How Each Sensor Works

Q: How does a capacitive humidity sensor work?

Uses a thin hygroscopic polymer film sandwiched between two electrodes, forming a capacitor.
The film absorbs water vapor; since water has a much higher dielectric constant than the dry polymer, moisture changes the film’s dielectric constant.
The resulting capacitance change is measured and converted into a relative humidity (%RH) reading.

Q: How does a resistive humidity sensor work?

Uses a hygroscopic material (salt, polymer, or conductive polymer) whose resistance changes with moisture.
Absorbed water increases ion mobility in the material, lowering its electrical resistance or impedance.
The change in resistance/impedance is measured and correlated to %RH.

Q: How does an aluminum oxide humidity sensor work?

Uses an aluminum strip anodized to grow a thin, porous aluminum oxide (Al2O3) layer, topped with a thin conductive electrode.
Water vapor diffuses through the porous electrode and is absorbed into the oxide pores, changing the layer’s dielectric properties.
This produces a capacitance change that is measured — the sensor is essentially a specialized capacitive design tuned for trace moisture.

Side-by-Side Comparison

Feature	Capacitive	Resistive	Aluminum Oxide
Sensing property	Capacitance change	Resistance / impedance change	Capacitance change in a porous oxide layer
Measurement range	Full 0–100% RH, condensing OK	Narrower, struggles at extremes	Excellent at very low humidity (trace moisture / dew point)
Linearity	More linear	Non-linear	Non-linear; needs individual calibration curve
Accuracy / stability	Better long-term	More prone to drift	Very sensitive, but can drift and needs periodic recalibration
Cost	Higher	Lower	Higher — specialized manufacturing
Contamination resistance	Better	Weaker	Vulnerable to condensation and contamination
Typical use	HVAC, weather stations, industrial	Low-cost consumer devices	Trace moisture in gases, dew point measurement, semiconductor/gas industries

Q: When should I use a capacitive sensor?

Applications needing a wide measurement range, good linearity, and long-term stability.
HVAC systems, weather stations, and industrial process control where reliability matters.

Q: When should I use a resistive sensor?

Low-cost or non-critical applications where a simpler, cheaper sensor is acceptable.
Basic consumer devices or simple humidistats operating in a moderate humidity range.

Q: When should I use an aluminum oxide sensor?

Applications needing very low humidity or dew-point measurement, such as in dry gases or process lines.
Semiconductor manufacturing, natural gas pipelines, and other industries where trace moisture matters most.

Q: What is the key trade-off between the three?

Capacitive: best all-around choice — wide range, good linearity, strong long-term stability, moderate cost.
Resistive: cheapest and simplest, but narrower range and more prone to drift and contamination.
Aluminum oxide: unmatched sensitivity at very low humidity/dew point, but non-linear, costlier, and needs recalibration.

Reference sheet — for general technical orientation only.