Extremely fine metal oxide particles are sintered at 1300°C to create porous metal oxide ceramics. Electrodes are then added to the surface of this porous material, and wiring terminals are then formed into ceramic humidity sensors. Humidity sensors must be exposed to the test environment during use. Therefore, grease, dirt, dust, and harmful substances (gases and solids) can alter their physical and chemical adsorption properties, causing degradation of the sensor's performance. However, the physical and chemical stability of the sintered ceramic body of metal oxide ceramic humidity sensors allows for heat decontamination to restore the sensor's humidity-sensing properties. Furthermore, the surface structure of the sintered body greatly increases the contact area between the sensor and water vapor, facilitating its absorption and removal. By manipulating the microstructure of the device, physical adsorption can dominate, achieving optimal humidity-sensing properties. Therefore, ceramic humidity sensors offer a long service life and stable performance, making them one of the most promising humidity sensors for engineering applications. The operating temperature range for ceramic humidity sensors is 0-160°C.

Among various metal oxide ceramic materials, porous semiconductor ceramics composed of a magnesium chromate-titanium dioxide solid solution offer superior humidity-sensing performance. Their surface resistivity can vary with humidity over a wide range and can withstand repeated thermal cleaning at high temperatures without sacrificing performance. Figure 3 shows the structure of this ceramic humidity sensor.