In this work I present three types of humidity sensors and their field of application. The most commonly used humidity sensors are based on capacitive or resistive measurement. All of these types of humidity sensors have a comparable design, using an insulated substrate, electrode structures, and a sensing material. The choice of an appropriate sensor fabrication (in case of technical specifications) depends on the operating conditions. The impact of humidity plays an important role in all areas of human life such as biology or automated industrial processes because water vapor is a natural component of air. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay Moisture sensors are used in intelligent systems for monitoring soil moisture in agriculture or for monitoring corrosion and erosion in infrastructure. Additionally, humidity sensors are used for human comfort issues in home applications. Due to the different fields of application of humidity sensors and the different associated requirements, there are different sensor principles. Based on the units of measurement, these sensors are divided into two main groups: relative humidity sensors (abbreviated RH) and absolute humidity sensors (abbreviated AH). ) sensors. These types of sensors are called hygrometric sensors. However, in many humidity measurement applications, relative humidity measurement is preferred because relative humidity measurement is simpler and therefore less expensive, and is widely used in the areas of indoor air quality and comfort issues . To make humidity sensors flexible for a wide range of application, the following requirements are established for these sensors, such as short response time, small hysteresis and good sensitivity in a wide humidity and temperature range. Since the most commonly used method is measuring relative humidity, relative humidity is explained below. In general, humidity is defined as the amount of water vapor in an air atmosphere. Since relative humidity is a temperature-dependent variable, in hygrometry it is customary to measure humidity together with temperature. Relative humidity is given in percentage and determined as follows: where: pw: water vapor pressure, ps: saturation pressure at the same temperature given in Bar. Humidity sensors according to the change in their electrical properties are divided into two groups: resistive- type and capacitive type. The structure of capacitive and resistive sensors are comparable, but the measurement principle is different. Capacitive type sensors are based on the variation of their dielectric properties, while resistive type sensors are based on the variation of their conductivity. Both types of sensors have a pair of electrodes on a substrate coated with a moisture-sensitive layer. The adsorption of water vapor causes a variation in the dielectric constant of the material (of capacitive type) and this leads to a variation of the capacitance between the electrodes, or a variation of the conductivity of the material (of resistive type) whereby the resistance changes. As the substrate, an alumina substrate was selected. The electrodes are located on this substrate, which is why an interdigital structure (or comb structure) with intervals of 0.15 mm was chosen. The moisture-sensitive layer consists of a mixed aqueous solution of styrene sulfonate monomers, cross-linking agents and vinyl polymers that are centrifuged onto the substrate. Since styrene sulfonate is polymerized and cross-linkedBy ultraviolet irradiation, the coated film is irradiated with ultraviolet light in a nitrogen atmosphere. For protection, the moisture-sensitive layer is covered with a moisture-permeable film. This protective film serves to suppress influences such as cigarette smoke, oil and other impurities and to protect the moisture-sensitive film from them. The size of the sensor is 5 mm x 7 mm. Using a thermostatic humidity generator, resistance was measured at various relative humidities. For the measurement the sensor was connected to a load resistor and an AC voltage of less than 3 V was applied. The accuracy of the humidity generated in the thermostatic test chamber is better than 2% relative humidity. The sensor shows high sensitivity to relative humidity and, as expected, logarithmic behavior and has the advantage of being linear in the range from 30% to 100%. Since many sensors were produced on the same substrate, the response characteristics are comparable and show the same behavior. The curve with the solid line shows the response for the sensor with a protective layer and the curve with the dotted line shows the response for the sensor without a protective layer. The response time is measured for a rapid change in relative humidity from 30% to 90% and vice versa. For the sensor with protective layer, the response time for adsorption and desorption is a few seconds. For the sensor without protective layer, the response time is 100 seconds for adsorption and 150 seconds for desorption. This sensor is called "Humicape" and was developed by Vaisala in Finland and is used in many humidity measuring instruments, such as radiosondes. A glass substrate was selected as the substrate. On this substrate, the lower twin electrodes are fixed by indium evaporation. The moisture-sensitive thin film material used is cellulose acetate with a thickness of approximately 1 µm. At the top is the top electrode made by evaporating gold. This top electrode is approximately 10 nm to 20 nm thick and is porous enough to transport water vapor. The upper electrode, which acts as a counter electrode for the lower twin electrodes, results in a series connection of two capacitances. This construction has the advantage that contact difficulties with the thin upper electrode are eliminated. Capacity is approximately proportional to ambient humidity in the range of 0% to 100%. The sensor has good accuracy and a response time of approximately 1 s to reach 90% of the steady-state value. An alumina substrate was selected as the substrate. The electrodes are located on this substrate, which is why an interdigital gold structure (or comb structure) with a thickness of 8 µm to 10 µm was chosen. The moisture sensitive layer was prepared with different mixing ratios of GTMAC (glycidyl trimethyl ammonium chloride), PPGDE (polypropylene glycol diglycidyl) and MTHPA (methyl tetrahydrophthalic anhydride). Figure 7 shows the sensor response characteristics at 25 °C and 1 kHz for a mixed ratio of GTMAC/PPGDE/MTHPA = 100/0/70. For the measurements, an alternating voltage of 1 V was applied between the electrodes. The sensor impedance was measured in the range from 30% to 100%. The absorption and desorption curve shows proportional behavior. To determine hysteresis, two dashed lines in the +- 2% RH range are shown in Figure 2. Due to sensor hysteresis, this results in a value of <2% RH. Figure 8 shows the sensor response time. This sensor has a response time of 55 s for adsorption and approximately the same response time for desorption..