Your cooling tower water report almost certainly includes a number labeled LSI. Most plant engineers scan right past it.
That’s a mistake. The Langelier Saturation Index is one of the few numbers on that page that combines five separate test results into a single answer. Is this water trying to deposit scale on your heat exchanger tubes, or trying to strip protective deposits off your piping? The LSI tells you before either problem shows up as a rising approach temperature, a leaking tube, or a chiller working harder than it should.
It’s also one of the most misread numbers on the report. A positive LSI in a treated cooling tower isn’t automatically a problem, and a “balanced” zero isn’t automatically the goal. Here’s how to read it correctly.
The Langelier Saturation Index (LSI) is a calculated value that shows whether water will deposit or dissolve calcium carbonate. It equals measured pH minus saturation pH (pHs). A positive LSI means scale-forming water, a negative LSI means water that dissolves scale and protective deposits, and zero means equilibrium.
Wilfred F. Langelier, a sanitary engineering professor at the University of California, Berkeley, introduced the index in a 1936 paper in the Journal of the American Water Works Association. Nearly 90 years later, it remains the most widely used screening tool for calcium carbonate scaling tendency in cooling water, municipal distribution systems, and pools.
The LSI measures tendency, not outcome. It tells you which direction calcium carbonate wants to move. It doesn’t tell you how fast, whether your inhibitor is holding it in solution, or what’s happening with other deposits like calcium phosphate or silica. That’s why it has to be read alongside the rest of your program data.
The formula itself is short:
pH is your measured value. pHs, the saturation pH, is the pH at which your water would sit in exact equilibrium with calcium carbonate, given its current mineral content and temperature. The widely used approximation calculates pHs from four factors:
| Factor | Input | Formula |
|---|---|---|
| A | Dissolved solids | A = (log10[TDS, mg/L] − 1) ÷ 10 |
| B | Temperature | B = −13.12 × log10(°C + 273) + 34.55 |
| C | Calcium hardness | C = log10[Ca hardness as CaCO3, mg/L] − 0.4 |
| D | Alkalinity | D = log10[M-alkalinity as CaCO3, mg/L] |
Then: pHs = (9.3 + A + B) − (C + D)
Recirculating water: pH 8.4 · TDS 1,200 mg/L · 95°F (35°C) · calcium hardness 400 mg/L · M-alkalinity 300 mg/L (both as CaCO3)
Same water, higher temperature, the LSI climbs to +2.12, without a single change in the water itself.
That second calculation is the one most reports skip. Calcium carbonate becomes less soluble as temperature rises, so the hottest surface in your system is always where scale forms first. An LSI calculated at basin temperature describes the basin. It doesn’t describe your chiller condenser tubes. A well-run program evaluates the index at maximum skin temperature, not just bulk water temperature.
The sign tells you the direction. The magnitude tells you how strong the push is. Here’s how to read the number in a cooling tower context:
| LSI value | What the water is doing | Risk in untreated water | What it means in a treated cooling tower |
|---|---|---|---|
| Below −0.5 | Strongly undersaturated; dissolves calcium carbonate | No protective film; aggressive toward mild steel and galvanized surfaces | Uncommon. Usually acid overfeed, very soft makeup, or very low cycles. Investigate now. |
| −0.5 to 0 | Mildly undersaturated | Mild corrosive tendency | Check corrosion coupon rates and corrosion inhibitor residual |
| 0 to +1.0 | Mildly supersaturated | Light scale over time | Easily controlled by nearly any inhibitor program |
| +1.0 to +2.5 | Supersaturated; scale-forming | Significant scale on heat transfer surfaces | Common operating zone for treated alkaline programs, but safe only with inhibitor at target residual |
| Above ~+2.5 | Strongly scale-forming | Heavy, rapid scale | At or beyond the limit of many conventional inhibitor programs; scale starts at the hottest surfaces |
Notice the gap between the last two columns. That gap is the entire reason cooling tower chemical programs exist.
There isn’t one universal number. If someone gives you “−0.5 to +0.5” as a cooling tower target, they’re quoting guidance written for untreated water.
For water with no chemical treatment, an LSI near zero to slightly positive is the goal. That’s enough saturation to avoid dissolving protective deposits, but not enough to build scale. Pool and drinking water guidance lives in this range.
Treated cooling towers deliberately run higher. Evaporation concentrates calcium and alkalinity with every cycle, and pH rises as CO2 strips out across the fill. A tower running several cycles of concentration on moderately hard makeup water will naturally land well above +1.0. Running there is intentional. It saves water, and mildly scale-forming water is far less aggressive toward mild steel than corrosive water.
What keeps that water from scaling is the inhibitor. Corrosion and scale inhibitors such as phosphonates and polymer dispersants hold calcium carbonate in a supersaturated state. They also distort the crystals that do form, so those crystals don’t bond to heat transfer surfaces. Many treated alkaline programs operate between roughly +1.0 and +2.5. Somewhere around +2.5 to +3.0, conventional inhibitor chemistry reaches its limit, and scale starts forming at the hottest surfaces first.
