Langelier Saturation Index Calculator

Calculator Inputs

Sample name

Used in downloads and reports.

Measured pH

Enter a valid pH (0–14).

Use the field measurement at sampling time.

Temperature (°C)

Enter a valid temperature.

Water temperature influences carbonate solubility.

Total dissolved solids (mg/L)

Enter a positive TDS value.

Higher ionic strength shifts saturation behavior.

Calcium hardness (mg/L as CaCO₃)

Enter a positive calcium hardness value.

Report hardness as CaCO₃ equivalent.

Total alkalinity (mg/L as CaCO₃)

Enter a positive alkalinity value.

Use total alkalinity as CaCO₃ equivalent.

Reset

Example Data Table

Sample	pH	Temp (°C)	TDS (mg/L)	Ca Hardness (mg/L as CaCO₃)	Alkalinity (mg/L as CaCO₃)	pHs	LSI	Trend
Cooling Tower	7.60	25	500	150	120	7.70	-0.10	Near equilibrium
Boiler Make-up	8.20	30	800	250	180	7.23	0.97	Scaling
RO Permeate	6.80	20	80	20	25	7.91	-1.11	Strongly corrosive

Example outputs are illustrative and assume inputs are reported as CaCO₃ equivalents.

Formula Used

Core equations

LSI = pH − pHs
pHs = (9.3 + A + B) − (C + D)

pHs estimates the pH at which water is saturated with calcium carbonate.

Factors

A = (log10(TDS) − 1) / 10
B = −13.12·log10(T + 273) + 34.55
C = log10(CaH) − 0.4
D = log10(Alk)

T in °C, TDS in mg/L, CaH and Alk in mg/L as CaCO₃.

LSI is an index, not a guarantee. Use alongside corrosion coupons, scaling tests, and system metallurgy guidance.

How to Use This Calculator

Measure pH and temperature at the sampling point.
Use lab results for TDS, calcium hardness, and alkalinity.
Ensure hardness and alkalinity are reported as CaCO₃ equivalents.
Click Calculate LSI to view results above the form.
Interpret LSI: negative suggests corrosion, positive suggests scale.
Download CSV for logs or PDF for reporting and sharing.

Operational meaning of the index

LSI converts routine water tests into a stability signal for cooling systems, boilers, and potable distribution. It compares measured pH with saturation pH (pHs), the point where calcium carbonate neither dissolves nor precipitates. The index supports decisions about scaling control, corrosion control, and maintaining balance across changing operating conditions.

Use the index as a screening tool, not a laboratory substitute. Pair it with measured calcium, alkalinity, and pH quality checks, and record sampling time and location. Consistent inputs reduce false swings and improve comparability between shifts and sites during seasonal changes and after chemical feed maintenance.

Working bands used in practice

Many programs treat −0.1 to +0.1 as near equilibrium, where neither heavy scale nor aggressive dissolving dominates. Values below −0.5 often align with higher metal loss potential, while values above +0.5 indicate strong deposition tendency. Site bands should be validated with coupons, inspections, and heat‑transfer performance metrics.

Temperature and salinity effects

As temperature rises, carbonate solubility decreases, typically lowering pHs and increasing LSI for the same measured pH. Higher TDS alters ionic strength through factor A, shifting pHs by tenths when conductivity climbs during concentration cycles. This is why a stable pH reading can still correspond to different scaling risk in summer operation.

Hardness and alkalinity drivers

Calcium hardness reduces pHs through factor C, raising LSI and increasing the likelihood of deposits on heat exchanger surfaces. Alkalinity reduces pHs through factor D and improves buffering, which can reduce corrosion in very soft waters when managed properly. Reporting both as CaCO₃ equivalents keeps calculations consistent across labs.

Control actions tied to direction

When LSI is negative, operators may increase alkalinity, adjust pH within material limits, or improve corrosion inhibition, then confirm via iron, copper, and coupon trends. When LSI is positive, actions can include softening, increasing blowdown, enhancing dispersants, or lowering pH to keep carbonate in solution and protect heat‑transfer efficiency.

Logging and audit-ready reporting

Track inputs, pHs, and LSI alongside temperature, conductivity, and inhibitor residuals to connect chemical control to outcomes such as pressure drop, approach temperature, and inspection findings. Use the CSV export for trending dashboards and the PDF report for consistent communication with maintenance, safety, and compliance stakeholders.

FAQs

1) What does a negative LSI mean?
Negative LSI indicates under‑saturation with calcium carbonate, so water tends to dissolve scale films and can increase corrosion risk, especially with low alkalinity.

2) What does a positive LSI mean?
Positive LSI indicates over‑saturation, so calcium carbonate may precipitate as scale, reducing heat‑transfer efficiency and increasing cleaning frequency.

3) Which units should be used?
Use TDS in mg/L, temperature in °C, and report calcium hardness and total alkalinity as mg/L expressed as CaCO₃ equivalents.

4) Why is pHs different from measured pH?
pHs is the computed equilibrium pH for carbonate saturation. The difference between measured pH and pHs determines whether water tends to scale or corrode.

5) Can LSI alone determine corrosion control?
No. Flow, oxygen, microbiology, and metallurgy influence corrosion. Use LSI with coupons, inhibitor residuals, and inspection data.

6) How often should LSI be recalculated?
Recalculate after any chemistry change: source shifts, dosing adjustments, blowdown changes, seasonal temperature swings, or major maintenance events.