Humidity Sensors Principle, Mechanism, and Fabrication Technologies: A Comprehensive Review

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.

Photopatternable PEDOT:PSS/PEG hybrid thin film with moisture stability and sensitivity

“…In addition, the influence of the thickness with the PEG matrix to the response time was also studied, and the results are summarized in Supplementary Figure S4. No appreciable difference in response time was observed, which suggests that either the diffusion of water molecules was rapid or the surface absorption process dominated the response time of the humidity sensor (that is, diffusion time is negligible) 45,51 .…”

“…until an equilibrium is established 45 . The alternation of moisture content in the matrix leads to the change in the permittivity of the PEG hydrogel (Supplementary Figure S3b), which can be detected as a capacitive shift in a single-sided capacitor configuration 46 .…”

Highly Response and Sensitivity Chitosan-Polyvinyl alcohol Based Hexanal Sensors

“…This process will form a number of stack layers. This will lead the vapor molecules have more mobile protons resulting the increasing in the conductivity values as well as increase the output voltage during the electrical testing [12]. During recovery process, oxygen molecules will be chemisorbed onto the surface of the chitosan particles when the sensor is exposed to normal air.…”

Low Humidity Characteristics of Polymer-Based Capacitive Humidity Sensors

“…So, the optimum thickness for used metals (gold, chromium, nickel) is ca. 10 nm [5]. Connection of such an ultra-thin metal layer to an electric tap is a difficult technological operation.…”