Ryznar Stability Index(RSI)

ryznar stability index calculator

What is Ryznar Stability Index?

Ryznar Stability Index(RSI) was developed by John Ryznar in 1940. It is also called Rznar Index.

The reason behind the development of ryznar index is to get more accurate prediction of calcium carbonate scale than Langalier Saturation Index which was developed in 1936.

Ryznar Index has its own concept & basis of prediction. However the calculation of saturation pH is same as LSI but the difference is in the formula & prediction values.

In Ryznar Index, any value above 6 indicates the water is likely to form calcium carbonate scale while on the other hand any value below 6 water will dissolve calcium carbonate scale and considered the tendency of water is corrosive.

Prediction Based on RSI:

RSI <5.5Heavy scale will form
5.5< RSI <6.2Moderate scale will form
6.2< RSI <6.8Slight scale will form
6.8< RSI <8.5Corrosive water
RSI <8.5Very corrosive water

Ryznar Stability Index Calculator:

Ryznar Stability Index Calculation Formula:

RSI Formula
A =( LOG10 * (TDS)-1) / 10
B = (-13.12 * LOG10(temp +273)) + 34.55
C = LOG(CaH) – 0.4
D = LOG10(M-Alk)
pHs = (9.3 + A + B) – (C + D)
RSI = 2pHs – pHa


TDS = Total Dissolved Solids(ppm)

temp = Temperature(dEG C)

CaH = Calcium Hardness(ppm as CaCO3)

M-Alk = M-Alkalinity(ppm as CaCO3)

pHs = Saturation pH

pHa = Actual pH

Ryznar Index Calculation Example:

Total Dissolved Solids500 mg/L
Temperature30 dEG C
Calcium Hardness250 ppm as CaCO3
M-Alkalinity100 ppm as CaCO3

Lets Calculate..

Put cooling water analysis values in above Formula to calculate RSI. You get below values for constants A,B,C & D as per below table:


Finally, RSI = 2pHs – PHa

RSI = (2 * 7.49) – 8.0 = 6.98

The tendency of this water is slightly scale forming.

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