Humidity sensors are divided into resistive and capacitive types, and the basic form of the product is to coat the substrate with moisture-sensitive materials to form a moisture-sensitive film. After the water vapor in the air is adsorbed on the moisture-sensitive material, the impedance and dielectric constant of the element are greatly changed, so as to make a moisture-sensitive element.
1. Accuracy and long-term stability
The accuracy of the humidity sensor should reach ±2%-±5%RH, it is difficult to use it as a metering tool if it does not reach this level, and it is difficult for the humidity sensor to achieve the accuracy of ±2%-±3%RH, and the characteristics given in the general product information are measured at room temperature (20 °C±10 °C) and clean gas. In practical use, because of the influence of dust, oil and harmful gases, the use of a long time, will occur, the accuracy of the decline of the accuracy of the humidity sensor to judge in combination with its long-term stability, generally speaking, long-term stability and service life is the first problem affecting the quality of the humidity sensor, the annual drift control at the level of 1% RH products are very few, generally around ±2%, or even higher.
2. The temperature coefficient of the humidity sensor
In addition to being sensitive to environmental humidity, moisture-sensitive elements are also very sensitive to temperature, and their temperature coefficients are generally in the range of 0.2-0.8%RH/°C, and some moisture-sensitive elements have different temperature coefficients under different relative humidity. The temperature drift is nonlinear, which requires a temperature-compensated formula to be added to the circuit. The use of single-chip microcomputer software compensation, or the humidity sensor without temperature compensation can not guarantee the accuracy of the full temperature range, the linearization of the temperature drift curve of the humidity sensor directly affects the effect of compensation, and the nonlinear temperature drift often does not compensate for a good effect, and only the use of hardware temperature compliance compensation will obtain the real compensation effect. The temperature range in which the humidity sensor operates, is also an important parameter. It is difficult for most moisture-sensitive elements to operate normally above 40°C.
3. Power supply of humidity sensor
Moisture-sensitive materials such as metal oxide ceramics, polymers, and lithium chloride cannot use DC voltage or DC component communication voltage when DC voltage is applied, which can cause performance change or even failure. It is necessary to be powered by AC electricity.
4. Interchangeability
At present, there is a phenomenon of poor interchangeability throughout the humidity sensor, the same type of sensor can not be interchangeable, which seriously affects the use of the role, increases the difficulty of maintenance and debugging, and some manufacturers have made various efforts in this regard, (but the interchangeability is still very poor) has achieved a good effect.
5. Humidity proofreading
Calibrating humidity is much more difficult than calibrating temperature. Temperature calibration is often used as a standard thermometer as a standard, and the humidity calibration standard is more difficult to achieve, wet and dry bulb thermometer and some common pointer hygrometer can not be used for calibration, accuracy can not be guaranteed, because it requires environmental conditions are very severe, in general, (under the condition that the humidity environment is suitable) in the lack of perfect verification equipment, generally use a simple saturated salt solution verification method, and measure its temperature.
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