So the right question isn’t “Is my LSI positive?” It’s “What is my LSI ceiling, and how close am I running to it?”
That ceiling depends on three things specific to your system:
If your water treatment provider hasn’t told you what LSI range they’re managing to, and why, that’s worth a direct question. Our complete cooling tower water treatment guide covers how LSI targets fit alongside cycles, biocide, and corrosion control.
ChemREADY’s cooling tower services set LSI and cycles targets from your makeup water, heat load, and metallurgy. We test against those targets at every visit and adjust when conditions shift, so the number on your report leads to a decision.
The LSI moves whenever any of its five inputs move, and in a cooling tower all five are moving constantly. The most common drivers:
That last point is why the LSI can’t be read in isolation.
The LSI is a screening tool. It’s valuable, but it has blind spots every plant engineer should know:
Two companion indices are often reported alongside it:
| Index | Formula | How to read it | Best used for |
|---|---|---|---|
| Langelier Saturation Index (LSI) | pH − pHs | Above 0 scale-forming; below 0 undersaturated | Direction of calcium carbonate tendency |
| Ryznar Stability Index (RSI) | 2(pHs) − pH | Below 6.0 scale-forming; 6.0–7.0 near balance; above 7.0 increasingly corrosive | Gauging severity, not just direction |
| Puckorius Scaling Index (PSI) | 2(pHs) − pHeq | Same scale as RSI, but uses an equilibrium pH calculated from alkalinity | Highly buffered cooling water where measured pH can mislead |
None of these replaces trend data. Digital remote monitoring tracks pH, conductivity, and chemical feed continuously, so drift shows up as a trend line instead of a surprise on next month’s report.
Start by identifying which input moved. Adjusting chemistry before you know the cause usually creates a second problem.
Every water treatment report can print an LSI. The software calculates it automatically. The difference between programs is what happens after the number is printed.
One of the most common things we find when we walk a cooling tower for the first time is an LSI that has been drifting upward for months. It’s visible on every report, and no one has acted on it. The makeup water changed, or a conductivity probe fouled and cycles crept up. Each monthly visit recorded the number without asking what was driving it. The first thing anyone noticed was a rising approach temperature on the chiller.
A drop-off arrangement delivers inhibitor and prints a report. A managed program sets an LSI range based on your makeup water, heat load, metallurgy, and the treatment chemistry in your tower. It then treats a reading outside that range as a problem to diagnose, not a number to file. That’s the difference between having data and having control.
Our cooling tower water treatment program is built on that second model.
A calculated value showing whether water will deposit or dissolve calcium carbonate. It equals measured pH minus saturation pH (pHs). Positive values indicate scale-forming water, negative values indicate water that dissolves calcium carbonate, and zero indicates equilibrium.
It depends on treatment. Untreated water should sit near zero to slightly positive. Treated alkaline cooling towers commonly run between about +1.0 and +2.5 because inhibitors hold calcium carbonate in solution. The safe ceiling depends on inhibitor chemistry, residual, skin temperature, and metallurgy.
The water is undersaturated with calcium carbonate. It won’t form a protective carbonate film and will dissolve existing deposits, which increases corrosion tendency. Below −0.5 in a cooling tower usually points to acid overfeed, very soft makeup, or very low cycles, and should be investigated with corrosion coupon data.
LSI = pH − pHs. The saturation pH is calculated as pHs = (9.3 + A + B) − (C + D), where A reflects total dissolved solids, B temperature, C calcium hardness, and D alkalinity. Calculate it at the hottest heat transfer surface temperature, not just basin temperature.
Changes in any of its inputs: rising cycles of concentration, pH drift from acid feed problems, higher heat load and skin temperature, and seasonal makeup water changes. Rising cycles caused by blowdown or controller problems are the most common driver.
Calcium carbonate becomes less soluble as temperature rises, so scale forms first at the hottest surface. In a worked example, the same cooling tower water read +1.67 at a 95°F basin and +2.12 at a 140°F tube wall, without any change in the water itself.
The LSI, pH − pHs, shows the direction of calcium carbonate tendency. The Ryznar Stability Index, 2(pHs) − pH, gauges severity: below 6.0 is scale-forming, 6.0 to 7.0 is near balance, and above 7.0 is increasingly corrosive.
Lay it up right so it restarts clean in spring, with chemistry targets ready for the new season.
Book a 30-minute conversation about your tower, your water reports, and what the numbers are telling you. We listen first. No high-pressure close, ever.
Adeline Johnston writes WaterFACTS for ChemREADY, covering cooling tower and boiler water treatment, scale and corrosion control, and water treatment costs.
Technically reviewed by the ChemREADY water treatment team.