How to choose a humidity sensor
The following article is for your reference.
First, the classification and humidity characteristics of humidity sensors
The humidity sensor is divided into two types: resistive type and capacitive type. The basic form of the product is to form a moisture sensitive 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 the dielectric constant of the element are greatly changed, thereby forming a humidity sensitive element.
The humidity sensor products of various manufacturers at home and abroad are of different levels, and the quality and price are quite different. How to choose the performance price is better than the ideal product of Zui, and it is necessary to have a deep 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 gas. In actual use, due to the influence of dust, oil and harmful gases, aging will occur 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 very 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 sensors are also sensitive to temperature. The temperature coefficient is generally in the range of 0.2 to 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 with 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 moisture sensitive components 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, (in the case of a suitable humidity environment), in the absence of a complete calibration device, a simple saturated salt solution test is usually used and the temperature is measured.
Second, several methods for preliminary judgment on the performance of humidity sensors
In the case that the humidity sensor is actually difficult to calibrate, the humidity sensor performance judgment and inspection can be performed by some simple methods.
1. Consistency judgment, the same type, the same manufacturer's humidity sensor product zui better buy more than two at a time, the more the more the problem is explained, put together the power supply to compare the detection output value, under relatively stable conditions, observe the test consistency . If it is further tested, it can be recorded at intervals of 24 hours. Generally, there are three kinds of humidity and temperature conditions in one day, such as high, medium and low, which can comprehensively observe the consistency and stability of the product, including temperature compensation characteristics.
2, use the mouth to breathe or use other humidification means to humidify the sensor, observe its sensitivity, repeatability, moisture and dehumidification performance, and resolution, the product's high range of Zui.
3. Test the product in both open and closed cases. Compare the consistency and observe the thermal effect.
4. Test the product in the high temperature state and low temperature state (according to the standard of the specification), and return to the normal state to compare the test with the pre-experiment record, check the temperature adaptability of the product, and observe the consistency of the product.
The performance of the product is ultimately based on the formal and complete testing methods of the quality inspection department. Using saturated salt solution for calibration, brand-name products can also be used for comparison detection. The product should also be subjected to long-term calibration during long-term use to judge the quality of the humidity sensor more comprehensively.
Third, the analysis of the humidity sensor products on the market
There are many domestic and foreign humidity sensor products on the domestic market. Capacitive humidity sensors are more common. The types of moisture sensitive materials are mainly high molecular polymers, lithium chloride and metal oxides.
The advantages of the capacitive humidity sensor are fast response, small size, good linearity, and stability. Some products in foreign countries also have high temperature performance. However, most of the products that achieve the above performance are foreign famous brands, and the prices are relatively expensive. Some low-priced products of capacitive humidity sensors sold on the market often fail to reach the above-mentioned level, and the linearity, consistency and repeatability are not ideal. Below 30% RH, the deformation of the wetted section above 80% RH is severe. Some products use MCU compensation correction, which makes the "step" jump of humidity, which reduces the accuracy, and has the disadvantages of poor consistency and linearity difference. Regardless of high-grade or low-grade capacitive humidity sensors, long-term stability is not ideal, most long-term use drift is severe, humidity capacitance change is pF level, 1% RH change is less than 0.5pF, and capacitance drift changes. Often caused by tens of RH% error, most capacitive humidity sensors do not have the performance of working at temperatures above 40 °C, often fail and damage.
Capacitive humidity sensors are also lack of corrosion resistance, and often require high environmental cleanliness. Some products also have illumination failure, electrostatic failure, etc. Metal oxides are ceramic humidity sensitive resistors with the same humidity sensitive capacitance. Advantages, but in the dust environment, the ceramic pores will be blocked by the blocking components, often treated by electric dust removal, but the effect is not ideal, and can not be used in flammable and explosive environment, the alumina moisture sensitive material can not be overcome The surface structure "natural aging" is weak, the impedance is unstable, and the metal oxide ceramic humidity varistor also has the weakness of long-term stability.
Lithium chloride humidity-sensitive resistors have the advantage of outstanding Zui's long-term stability. Therefore, through strict process, the instrument and sensor products can achieve high precision, and the stability is strong. The product has good linearity. , precision and consistency, is a reliable guarantee for long-term service life. The long-term stability of lithium chloride humidity sensor is not replaceable with other moisture sensitive materials.
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