Calibrating relative-humidity sensors
Humidity calibration is a temperature calibration in disguise: small temperature differences between sensor and reference become large RH errors.
Relative humidity depends strongly on temperature. A 0.5 °C difference between the unit under test and the reference at 50 %rh produces an RH error of the order of 1.5 %rh, so humidity calibration depends first on thermal equality.
Methods
- Comparison with a reference hygrometer (chilled-mirror or calibrated capacitive) in a humidity chamber at several RH and temperature points.
- Saturated salt solutions in sealed containers for fixed points, with long equilibration times.
- Two-pressure or two-temperature humidity generators in specialist laboratories.
Points to note
Allow long stabilisation, keep sensor and reference close together, and report the temperature at which each RH point was calibrated. Capacitive sensors drift and can be contaminated; a calibration history is essential.
Frequently asked questions
- What uncertainty is typical for humidity sensor calibration?
- Around 1–2 %rh at k = 2 in a good chamber against a chilled-mirror reference; 2–3 %rh with salt solutions or lower-grade references.
- How often should humidity sensors be calibrated?
- Annually is common for capacitive sensors because they drift with contamination and exposure; the interval should be adjusted from as-found history.
References
- [1]EURAMET cg-20: Calibration of Climatic Chambers
General technical guidance written against the cited sources. It is not regulatory or legal advice and does not replace the applicable standard, guideline or a qualified reviewer's judgement.
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