Resistive humidity sensors

Ink-jet printing provides another opportunity to fabricate resistive humidity sensors. Different sensor configurations have been investigated using an ink with silver nanoparticles (Weremczuk et al., 2012). Likewise, thin film technologies were applied to develop resistive temperature elements onto Kapton strips, which were subsequently woven into a textile (Zysset, 2013). 

Cho N, Lim T, Jeon Y, Gong M (2008) Inkjet printing of polymeric resistance humidity sensor using UV-curable electrolyte inks. Macromol Res 16 149-154... 

Hijikigawa M, Miyoshi S, Sugihara T, Jinda A (1983) A thin-film resistance humidity sensor. Sens Actuators 4 307-315... 

Ueda Masahiro, Nakamura Kazumasa, Tanaka Kazuhiro, Kita Hidetoshi, and Okamoto Ken-ichi. Water-resistant humidity sensors based on sulfonated polyimides. Sens. Actual. B. Ill (2007) 463-470. 

If solids are wetted, then their lattice components become hydrated, and mobile ions can form. As a result, their conductance increases. This behaviour is the basis for resistive humidity sensors. Commonly, a thin layer of the sensitive solid is placed between two electrodes. A useful material e.g. is phoshorous pentoxide. 

Fig. 1 Representative polymers that are used for electrical-resistance humidity sensors.

Fig. 3 The temperature dependence of the impedance of an electrical-resistance humidity sensor.

Hijikigawa, M., Miyoshi, S., Sugihara, T, and Jinda, A. (1983). A thin-film resistance humidity sensor. Sensors Actuators 4 307. 

An interesting system based on PEDOT PSS with embedded iron oxide nanoparticles has been proposed as resistive humidity sensor due to the water adsorbing properties of this conducting polymer [48]. In particular a free standing nanocomposite films were prepared to be easily transferred onto different substrates which favor its applicability. Remarkably, the sensitivity to humidity increased with the nanoparticle concentration probably due to the augment of the surface roughness of the film that enlarges the exposed area to interact with water vapor. 

Surface conduction is monitored in most humidity sensors through the use of porous ceramics of MgCr204—Ti02 that adsorb water molecules which then dissociate and lower the electrical resistivity. 

Humidity sensors may be animal or plastic skins varying in length with changes in rh or lithium chloride coating changing in electrical resistance. The former is prone to lose calibration. Other comments above apply equally to rh control. 

Conventional humidity sensors of the electric resistance variable type use hydrocarbon polyelectrolyte as a moisture sensing material. Therefore, the sensors usually have insufficient heat resistance, and cannot be used at temperatures of 60°C or more. Another problem is that they deteriorate when in contact with cigarette smoke and oil contained in the air [64,65]. When the fluorinated pitch-deposited coating was breathed upon, the electrical resistance quickly decreased, but electrical resistance quickly recovered when this action was stopped. Then, how to develop a humidity sensor excelling in humidity response sensitivity, heat resistance and durability was attempted [66]. Two kinds of comb-like electrodes with different electrode gaps were made, and a thin film was formed on the surfaces by vacuum deposition of fluorinated pitch. The obtained fluorinated pitch sensors were left at rest in a thermostatic chamber, and electrical resistance was determined under the following conditions. 

Other mechanisms affecting resistance changes in the semiconductor are adsorption of ions other than oxygen at the surface, changes in ambient humidity, or water formed by combining with adsorbed oxygen [1]. These last two related mechanisms are the underlying principles for the development of several metal oxide-based humidity sensors. 

A humidity sensor has to satisfy the following practical requirements (1) high sensitivity over a wide humidity range, (2) quick response, (3) good reproducibility and no hysteresis, (4) robustness and long life, (5) resistance to contaminants, (6) insigniHcant dependence on temperature, and (7) simple structure and low cost. For particular applications, other requirements should be satisHed, such as low power, low weight, or microprocessor compatibility. 

Resistance-humidity characteristics of the Ti02-Sn02 ceramic sensor at 40 °C. 

The resistance of the Ni, Fe2. 04 humidity sensor increased with a rise in water vapor pressure as shown in Figure 20-21. The maximum sensitivity was obtained at x = 0.4. This sensor exhibits a fairly good linearity with humidity and reproducibility but lacks interchangeability and productivity [28]. 

Figure 20-24 shows the resistance of Zr02-MgO as a function of water vapor content (ppmw). The resistance decreases rapidly with an increase in water vapor from 10 to 10 ppmw. Compared with the ionic-type humidity sensor, the response of the semiconductor-type is rather slow because of the slow rate of chemisorption or the subsequent electron transfer process on the oxide surface. The microstructure of the elements as defined by surface area and average particle size, has a less pronounced effect on sensing characteristics than is the case in the ionic-type humidity sensors [31]. 

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

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. 

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