Resistivity and conductivity calibration
Resistivity covers two different jobs: solutions (conductivity cells, ultrapure water) and materials (wires, wafers, insulators). Both rest on a geometry factor and a temperature correction.
Instruments and typical accuracy
| Instrument | Typical range | Typical accuracy | Where it is used |
|---|---|---|---|
| Two-electrode conductivity cell | 0.01 µS/cm to about 20 mS/cm | ±0.5 to ±2 % of reading | Water treatment, laboratory |
| Four-electrode or toroidal cell | 100 µS/cm to over 1 S/cm | ±0.5 to ±2 % | Brines, seawater, process liquids |
| Ultrapure water resistivity meter | Up to 18.2 MΩ·cm | ±1 to ±2 % | Semiconductor and pharmaceutical water |
| Four-point probe | 0.001 to 1000 Ω·cm for semiconductors | ±1 to ±5 % | Wafers, thin films, metals |
| Kelvin bridge or micro-ohmmeter on a sample | Low resistance of wire and bar | 0.1 to 1 % | Wire, bus bar and conductor testing |
| Surface and volume resistivity test | 10⁶ to 10¹⁷ Ω | 10 to 20 % typical | Insulators, plastics, ESD materials |
Resistivity is the inverse of conductivity. Pick the unit that suits the range. The accuracy figures are typical ranges to help you compare instruments. Always use the specification of the instrument in front of you.
Standard solutions and pure water at 25 °C
| Standard | Conductivity | Resistivity |
|---|---|---|
| Ultrapure water | 0.0550 µS/cm | 18.18 MΩ·cm |
| 0.001 mol/L KCl | about 147 µS/cm | about 6.8 kΩ·cm |
| 0.01 mol/L KCl | about 1413 µS/cm | about 708 Ω·cm |
| 0.1 mol/L KCl | about 12.88 mS/cm | about 77.6 Ω·cm |
Round figures for orientation. Use the certified value and temperature on your standard solution certificate.
Reference standards and traceability
- Standard solutions. Potassium chloride solutions of known concentration reproduce conductivity at 25 °C (OIML R 56). Common values are about 147 µS/cm (0.001 mol/L), 1413 µS/cm (0.01 mol/L) and 12.88 mS/cm (0.1 mol/L): use the value on your certificate.
- Water methods. ASTM D1125 and ISO 7888 cover electrical conductivity of water, and ultrapure water standards describe temperature-dependent limits.
- Material methods. ASTM F84 (four-point probe for silicon), ASTM D257 (insulating materials) and IEC and ASTM wire resistivity methods.
- Quality system. ISO/IEC 17025, with scope listing conductivity or resistivity with range and uncertainty.
How it is calibrated
Typical procedure
- Rinse the cell with deionised water, then with a little of the standard.
- Immerse in a standard that brackets the working range and wait for temperature to stabilise.
- Read conductance or conductivity with temperature compensation set as the standard certificate requires.
- Compute the cell constant, or adjust the meter, and check a second standard.
- For wires and bars, measure resistance with a 4-wire method, length and cross-section, and calculate ρ.
- For wafers, use four-point probe measurements with correction factors for size and edges.
Where it goes wrong
- Temperature. Conductivity changes about 2 % per °C, so a 1 °C error is a 2 % error.
- CO₂ absorption. Ultrapure water exposed to air jumps from 0.055 to over 1 µS/cm within minutes.
- Polarisation and fouling. Electrodes coated with oil or scale read low.
- Probe spacing and sample size. Small wafers need correction factors.
- Moisture on insulators. Humidity can change surface resistivity by decades.
What goes in the uncertainty budget
- Standard solution uncertainty and its temperature dependence.
- Temperature measurement and compensation model.
- Cell constant stability and fouling.
- Geometry for wires and bars (length, diameter) and probe spacing for wafers.
- Resolution and repeatability of the meter.
Combine the contributions as in the uncertainty budget calculator, then decide pass or fail with a guard band as explained on the calculators page.
Calculators
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