Pure water conducts almost no electricity; as dissolved salts increase, conductivity rises. That is why conductivity is the field-measurable, instant equivalent of TDS. The measurement depends on temperature; instruments usually compensate to 25 °C.
Reference values
- RO product water: 5–50 µS/cm
- Mains water: 200–600 µS/cm
- Dialysis water limit: <10 µS/cm
- Purified water (Ph. Eur.): ≤4.3 µS/cm @20 °C
In RO systems, membrane performance (salt rejection) is monitored by continuously measuring feed and product conductivity.
