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                Characteristics and Differences of Humidity Sensitive Resist
                發布時間:2019-02-21 15:42
                Humidity sensor is the simplest humidity sensor. Humidity sensors are mainly resistive and capacitive. Humidity capacitor humidity capacitor is usually made of polymer film capacitor. Polystyrene, polyimide and acetic acid fibers are commonly used as macromolecule materials. When the ambient humidity changes, the dielectric constant of the humidity-sensitive capacitive sensor changes, which makes the capacitance change. The capacitance change is proportional to the relative humidity. The main advantages of humidity-sensitive capacitive sensor are high sensitivity, good product interchangeability, fast response, less hysteresis of humidity, easy manufacture, miniaturization and integration, and its accuracy is generally lower than that of humidity-sensitive resistor.
                Characteristics and Differences of Humidity Sensitive Resistor and Humidity Sensitive Capacitor
                The characteristic of humidity-sensitive resistor is that the substrate is covered with a film made of humidity-sensitive material. When water vapor in the air is adsorbed on the film, the resistivity and resistance of the element change. Humidity can be measured by using this characteristic. Humidity-sensitive capacitors are generally made of polymer thin film capacitors. Polystyrene, polyimide, butyric acid acetate fibers and other common polymer materials are used.
                When the ambient humidity changes, the dielectric constant of the humidity-sensitive capacitor changes, which makes the capacitance change. The capacitance change is proportional to the relative humidity. There are many kinds of humidity-sensitive resistors, such as metal oxide humidity-sensitive resistors, silicon humidity-sensitive resistors, ceramic humidity-sensitive resistors and so on.  The advantages of humidity resistors are high sensitivity, and the main disadvantages are poor linearity and interchangeability of products. In addition to resistive and capacitive humidity sensor elements, there are electrolyte ion humidity sensor, weight humidity sensor (using the change of humidity film weight to change the oscillation frequency), light intensity humidity sensor, surface acoustic wave humidity sensor and so on. The linearity and anti-pollution of humidity sensor are poor. When measuring environmental humidity, humidity sensor should be exposed to the environment under test for a long time, which can easily be polluted and affect its measurement accuracy and long-term stability.
                Characteristics and Differences of Humidity Sensitive Resistor and Humidity Sensitive Capacitor
                Humidity sensitive resistor can only use AC, DC will lead to humidity sensitive failure, because DC electric field will lead to charged particles in polymer materials biased toward the poles, humidity sensitive resistor will fail after a certain time. Therefore, AC must be used to maintain its dynamic balance, which is why the resistance value of humidity sensitive resistor should be measured by bridge instead of ordinary multimeter.
                Water molecule is a polar molecule, which can be decomposed into H2 and O2 in DC electric field, affecting the measurement. Conductive ions exist in humidity sensor. In high humidity, if DC electric current is used, it will drift, resulting in conductivity drift, affecting the service life of the sensor. RC charging and discharging, timing mode can be used. Some chips have RFC function, can also be used, or can directly generate 1KHZ square wave with IC, superimposed on the sampling resistance and humidity sensor, through AD, the partial voltage can also be measured in the positive cycle. Or use analog circuit, through logarithmic operation for R/V conversion, and then AD sampling. Application of humidity resistors: humidity resistors are widely used in household appliances such as washing machines, air conditioners, video recorders, microwave ovens, industry and agriculture for humidity detection and humidity control.
                At present, there are two kinds of conventional production methods of humidity-sensitive capacitors: one is to fabricate the welding pad on the base material, the other is to fabricate the porous electrodes on the polymer humidity-sensitive film to form two capacitors in series, which enlarges the size of the components, on the other hand, reduces the volume value and reduces the sensitivity under the same size. Later modification design is to use two times metal film forming method to deposit a thick metal film between porous and permeable metal film and the pad made at the same time with the bottom electrode, and connect the permeable metal film to the pad, so as to ensure that the sensitivity of the element is improved without increasing the size, but the thickness of the permeable metal film can only be tens of nanometers, and the process can be controlled. Not good, and this layer can not effectively protect sensitive material layer, easy to scratch, follow-up application is more inconvenient, resulting in poor reliability of this capacitive polymer humidity sensor.