Swelling-type humidity sensors

An electronic conduction-type humidity sensor was developed by Ishida et al. [56J. The sensor structure is very similar to the one illustrated in Figure 20-41. A cross-linked hydrophilic polymer in which carbon particles are dispersed is used as the humidity-sensitive film. The swelling of the polymer disturbs the ohmic contacts between dispersed carbon particles and thus the electronic resistance of the element increases sharply as the relative humidity approaches 100< o, as shown in Figure 20-41. This element was put into practical use as a dew detector for the VTR cylinder in 1978. 

This type of sensor utilizes a drastic increase in resistance at the point of dew condensation, since water absorption swells the polymers to counteract ohmic contact between carbon particles. This type sensor is now widely applied to humidity controling systems of video tape recorders or car windows. 

Resistance-humidity characteristics of a swelling-type sensor consisting of polyethylen-oxide-sorbitol. 

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