Ryznar Stability Index (RSI)

John Ryznar worked at Nalco in 1944 and noticed that Langelier's index told you the direction of a water's tendency but not how hard it would push. He rebuilt it as an empirical scale calibrated against real municipal distribution data: RSI = 2·$\mathrm{pH}_s$ − pH, always positive, running roughly from 4 to 10. Below about 6 the water lays down scale; 6 to 7 is the sweet spot most cooling and distribution systems aim for; above 7 the water is increasingly aggressive and above 8.5 it is downright corrosive. A tower at pH 7.8 with $\mathrm{pH}_s$ 7.31 gives RSI = 14.62 − 7.8 = 6.82: mildly aggressive, contradicting the +0.49 LSI that called the same water scaling.

That contradiction is the point, not a bug. LSI says the water can deposit calcium carbonate; RSI says there is not enough driving force to build a protective film fast enough to matter. Field practice is to run both and treat a water only when they agree, or to lean on RSI in distribution mains and on LSI in heated equipment where the skin temperature does the deciding. The one thing you should never do is compute RSI from a $\mathrm{pH}_s$ you looked up at 25 °C and a pH you measured on hot water: both numbers have to describe the same fluid at the same temperature.