The Ryznar stability index (RSI) uses a database of scale thickness measurements in municipal water systems to predict the effect of water chemistry. It was developed from empirical observations of corrosion rates and film formation in steel mains.
This index is defined as:
:RSI = 2 pH – pH (measured)
* For 6.5 < RSI < 7 water is considered to be approximately at saturation equilibrium with calcium carbonate
* For RSI > 8 water is undersaturated and, therefore, would tend to dissolve any existing solid CaCO
* For RSI < 6.5 water tends to be scale form