The Ryznar Stability index is an empirical method for predicting scaling tendencies of water based on a study of operating results with water of various saturation indices.

Stability index = 2pH_s - pH = pH_s - Langelier's Saturation pH

This index is often used in combination with the Langelier index to improve the accuracy in predicting the scaling or corrosion tendencies of a water. The following chart Illustrates how to use this index.