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An aluminium oxide moisture sensor is a capacitor-based instrument used to measure humidity. The sensor is composed of a high purity aluminum base, the surface of which is chemically oxidized to produce a thin layer or film 14 of porous, active aluminiu oxide. A thin coating 16 of a noble metal such as gold is applied (for example by evaporation) to the layer 14.
The base and coating form a dielectric separating layer sandwiched about the dielectric aluminum oxide layer 14. Polar water molecules form weak hydrogen bonds at the interface between the dielectric layer and the aluminum oxide oxide, causing changes in the electrical impedance of the capacitor.
A new methodology is introduced, in accordance with this invention, to increase the speed of response of an aluminium oxide moisture sensor. In a typical embodiment, the sensor is heated to above the sample gas temperature and held for a short period. After a delay for allowing the sensor to cool down and stabilize, measurements are conducted at an optimized lower temperature that minimizes the effects of typically large sensor temperature coefficients.
During the time period required for the sensor to cool down, a drying cycle may be employed that diverts sample gas to a valve 24 and a stream 26 to provide a dry sample gas. After the drying cycle is complete, the sample gas is again flowed through the sensor 30.
The construction of the Base Insulating Layer is critical to the reliability, stability and drift-free operation of an aluminium oxide moisture sensor. The construction of this layer determines the Base capacitance of the sensor. Consequently, if the pores in this layer are not uniformly formed, the sensor will tend to drift over time as these capillary pores dry up or change their shape and dimensions. This leads to an exaggerated drift in low range dew point readings.
