Humidity sensor classification and moisture characteristics
Humidity sensor classification and moisture characteristics
   The humidity sensor is divided into two types: resistive type and capacitive type. The basic form of the product is to form a wet film on the substrate coated with the moisture sensitive material. After the water vapor in the air is adsorbed to the moisture sensitive material, the impedance and dielectric constant of the element are greatly changed, thereby forming a humidity sensitive element. Welcome to the company's official website http://
The humidity sensor products of various manufacturers at home and abroad are of different levels, and the quality and price are quite different. It is difficult for users to choose the ideal product with the best price/performance ratio. It is necessary to have an in-depth understanding in this aspect. The humidity sensor has the following features:
　　 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 measuring instrument. It is difficult to achieve the accuracy of ±2%~±3% RH for the humidity sensor . The characteristics given are measured at room temperature ( 20 °C ± 10 °C ) and in clean gases. In actual use, due to the influence of dust, oil and harmful gases, the use time will lead to aging and the accuracy will decrease. The accuracy level of the humidity sensor should be judged by its long-term stability. Generally speaking, long-term stability and use Lifetime is the first problem affecting the quality of humidity sensors. There are few products with annual drift control at 1% RH level, generally around ±2% or even higher.
　　 2 , the temperature coefficient of the humidity sensor
In addition to sensitivity to environmental humidity, humidity sensitive components are also very sensitive to temperature. The temperature coefficient is generally in the range of 0.2~0.8% RH/ °C , and some humidity sensors have different temperature coefficients under different relative humidity. . Temperature drift is nonlinear, which requires temperature compensation on the circuit. The humidity sensor using MCU software compensation or 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 compensation effect. The nonlinear temperature drift often does not compensate for the better effect. Only the hardware temperature follow-up compensation will be used to obtain the real compensation effect. The temperature range in which the humidity sensor operates is also an important parameter. Most humidity sensors are difficult to work properly above 40 °C .
　　 3 , the power supply of the humidity sensor
When a DC voltage is applied to a moisture sensitive material such as a metal oxide ceramic, a high molecular polymer or a lithium chloride, it may cause a change in performance or even a failure, so such a humidity sensor cannot use a direct current voltage or an alternating current voltage having a direct current component. Must be AC ​​powered.
　　 4 , interchangeability
At present, humidity sensors generally have poor interchangeability. The same type of sensors cannot be interchanged, which seriously affects the use effect and adds difficulties to maintenance and debugging. Some manufacturers have made various efforts in this respect (but interchangeability). Still poor) has achieved good results.
　　 5 , humidity correction
Correcting humidity is much more difficult than correcting the temperature. Temperature calibration often uses a standard thermometer as the standard, and the humidity calibration standard is difficult to achieve. Dry and wet bulb thermometers and some common pointer hygrometers cannot be used for calibration. The accuracy cannot be guaranteed because of the required environment. The conditions are very strict, in general, (preferably in a suitable environment for humidity). In the absence of a complete calibration device, a simple saturated salt solution is usually used and the temperature is measured.
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