Two hygrometers can show different relative-humidity values because of sensor tolerance, temperature, placement, stabilization time or drift. Place them side by side in stable air, wait until both readings settle and compare the difference in percentage points of RH. Without a calibrated reference you cannot infer which instrument is correct from the disagreement alone.
How to interpret two instruments against a reference
If the two hygrometers read 47% and 52% RH while a suitably calibrated reference reads 49% RH under the same conditions, their deviations from the reference are -2 and +3 percentage points respectively. Whether either deviation is acceptable depends on its own specified tolerance and the reference uncertainty; agreement between two unknown sensors proves nothing by itself.
First separate different air conditions from instrument error
Hygrometers installed at different heights, near windows, humidifiers, supply vents or cold walls may sample different microclimates. Relative humidity also changes with temperature. Even if water-vapor content stays almost the same, a cooler local air mass can have a higher RH value. A valid side-by-side comparison starts with the same environment, not the same nominal room. The relationship between humidity, cold surfaces and local temperature is also explained in why condensation forms on one wall but not another.
Side-by-side verification procedure
- Place the instruments together without touching, allowing air around both sensors.
- Keep them away from direct sunlight, condensation, breathing and concentrated airflow.
- Allow sufficient stabilization based on their response characteristics, then record RH and air temperature.
- Calculate the absolute difference in RH percentage points; record the devices’ serial numbers.
- Compare with their separate accuracy conditions and, if necessary, with an independently calibrated humidity reference.
- Follow each manufacturer’s calibration or adjustment procedure only if the model supports it.
Worked example: how to judge four percentage points
Hypothetical: Hygrometer A indicates 48% RH at 22.0 °C; B indicates 52% RH at 22.1 °C. Their disagreement is 52 − 48 = 4 percentage points, not a 4% relative difference. If both are specified at ±3 percentage points in these conditions, a four-point separation could be consistent with the combined possible errors; this is not a calibration pass or proof that either is accurate.
Testo options for an independent comparison
The testo 605i measures room or duct air temperature and relative humidity, and its app calculates dew point. The current testo 625 is a professional temperature/RH meter. Either may be used as a comparison instrument only when its accuracy and calibration status suit the task. Professional branding alone does not make it an accredited reference.
Persistent disagreement: next checks
| Finding | Next step |
|---|---|
| Difference disappears when placed together | Investigate location, airflow and temperature difference |
| One sensor responds much more slowly | Check fouling, response time and equilibration |
| Both agree with each other but not with a reference | Investigate common bias or poor reference/test conditions |
| One remains outside verified tolerance | Use the model’s approved service or replacement path |
Limits of a salt-test shortcut
A saturated-salt check needs a validated recipe, temperature control, a suitable chamber and enough equilibration time. An improvised bag and unspecified salt are not a traceable calibration. NIST describes humidity calibration using controlled, well-characterized air; use a qualified lab when decisions require documented uncertainty.