely form |
| Above +2 | Heavy scaling likely | How thick, or how fast |
That right-hand column is the point of this article. LSI is thermodynamics — it tells you which way the reaction wants to run. It contains no kinetics whatsoever. Two waters with an identical LSI of +1.5 can behave completely differently: one scales a condenser in a month, the other sits there for a year, because scale formation depends on nucleation sites, surface temperature, flow velocity, and whether you are dosing an inhibitor that keeps the minerals in solution past their saturation point.
In fact, a well-inhibited programme deliberately runs positive LSI. That is the whole trick: threshold inhibitors let you hold water supersaturated without depositing, which is what makes higher cycles of concentration possible. A positive LSI on a properly dosed system is not a fault. A positive LSI on a system with no verified inhibitor residual is a countdown.
This is where LSI gets misused, usually by treating it as a general water-quality score. It is not. It is a calcium carbonate index, and nothing more. Hach's own method sheet says it plainly: the index “is not related directly to corrosion”.
The honest position: LSI is a useful, cheap, fast directional indicator that has earned its place. It is not a verdict, it is not a corrosion index despite constantly being used as one, and a system managed on LSI alone is managed on one quarter of the evidence.
There is an intuitive but wrong response to all this: if positive LSI scales, run negative and you are safe. You are not.
Undersaturated water is chemically hungry for calcium carbonate. Where it finds carbonate, it dissolves it — including any protective carbonate-containing film on the inside of your pipework. Strip that film and you expose bare metal to oxygenated water. The result is not a clean system; it is a corroding one, and corrosion is the failure mode that ends in replacing pipe rather than cleaning it.
This is the trade-off at the centre of every cooling water programme, and it is why treatment is a balancing act rather than an optimisation. Push pH down to stop scale, accelerate corrosion. Push pH up to protect metal, precipitate scale — and cripple your chlorine on the way. There is no setting that is simply correct. There is only the setting that is right for your water, your metals and your inhibitor, held there deliberately and verified. We put numbers on the cost of getting that balance wrong in the real cost of doing nothing.
Most programmes aim to sit near equilibrium, roughly between minus 0.5 and plus 0.5, but there is no universally correct target. A well-inhibited system may deliberately run positive LSI, because threshold inhibitors hold minerals in solution past saturation and that is what allows higher cycles of concentration. What matters is whether the operating point is deliberate and whether the inhibitor residual supporting it is verified.
LSI equals measured pH minus saturation pH. Saturation pH is calculated as (9.3 + A + B) minus (C + D), where A comes from total dissolved solids, B from temperature, C from calcium hardness as calcium carbonate, and D from total alkalinity as calcium carbonate. You need five measurements: pH, temperature, calcium hardness, total alkalinity and TDS.
No — it usually means the opposite. Negative LSI means the water is undersaturated and will tend to dissolve calcium carbonate, including protective carbonate films inside pipework. Stripping that film exposes bare metal to oxygenated water. Negative LSI is a corrosion indication, not a clean bill of health.
Not really, although it is frequently used as one. LSI only predicts calcium carbonate saturation. It has no term for chloride or sulphate, which drive pitting by a separate mechanism, and no term for biological activity or under-deposit corrosion. Water can sit at a perfect LSI of zero and still be corroding.
Because calcium carbonate becomes less soluble as temperature rises, unlike most solids. The water film against a hot heat-transfer surface is at a higher temperature than the bulk water, so it is more supersaturated and deposits first. This is also why calculating LSI only at bulk temperature underestimates the real scaling tendency.
LSI is five minutes of arithmetic on a full analysis. Getting the analysis right is the part that matters — we will do both.