What the number measures
Resistivity measures how well water conducts electricity. The purer the water, the fewer charge carriers remain, and the higher the resistance. It is an inverse indicator — a larger number means purer water.
The unit, MΩ·cm, can be pictured as the resistance of a one-centimetre column of water to current flow.
Why the ceiling is 18.2
Even with every dissolved ion removed, water still dissociates weakly:
H₂O ⇌ H⁺ + OH⁻
At 25 °C this self-ionisation yields roughly 10⁻⁷ mol/L of H⁺ and OH⁻, which are themselves charge carriers. Pure water therefore cannot be infinitely insulating; the theoretical limit is 18.2 MΩ·cm at 25 °C. This is a physical constant of water, not a marketing figure.
One useful corollary: any instrument claiming more than 18.2 MΩ·cm is not telling the truth. It is physically impossible.
Temperature must be stated
Ionisation varies strongly with temperature, and so does resistivity:
- 25 °C: 18.2 MΩ·cm
- 20 °C: about 15 MΩ·cm
- 30 °C: about 21 MΩ·cm (even though the water is actually worse — ionisation is simply stronger)
A resistivity figure without a temperature is meaningless. The convention is to normalise to 25 °C, and reputable instruments apply temperature compensation automatically. If a reading is quoted at another temperature, ask whether compensation was applied.
Why "display reads 18.2" is not "you received 18.2"
This is the most common trap. Three causes:
- Sensor location. The probe sits inside the unit, measuring the water at that moment. By the time water travels through tubing and the final filter and meets air at the outlet, it has absorbed CO₂ — which forms carbonic acid and drops resistivity to 5–10 MΩ·cm.
- Dispensing practice. Open vessels, unclean containers and touching the bottle mouth all contaminate. Ultrapure water should be drawn into dedicated containers with minimal air exposure.
- Instantaneous readings. Values look best right after start-up or a cartridge change. Only the reading after a period of stable operation reflects reality.
How to judge whether a reading is trustworthy
- Check for temperature compensation. No stated compensation means none was applied.
- Check for a trend log. With only a live number you cannot tell whether the unit is stable. Trend history is what makes a reading meaningful.
- Verify independently. Measure at the outlet and in the container with a portable conductivity meter, then compare with the unit's display. A large gap points to tubing or dispensing problems.
- Never rely on resistivity alone. It is blind to uncharged contaminants — TOC, bacteria and endotoxin do not affect it. Resistivity must be read alongside TOC and microbiological data.
A useful analogy
Think of resistivity as "how much salt is left" and TOC as "how much organic dirt is left". They are independent dimensions: water can be salt-free (18.2 MΩ·cm) while still organically dirty (high TOC). For chromatography and cell culture, the latter is often the more damaging.
Summary
18.2 MΩ·cm is water's physical limit — necessary but not sufficient. The three questions that matter are: is it sustained, is temperature compensated, and what is the TOC? Answer those and you have actually understood the instrument.
