Ryznar Stability Index(RSI)

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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.

RSI Prediction

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

pH 8.0
Total Dissolved Solids 500 mg/L
Temperature 30 dEG C
Calcium Hardness 250 ppm as CaCO3
M-Alkalinity 100 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:

A 0.17
B 2.02
C 2.0
D 2.0
pHs 7.49

Finally, RSI = 2pHs – PHa

 RSI = (2 * 7.49) – 8.0 = 6.98 

The tendency of this water is slightly scale forming.

Also, read:

Langelier Saturation Index Calculator (LSI)
Cooling Tower Calculations

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