Capacitance-humidity characteristics

A capacitance-type humidity sensor developed by Vaisala consists of a comb shaped Au electrode and cellulose acetate dissolved in ethylene dichloride as humidity-sensitive materials. A schematic view of this sensor is shown in Figure 20-31 [47]. This sensor is now widely used in meteorological observations and in many other humidity measuring instruments. As illustrated in Figure 20-32, the capacitance-humidity characteristics show a linear relation from 0 to 100<7o r. h. [48]. This sensor has the advantages of good accuracy, low hysteresis, and fast response time. 

Frequency dependence of capacitance-humidity characteristics of Humicape . 

Capacitance-humidity characteristics at various temperatures of the temperature-humidity sensor. 

Delapierre G, Grange H, Chambaz B, Destannes L (1983) Polymer based capacitive humidity sensor—characteristics and experimental results. Sens Actuators A 4 97-104... 

Fig. 18.6 Operating characteristics of a-Al203-based capacitance humidity sensors during long-term stability test (Data from Chen and Jin 1992)...

The most important considerations with respect to sensor characteristics are surface properties (hydrophilic-hydrophobic), pore-size distribution, and electrical resistance. To ensure adequate sensitivity and response a sensor of this type should consist of a very thin film of porous ceramic with a porosity >30%. The contacting electrodes may be interdigitated or porous sandwich-type structures made from noble metals (e.g., Pd, Pt, Au) and so constructed that they do not obstruct the pores of the oxide film. Humidity affects not only the resistance of a porous ceramic but also its capacitance by extending the surface area in contact with the electrodes. The high dielectric constant of adsorbed water molecules also plays an important role. 

The dielectric constant is an expression of the ratio of capacitance of a capacitor with plastic as the dielectric to the capacitance of the same capacitor with air as the dielectric. The loss factor, by contrast, is a measure of the energy that an insulating substance in an alternating field transforms into heat and is therefore lost as electrical energy. The dielectric characteristic values are material constants. They are dependent on temperature and frequency range and also on ambient conditions such as humidity, for example [8]. 

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