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Review
. 2014 Apr 30;14(5):7881-939.
doi: 10.3390/s140507881.

Humidity sensors principle, mechanism, and fabrication technologies: a comprehensive review

Hamid Farahani  1 , Rahman Wagiran  2 , Mohd Nizar Hamidon  3
Affiliations
Review

Humidity sensors principle, mechanism, and fabrication technologies: a comprehensive review

Hamid Farahani et al. Sensors (Basel). .

Abstract

Humidity measurement is one of the most significant issues in various areas of applications such as instrumentation, automated systems, agriculture, climatology and GIS. Numerous sorts of humidity sensors fabricated and developed for industrial and laboratory applications are reviewed and presented in this article. The survey frequently concentrates on the RH sensors based upon their organic and inorganic functional materials, e.g., porous ceramics (semiconductors), polymers, ceramic/polymer and electrolytes, as well as conduction mechanism and fabrication technologies. A significant aim of this review is to provide a distinct categorization pursuant to state of the art humidity sensor types, principles of work, sensing substances, transduction mechanisms, and production technologies. Furthermore, performance characteristics of the different humidity sensors such as electrical and statistical data will be detailed and gives an added value to the report. By comparison of overall prospects of the sensors it was revealed that there are still drawbacks as to efficiency of sensing elements and conduction values. The flexibility offered by thick film and thin film processes either in the preparation of materials or in the choice of shape and size of the sensor structure provides advantages over other technologies. These ceramic sensors show faster response than other types.

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Figures

Figure 1.
Figure 1.
Proton conduction of the H-bonded networks between water molecules [74]. The express substitution of hydrogen (H+) between hydronium cation (H3O+) and water molecule (H2O) without diffusive action of an individual hydrogen or oxygen atom.
Figure 2.
Figure 2.
Proton transfer mechanism of hydroxide ions [75].
Figure 3.
Figure 3.
Adsorption sites on the silica surfaces, and formation of hydroxyl pairs to hold water molecules [87].
Figure 4.
Figure 4.
Illustration of water vapour chemisorption and hydroxyl layers on the surface of SnO2.
Figure 5.
Figure 5.
Multilayer structure of adsorbed water vapour molecules on the surface of iron oxide [83].
Figure 6.
Figure 6.
Layout of nanocrystalline ceramic with the different superficial components.
Figure 7.
Figure 7.
Sketch of a planar thick/thin film-based humidity sensor based on the interdigitated structure with the porous sensing element.
Figure 8.
Figure 8.
Schematic view of the ‘Hument HPR’ type humidity sensor [47].
Figure 9.
Figure 9.
Construction sketch of MgCr2O4-TiO2 ceramic humidity sensor [86].
Figure 10.
Figure 10.
Configuration of ‘Humicape’ humidity sensor.
See this image and copyright information in PMC

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