Source TDS, post-RO TDS, post-DI TDS, system pressure, production flow, water temperature, membrane spec, resin age. The math names which component has failed — the membrane, the resin, the pre-filter, or a downstream contamination — and surfaces the right intervention before the wrong part gets thrown at the problem.
Methodology: TFC membrane rejection ratios, temperature- and pressure-corrected production curves, DI-resin polish thresholds. Calibrated against Bluegrass and Ohio Valley commercial route data. The math is in the methodology guide.
RO rejection is 96.4% (healthy ≥ 95%), production is 99% of the temperature-corrected nominal (healthy ≥ 85%), and the post-DI reading is 2 ppm (residential threshold ≤ 10 ppm). The rig is doing exactly what it was built to do: the pre-filter takes the sediment and the chlorine, the membrane takes the bulk of the dissolved solids, and the resin polishes the last fraction. Log the readings and re-test at the next route.
The mistake the shop made for the first three years on the commercial book was reading a high post-DI number and replacing the resin. The customer was happy for a route or two, the post-DI dropped, and we wrote it off as a routine service interval. What we missed was that the resin was exhausting faster every cycle because the membrane behind it had been slowly failing all along — and we were buying two-hundred-dollar resin canisters every five weeks to mask a forty-dollar membrane problem. The diagnostic that catches it is the one this tool runs: read the rejection number first, the resin number second. The resin reading is downstream of the membrane reading, both literally and diagnostically.