Water Resistivity and Conductivity

Above about 10 µS/cm nobody in water treatment talks about resistivity; below it, nobody talks about anything else. The two are exact reciprocals of the same physical property, but the resistivity scale spreads out the ultrapure end where the interesting decisions live. Theoretically perfect water at 25 °C, ionized only by its own dissociation into H⁺ and OH⁻, has a conductivity of 0.055 µS/cm and therefore a resistivity of 18.2 MΩ·cm: the number stamped on every laboratory polisher. Drop to 1 MΩ·cm and you are at 1 µS/cm, roughly 0.5 mg/L of dissolved solids; at 100 kΩ·cm (10 µS/cm) a mixed-bed cartridge is exhausted and needs changing.

The trap is the centimetre. Instrument resistivity is quoted in ohm-centimetres and conductivity in siemens per centimetre, so 18.2 MΩ·cm is 1.82 × 10⁵ Ω·m in SI, and mixing the two scales throws a factor of 100 into your answer. Temperature matters even more here than in ordinary conductivity work: water's self-ionization roughly doubles for every 10 °C, so uncompensated ultrapure water reads dramatically "dirtier" when warm. And resistivity says nothing about dissolved gases or organics: an 18.2 MΩ·cm loop can still be carrying enough TOC or dissolved CO₂ to ruin a semiconductor rinse.