handbook of modern sensors

13.3 Electrical Conductivity Sensors 399
Fig. 13.5. Simplified equivalent electric
circuit of a capacitive thin-film humidity
sensor.
vapor-deposited SiO 2 or phosphorosilicate glass (CVD PSG). The thickness of the
layer is in the range 300–4000 Å.
Asimplified equivalent electrical circuit is shown in Fig. 13.5. Each element of the
circuit represents a RC transmission line [9]. When the relative humidity increases, the
distributed surface resistance drops and the equivalent capacitance between terminals
1 and 2 grows. The capacitance is frequency dependent; hence, for the low-humidityrange
measurement, the frequency should be selected near 100 Hz, whereas for the
higher humidities, it is in the range between 1 and 10 kHz.
13.3 Electrical Conductivity Sensors
Resistances of many nonmetal conductors generally depend on their water content, as
was discussed in Section 3.5.4 of Chapter 3. This phenomenon is the basis of a resistive
humidity sensor or hygristor. A general concept of a conductive hygrometric sensor is
shown in Fig. 13.6. The sensor contains a material of relatively low resistivity which
changes significantly under varying humidity conditions. The material is deposited
on the top of two interdigitized electrodes to provide a large contact area. When water
molecules are absorbed by the upper layer, resistivity between the electrodes changes
and can be measured by an electronic circuit. The first such sensor was developed
by F. W. Dunmore in 1935; it was a hygroscopic film consisting of 2–5% aqueous
solution of LiCl [10]. Another example of a conductive humidity sensor is the socalled
“Pope element,” which contains a polystyrene film treated with sulfuric acid
to obtain the desired surface-resistivity characteristics.
Other promising materials for the fabrication of a film in a conductivity sensor
are solid polyelectrolytes because their electrical conductivity varies with humidity.
Long-term stability and repeatability of these compounds, although generally not too
Fig. 13.6. Composition of a conductive humidity sensor.