A modification of the Ryznar Stability Index developed in 1983 by Brooke and Puckorius using water analysis data from operating cooling towers (as opposed to municipal waters). While very similar to the RSI as an equilibrium index for calcium carbonate saturation, it uses a new value, pHeq, the equilibrium pH which was developed from these studies.
pHeq = 1.465 x log10 (Total Alkalinity) + 4.54
PSI = 2pHS – pHeq
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