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Langelier Saturation Index (LSI) & Ryznar Stability Index (RSI)

The Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI) are water quality indicators used to assess whether water is likely to form calcium carbonate scale or cause corrosion in pipes and plumbing systems.

The Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI) are used to determine whether water is corrosive, stable, or scale forming.

TEMPERATURE
HARDNESS
ALKALINITY
TDS
PH

LSI "Langelier Saturation Index" Result Description

The Langelier Saturation Index (LSI) is a numerical indicator used to evaluate whether water is likely to be corrosive (negative LSI) or scale-forming (positive LSI).

It measures the tendency of calcium carbonate to either dissolve into the water or precipitate out, helping assess the water's scaling or corrosion potential in water systems.

LSI INDEX DESCRIPTION GENERAL RECOMMENDATION
-5 Severe Corrosion Strong Corrosion Inhibitor Treatment Required
-3 Moderate Corrosion Strong Corrosion Inhibitor Treatment Required
-2 Moderate Corrosion Corrosion Inhibitor Treatment Required
-1 Mild Corrosion Corrosion / Scale Treatment Required
-0.5 None- Mild Corrosion Corrosion / Scale Treatment Required
0 Near Balanced Balanced Treatment Required
0.5 Some Faint Coating Scale preventive Treatment Required
1 Mild Scale Coating Scale preventive Treatment Required
2 Mild to Moderate Coatings Scale preventive Treatment Required
3 Moderate Scale Forming Strong Scale preventive Treatment Required
4 Severe Scale Forming Strong Scale preventive Treatment Required

RSI "Ryznar Stability Saturation Index" Result Description

The Relative Strength Index (RSI) is a widely used momentum indicator in technical analysis that measures the speed and strength of an asset's recent price movements.

Displayed on a scale from 0 to 100, it typically uses a 14-period calculation to help identify potential market conditions, with readings above 70 indicating an overbought market and below 30 suggesting an oversold market.

RSI INDEX DESCRIPTION GENERAL RECOMMENDATION
4 - 5 Highly Scale Forming Polymer Scale Preventive Treatment Required
5 - 6 Slightly Scale Forming Genral Scale Preventive Treatment Required
6 - 7 Slightly Scale Forming / Corrosive Treatment according to other parameter Required
7 - 7.5 Corrosive Corrosion inhibitor Treatment Required
7.9 - 9 Highly Corrosive Strong Corrosion inhibitor Treatment Required
9 and Over Very Highly Corrosive Strong Corrosion inhibitor Treatment Required

LSI & RSI Formula

1. LSI Formula

LSI = pH − pHs

  • pH = Measured Water pH
  • pHs = Saturation pH

2. Saturation pH (pHs)

pHs = (9.3 + A + B) − (C + D)

A is TDS Factor

A = (log10(TDS) − 1) / 10

  • TDS = Total Dissolved Solids (mg/L)

B is Temperature Factor

Convert temperature into Kelvin:

K = Temperature (°C) + 273

B = -13.12 × log10(K) + 34.55

C is Calcium Hardness Factor

C = log10(Calcium Hardness) − 0.4

Calcium Hardness should be in mg/L as CaCO₃.

D is Alkalinity Factor

D = log10(Total Alkalinity)

Total Alkalinity should be in mg/L as CaCO₃.

Complete Saturation pH Formula

pHs = (9.3 + A + B) − (C + D)

3. Ryznar Stability Index (RSI)

RSI = (2 × pHs) − pH

Calculation Example

Parameter Value
Water Temperature 35°C
Water pH 7.2
TDS 1000 mg/L
Calcium Hardness 100 mg/L
Total Alkalinity 200 mg/L

Step 1: Calculate A

A = (log10(1000) − 1) / 10 = (3 − 1) / 10 = 0.20

Step 2: Calculate B

Kelvin = 35 + 273 = 308 K

B = -13.12 × log10(308) + 34.55 ≈ 1.99

Step 3: Calculate C

C = log10(100) − 0.4 = 2 − 0.4 = 1.60

Step 4: Calculate D

D = log10(200) ≈ 2.301

Step 5: Calculate pHs

pHs = (9.3 + 0.20 + 1.99) − (1.60 + 2.301) ≈ 7.589

Step 6: Calculate LSI

LSI = 7.2 − 7.589 = -0.389

Step 7: Calculate RSI

RSI = (2 × 7.589) − 7.2 = 7.978

LSI Interpretation

LSI Value Interpretation
Less than 0 Corrosive Water
0 Stable Water
Greater than 0 Scale Forming Water

RSI Interpretation

RSI Value Interpretation
Less than 6 Heavy Scale Forming
6.0 – 6.5 Scale Forming
6.5 – 7.0 Slight Scale Forming
7.0 – 7.5 Nearly Stable
7.5 – 8.0 Mild Corrosion
Greater than 8 Highly Corrosive