Salt Reading High On Cell, Low On Tester Decision Tree

Why this matters

A salt chlorine generator (SWG) judges its own salinity from a conductivity reading taken inside the cell. When that on-board number reads high (or "OK") while a calibrated drop test or independent meter reads low, you are looking at a measurement disagreement, not a chemistry fact. Add salt to satisfy the tester and you may push true salinity past the cell's tolerance, accelerating plate wear and corroding heater headers and stainless rails. Trust the cell and ignore the tester, and the generator may quietly throttle output, leaving the pool under-chlorinated for days. The whole point of this tree is to decide which instrument is lying before you move a single bag of salt.

Symptom presentation

Typical field picture: the controller (Pentair IntelliChlor, Hayward AquaRite, Jandy AquaPure) displays salinity in the normal band (roughly 2700 to 3400 ppm depending on model), yet a fresh K-1766 salt titration or a calibrated handheld meter reads 600 to 1200 ppm lower. Output may still show green, or the generator may be cycling between "Generating" and "Inspect Cell." Water temperature is often on the cool side of the season, or the cell has 300+ hours of runtime since last cleaning.

Quick checks

• Confirm the bench tester first. Rinse and zero a handheld salt meter in distilled water; verify a drop-test kit reagent is in date. A wrong tester reverses the whole tree, so this is non-negotiable before touching salt.
• Read water temperature. Cell conductivity is strongly temperature-compensated, and below about 60 F many cells over-report salt by 300 to 700 ppm because the compensation curve loses accuracy at the cold end.
• Pull cell runtime hours and last-clean date. Scale insulates the plates and skews the reading high; a cell past its expected coating life also drifts.
• Check that the cell is fully flooded with no trapped air and flow is above the cut-in threshold; a partly air-bound cell gives an erratic salinity read.
• Note CYA and stabilizer history. It is not part of the salt disagreement, but record it while you have the kit out so the visit captures the full chemistry.
• Inspect the cell's polarity-reversal behavior if the controller logs it. A cell stuck in one polarity scales unevenly and produces a lopsided conductivity read that the display interprets as high salt.

Isolation tree

Start at the tester, walk toward the cell.

1. Is the bench tester verified (in-date reagent or meter zeroed in distilled)? If no, fix the tester and re-read. If yes, continue.
2. Is water temperature below roughly 60 F? If yes, the cell's compensation is the prime suspect. Warm-weather re-test or compare against the OEM temperature-correction note before adjusting salt. If no, continue.
3. Inspect the cell plates. Visible white scale or flaky calcium? If yes, the insulating layer raises measured conductivity and the cell over-reports. Clean per OEM (see remediation) and re-read both instruments. If plates are clean, continue.
4. Disconnect and inspect the cell wiring harness and the salinity-sense leads. Corroded or loose sense terminals make the cell read erratically high. Repair and re-read. If clean, continue.
5. Compare two independent bench methods (titration plus meter). If both agree low and the cell still reads high on clean, well-flooded, warm water with good wiring, the cell's salinity sensor or board calibration has drifted. The cell is the liar.

Confirming diagnosis

Confirm before correcting. Draw a sample at the return (not the skimmer, where surface debris skews it), run a K-1766 titration, and cross-check with a calibrated meter; two methods within 200 ppm of each other establish true salinity. Then compare that figure to the cell display. If true salt is genuinely low and the cell reads high, the cell over-reports. If true salt is in range and the cell reads high, salt is fine and the cell is miscalibrated, do not add salt. Document both numbers, water temperature, and cell hours so the next visit can trend the drift. A cell that over-reports by a consistent offset visit after visit is calibration drift; one that over-reports more on cold mornings than warm afternoons is temperature compensation; the trend tells you which.

Remediation

• Cell over-reports from scale: clean per OEM. Most makers specify a one-part muriatic acid to four-or-five-parts water soak, plates vertical, until fizzing stops (typically a few minutes), then rinse. Never scrape plates; coating loss is permanent.
• Cell over-reports from cold water: do not chase it with salt. Re-verify when water rises above the model's compensation floor.
• Sense-lead or harness fault: clean and re-seat terminals, replace corroded leads, recheck.
• Genuinely low true salt: add pool-grade salt to the OEM target, run the pump a full turnover, and re-test before declaring done. Dose in stages toward target; you can always add more, you cannot remove it without dilution.
• Drifted cell board with clean plates and verified low true salt: recalibrate per OEM service menu if supported, otherwise replace the cell.

References

• Pentair, "IntelliChlor Salt Chlorine Generator Installation and User's Guide" (cell cleaning, salinity range, and inspect-cell conditions).
• Hayward, "AquaRite Salt Chlorination System Owner's Manual" (salt display, temperature compensation, and cell-inspection diagnostics).
• PHTA/APSP-11, "Standard for Water Quality in Public and Residential Pools and Spas" (chemical parameter ranges and testing practice).
• CDC, "Model Aquatic Health Code (MAHC) Code and Annex," latest edition (water-quality testing and disinfection guidance).