Aluminium Oxide Sensor

Author: Hans-Jürgen Schwarz

English Translation by Sandra Leithäuser

back to Air Humidity Measurement

Abstract

An oxide layer (Al₂O₃) is applied to an aluminium electrode at a few microns thickness. Such layers are always porous and therefore water molecules can be stored. The concentration of the water molecules depends on the ambient air.

A thin layer of gold, which is permeable to water molecules, is now deposited on the oxide layer. The whole set up works as a capacitor with the oxide film as a dielectric. Its impedance depends on the concentration of the stored water molecules and is measured by an external electronic device. The absorption of water molecules depends on temperature, therefore the temperature of the measurement gas must be either constant or must be determined. The measuring range is typically between -70 to +60°C dew point. The sensor is sensitive towards aggressive gases and shows a drift, therefore it has to be recalibrated.
Cermet- Sensors work in a similar way but use a conductive metal-ceramic as a dielectric. They may be more chemically resistant in certain applications.

Literature

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an electric insulator, e.g. the material between the metallic plates of a capacitor.

describes a measure of opposition to alternating current, comparable with the resistance to direct current in Ohm’s law.

The CERMET-sensor is a composite material of conductive metal and ceramics to detect the absolute humidity.