The general principle of operation of an aluminium oxide sensor for the measurement of moisture in air and gases is well documented but the actual construction of this type of sensor, which varies widely from manufacturer to manufacturer, is critical to the performance and reliability of systems in field operation.
Certain manufacturers have resorted to catchy marketing phrases such as Hyper Thin Film Technology, Ceramic and Silicon Sensing Technology (which are simply aluminium oxide sensors on a substrate) and Thin Film technology, to demonstrate their technical competence. However, in the final analysis the performance and stability in the field of the end product is the key test.
The basic construction of the Alpha Moisture Systems Dew-point Sensor consists of a high purity aluminium wire, the surface of which is chemically oxidised to produce a pore filled insulating layer of partially hydrated aluminium oxide. A porous but conductive gold film is then deposited on the oxide layer with the gold film and aluminium wire forming the two plates of a capacitor (see diagram below).
The oxide layer that forms the dielectric separating layer of the capacitor is in the form of a mass of tubular pores running up from the aluminium core to the exposed surface. Because a dynamic equilibrium is maintained between the water vapour outside the sensor and the absorbed water within it, the water absorbed into these pores is directly related to the moisture content of the gas surrounding the sensor.
The pore size of the aluminium oxide layer is specific to water vapour and smaller molecules, but due to the dielectric constant of water compared to that of other vapours which may enter the pores, such as hydrogen, the sensor response is resistant to many contaminants and specific to changes in water vapour, regardless of the matrix gas.
Wide dynamic range from 0.001 ppm to 0.2 per cent by volume
Stability, low hysteresis and temperature coefficients
Can be intrinsically safe (when used in conjunction with barrier or isolator)
High selectivity for moisture
Can operate over a wide range of temperature and pressure