← Buyer GuidesHow to Test TDS Rejection Rate (Field Method)
Rejection rate = (feed TDS − product TDS) ÷ feed TDS × 100%. It is the single most practical field metric for RO performance — measurable in 60 seconds with a calibrated TDS pen.
Key points
- Test conditions matter: feed pressure, water temperature, and feed TDS all shift the result. Compare readings taken under the same conditions.
- Calibrate the TDS meter (or verify against a reference solution) before any acceptance test.
- New membranes need a short flush/run-in before the reading stabilizes — don't judge the first liter.
- TDS measures quantity of dissolved solids, not which ones. Pair it with certifications for safety claims.
- Define a pass/fail threshold in your QC protocol (e.g., product water ≤50 ppm at 25 °C) — in writing, before inspection.
Action checklist
- Calibrate TDS meters before the inspection (reference solution or cross-check two meters).
- Record feed TDS, product TDS, water temperature, and feed pressure for each sample.
- Flush new systems per the manual before taking acceptance readings.
- Compute rejection rate per unit; flag any unit far below the lot average.
- Photograph meter readings with unit serial numbers visible.
- File the readings with the inspection report for traceability.
Common questions
My product TDS is 30 ppm from 300 ppm feed. Is that good?
That is a 90% rejection rate — acceptable for many residential systems but below what a new standard membrane should deliver (typically mid-to-high 90s). Check pre-filter condition and feed pressure before blaming the membrane.
Why do two meters give different readings?
Usually calibration drift or temperature compensation differences. Always cross-check with a second meter and a reference solution before rejecting a lot over TDS numbers.
Related reading
A note on honesty. CWS provides sourcing and QC services for water purification products. This guide is general information for overseas buyers — not a guarantee of any outcome. We don't promise the lowest price in China, and we don't promise zero defects. What we do promise: written inspection criteria, lot traceability, and telling you when something is outside what we can verify.