It’s an immutable law of RH measurement; relative humidity sensors drift. They do so for the simple reason that they are “air breathers.” Unlike temperature sensors, the internal structure of the humidity sensor must be in direct contact with the environment, which is constantly changing temperature and contains countless airborne contaminants. Both fluctuating temperatures and contaminants significantly affect the accuracy of any RH sensor, more so over time. This is why, even if the calibration process were perfect (it isn’t), once exposed to the real world, the measurement accuracy inevitably degrades.

In other words, to anticipate the drift of a device, you must achieve optimal accuracy in the calibration and the sensor measurement system. In effect, you need to reduce or virtually eliminate all other sources of error in the manufacture and maintenance of the device.

Calibration Uncertainty
All humidity calibration chambers have an associated uncertainty, a major source of which is temperature non-uniformity, which must be factored into a measuring device’s accuracy specification. Before humidity calibration, manufacturers of humidity data recorders must perform a high-accuracy temperature calibration. Each recorder’s measured temperature is then able to compensate for chamber non-uniformity during RH calibration — greatly reducing this source of error.

Temperature Effect and Mathematical Fit
Most RH measuring devices are calibrated to measure at one specific temperature (typically 25ºC). But, unless the device will only be used to measure humidity at that temperature, there can be significant temperature-related inaccuracies.

Hysteresis
Hysteresis is the tendency of measuring devices to not return completely to their original state after a change has been measured. It’s also a major source of error.

Measurement Resolution
Resolution is simply the smallest measurable increment that the device can detect. A good device will feature a 12-bit high-resolution system that detects changes of as small as 0.05%RH.

Electronics
A significant element that affects a device’s accuracy is its electronic components. Electronics systems are greatly impacted by temperature, which in turn affects overall accuracy. One challenge that manufacturers face is trying to get the electronic system to remain stable over wide temperature ranges.

From: “Catching The Drift”