Puckorius (Practical) Scaling Index

Paul Puckorius argued in the 1980s that measured pH is the wrong number to put in a scaling index for recirculating cooling water. Bulk pH drifts with dissolved CO₂, with acid feed swings, and with whatever the tower stripped out of the air that afternoon — but the water's real buffering capacity, and therefore the pH it will settle back to at the hot metal surface, is set by alkalinity. So he substituted an equilibrium pH, pHeq = 1.465·log₁₀(alkalinity as CaCO₃) + 4.54, and kept Ryznar's doubling. With pHs = 7.31 and alkalinity 180 mg/L, pHeq = 1.465 × 2.255 + 4.54 = 7.84 and PSI = 14.62 − 7.84 = 6.78.

Read PSI on the Ryznar scale: under about 6 scaling, 6–7 balanced, over 7 corrosive. Where it earns its keep is in high-cycle towers dosed with acid, where the operator has pushed pH down to 7.0 but the alkalinity is still 300 mg/L — LSI and RSI both look fine while the exchanger scales, because the surface pH never actually stayed at 7.0. The trap is feeding it alkalinity in raw bicarbonate mg/L instead of as CaCO₃; the 0.82 conversion factor is a fifth of a PSI unit and will flip your verdict.