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Puckorius (Practical) Scaling Index

Also known as PSI scaling index

PSI=2 pHs−pHeq,pHeq=1.465 log⁡10Alk+4.54\mathrm{PSI} = 2\,\mathrm{pH_s} - \mathrm{pH_{eq}}, \quad \mathrm{pH_{eq}} = 1.465\,\log_{10}\mathrm{Alk} + 4.54
sig figs

Worked example: pHs 7.31, Alk 180 mg/L as CaCO3 → PSI = 6.776. Press Try an example to run it live, then adjust anything.

Enter your known values, leave one input blank, and solves for the missing one. Tap a variable’s symbol to see what it means, with a typical value. Change any unit and the answer follows.

Where Puckorius (Practical) Scaling Index fits

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See also

Saturation pH (pHs) for Langelier's Index: Both use saturation pH and total alkalinity as CaCO₃.Langelier Saturation Index (LSI): Both use saturation pH.Ryznar Stability Index (RSI): Both use saturation pH.Saturation pH (pHs), Rigorous: from the Carbonate Equilibria: Both use saturation pH.Cooling Tower LSI at Cycles of Concentration: Both use total alkalinity as CaCO₃.Langelier Saturation Index from a Full Water Analysis: Both use total alkalinity as CaCO₃.
6 more relatedLarson–Skold Index: Both use total alkalinity as CaCO₃.Lime Dose for Softening (as CaCO₃): Both use total alkalinity as CaCO₃.Predicted Cooling Tower LSI and Maximum Cycles, from the Makeup Analysis: Both use total alkalinity as CaCO₃.Soda Ash Dose for Softening (as CaCO₃): Both use total alkalinity as CaCO₃.Total Alkalinity as CaCO₃ from Species: Both use total alkalinity as CaCO₃.Alkalinity Remaining After Alum: Both use total alkalinity as CaCO₃.
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Puckorius (Practical) Scaling Index explained

AlkpHsPSI

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\mathrm{pH}_{\text{eq}} = 1.465·log₁₀(alkalinity as CaCO₃) + 4.54, and kept Ryznar's doubling. With pHs\mathrm{pH}_s = 7.31 and alkalinity 180 mg/L, pHeq\mathrm{pH}_{\text{eq}} = 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.

Puckorius (Practical) Scaling Index formula

PSI=2 pHs−pHeq,pHeq=1.465 log⁡10Alk+4.54\mathrm{PSI} = 2\,\mathrm{pH_s} - \mathrm{pH_{eq}}, \quad \mathrm{pH_{eq}} = 1.465\,\log_{10}\mathrm{Alk} + 4.54
Where
  • PSI\mathrm{PSI}= Puckorius Scaling Index
  • pHs\mathrm{pH_s}= Saturation pH
  • Alk\mathrm{Alk}= Total alkalinity as CaCO₃ (mg/L)
Where this comes from
  • Puckorius, P. R., & Brooke, J. M. (1991). A new practical index for calcium carbonate scale prediction in cooling tower systems. Corrosion, 47(4). Read it
In this toolkit

Scaling and corrosion indices

6 formulasLangelier, Ryznar, Puckorius, Larson–Skold and coupon corrosion rate — the indices that say whether water will scale or eat the pipe.

Revised 31 October 2026
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formula.expert. (2026, October 31). Puckorius (Practical) Scaling Index Formula & Calculator. https://formula.expert/solvers/puckorius-scaling-index

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