Alpha Moisture Systems Dewpoint Sensor Technology

Alpha Moisture Systems Dewpoint Sensor Technology

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.