The Ryznar stability index (RSI) attempts to correlate an empirical database of scale thickness observed in municipal water systems to the water chemistry. Like the Langelier Saturation Index (LSI), the RSI has its basis in the concept of saturation level. Ryznar attempted to quantify the relationship between calcium carbonate saturation state and scale formation. The Ryznar index takes the form:

Where:

• pH is the measured water pH
• pHs is the pH at saturation in calcite or calcium carbonate

So, it gives an indication of the aggressive or encrusting trend (scale-forming) water. And also an indication about the scale and corrosion potential of the water.
The empirical correlation of the Ryznar stability index can be summarized as follows:

• RSI < 6,5 water tends to be scale forming (the scale tendency increases as the index decreases)
• 6,5 < RSI < 7 water is considered to be approximately at saturation equilibrium with calcium carbonate
• RSI > 8 water is under saturated and, therefore, would tend to dissolve any existing solid CaCO3 (mild steel corrosion becomes an increasing problem)

Attached table given by the author (in French):

---------------------
Perconnal indication for aggressive and corrosive tendency:

Ryznar index	Indication
<= 6.5	No aggressive trend
> 6.5 and <= 7.2	Slight aggressive
> 7.2 and <= 7.8	Slight corrosion (strong with T> 60 °C)
> 7.8 and <= 8.5	Corosion so T>15°C
> 8.5	Strong corrosion

---