← Technology WikiTDS (Total Dissolved Solids)
TDS measures the combined content of dissolved minerals, salts, and metals in water, expressed in ppm (mg/L) — the single most-used field metric for judging RO system performance.
How it works
- A TDS meter measures electrical conductivity and converts it to an estimated ppm value.
- It does not identify WHICH substances are dissolved — only the total amount.
- Typical tap water: 100–500 ppm depending on region; RO product water: usually below 50 ppm.
- TDS creep (rising product-water TDS over months) is the standard field indicator of membrane exhaustion.
Key specifications
| Unit | ppm (mg/L) |
| Typical tap water | 100–500 ppm (region-dependent) |
| Typical RO product | <50 ppm |
| Measurement method | Conductivity → estimated ppm |
What buyers and QC should check
- Ship a calibrated TDS pen with every B2B sample order — it turns an abstract spec into a demo the buyer can run in 10 seconds.
- For QC: define a pass/fail TDS threshold in the inspection protocol (e.g., product water ≤50 ppm at 25°C).
- Remind buyers that TDS alone does not prove safety — it must be read together with the system's certifications.
Free one-page QC checklist (PDF)
The buyer checks from this entry, condensed for the factory floor or the inspection. No signup needed.
Download free PDF checklistCommon questions
Does a low TDS reading mean the water is safe?
Not by itself. TDS measures quantity of dissolved solids, not which ones. Low TDS plus valid certifications (e.g., NSF/ANSI 58) is the combination that means something.
Why does my RO water TDS rise over time?
Usually membrane exhaustion or spent pre-filters letting chlorine through (which damages TFC membranes). Check pre-filter age first — it is the cheaper fix.
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