Engineering Basis, Formulas & Limits
All calculations on RO Water Check run locally in your browser using established residential water treatment formulas and public engineering standards.
1. TDS Rejection Formula & Meter Calibration
Rejection rate is calculated from feed (tap) total dissolved solids and permeate (RO) total dissolved solids:
Rejection (%) = (1 − (RO TDS / Tap TDS)) × 100
This aligns with standard residential evaluation practices under NSF/ANSI 58 (Reverse Osmosis Drinking Water Treatment Systems). Note that handheld TDS pens actually measure electrical conductivity (µS/cm) and multiply by a conversion factor—commonly 0.50 (NaCl scale) or 0.70 (442 natural water scale). Because different meter brands use different factors, always measure both Tap TDS and RO TDS with the same meter at roughly the same water temperature so the conversion factor cancels out in the ratio.
2. Benchmark Thresholds & Sources
| Parameter | Reference Range | Engineering Basis | Boundary / Exception |
|---|---|---|---|
| TFC Membrane Rejection | ≥ 90% (typical 92%–98%) | NSF/ANSI 58 & WQA residential TFC membrane guidance | Must be measured after 30–60 sec flush and before any remineralization stage. |
| Empty Tank Air Pre-Charge | 7–10 psi | Standard bladder tank manufacturer & WQA service practice | Measured only when cold feed is shut off and tank is 100% drained. |
| Minimum Feed Pressure | ≥ 40 psi | Non-pumped ASO shut-off and membrane driving pressure | Systems with an electric booster pump can operate on lower supply pressure. |
| Pre-Filter / Membrane Intervals | 6–12 mo / 2–4 yr | WQA residential maintenance benchmarks | Shortens on high-silt well water, extreme hardness, or chloramine supply. |
3. What a TDS Meter Cannot Detect
A low TDS reading confirms that your RO membrane's semi-permeable barrier is intact against dissolved inorganic ions (such as calcium, sodium, chloride, and many dissolved salts). However, a handheld TDS pen is not a comprehensive water safety lab test:
- It cannot detect uncharged organic molecules (certain VOCs, pesticides, or PFAS) or microbial contaminants (bacteria, viruses, cysts).
- If you are on private well water or suspect microbial/chemical contamination, send a sample to a state-certified laboratory rather than relying on TDS alone.
4. Primary References
- Water Quality Association (WQA) — Residential reverse osmosis technical & maintenance guidance.
- NSF International — NSF/ANSI 58 Reverse Osmosis Drinking Water Treatment Systems overview.
- University of Minnesota Extension — Residential RO operation and discharge considerations.
- U.S. EPA — Drinking water consumer guidance.