Ryznar Index (RSI)

Definition - What does Ryznar Index (RSI) mean?

The Ryznar index is a tool that is used to predict the likelihood of calcium carbonate scale to form in a given sample of water. It was created by John Ryznar in the 1940s in an attempt to provide the scientific community with a more accurate predictor of calcium carbonate scaling.

The Ryznar index is also known as the Ryznar stability index.

Corrosionpedia explains Ryznar Index (RSI)

The Ryznar index is based loosely off of the Langelier saturation index that was created in the 1930s.

The Ryznar index is logarithmic, similar to other indices that are used to calculate calcium carbonate formation. The "neutral" zone of the Ryznar index is roughly around the numerical value 6. Generally, any value above 6 indicates that the water is likely to form a calcium carbonate scale. A value below the number 6 indicates that the water will dissolve calcium carbonate formations.

The Ryznar index is calculated as follows:

2 x pHs - pH


pHs = the saturation pH

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