Here are some references for you on how to select humidity sensors:

Classification and characteristics of humidity sensors: Humidity sensors are divided into resistance-type and capacitance-type, and the basic form of the product is to coat a sensing material on the substrate to form a sensing membrane. After water vapor in the air is adsorbed on the sensing material, the impedance and dielectric constant of the element change significantly, thus forming a humidity-sensitive element.

Accuracy and long-term stability: The accuracy of humidity sensors should reach ±2% to ±5% RH. It is difficult to achieve this level, and usually, the drift is within ±2%. Even higher.

Temperature coefficient of humidity sensors: In addition to being sensitive to environmental humidity, humidity sensors are also very sensitive to temperature. The temperature coefficient is generally within 0.2 to 0.8% RH/℃, and some may vary depending on the relative humidity. The linear temperature drift of humidity sensors directly affects the compensation effect, and non-linear temperature drift often fails to achieve good compensation results. Only with hardware temperature tracking compensation can true compensation effects be achieved. The operating temperature range of most humidity sensors is difficult to exceed 40 ℃.

Power supply of humidity sensors: Most moisture-sensitive materials such as metal oxide ceramics, polymers, and lithium chloride undergo performance changes or even failure when applying a DC voltage. Therefore, these humidity sensors must be powered by AC power.

Interchangeability: Currently, there is a significant problem with the interchangeability of humidity sensors. Sensors of the same model cannot be interchanged, which seriously affects the use effect and adds difficulties to maintenance and commissioning. Some manufacturers have made various efforts in this regard and have achieved good results.

Humidity calibration: Calibration of humidity is more difficult than calibration of temperature. Standard thermometers are usually used for temperature calibration, but for humidity calibration, saturated salt solution calibration methods are usually used, and the temperature should also be measured.

Several methods for initially judging the performance of humidity sensors: In the absence of difficult calibration of humidity sensors, some simple and convenient methods can be used to judge the performance of humidity sensors.

Consistency determination: Purchase more than two humidity sensors of the same type and manufacturer. The more, the better. Place them together and compare the output values. Under relatively stable conditions, observe the consistency of the test. Further testing can be carried out by recording at intervals within 24 hours, and observe in different humidity and temperature conditions, such as high, medium, and low humidity, to fully observe the consistency and stability of the product, including temperature compensation characteristics.

Humidity sensing by blowing with the mouth or using other humidification methods: Observe its sensitivity, reproducibility, moisture absorption and desorption performance, as well as resolution and the maximum range of the product.

Testing in the open and closed boxes: Compare and test whether they are consistent, and observe the thermal effect.

Testing at high and low temperatures (according to the standard in the manual): Test and compare with the records before and after returning to normal, to examine the temperature adaptability of the product and observe the consistency of the product.

The performance of the product ultimately depends on the complete and proper detection methods of the quality inspection department. The saturation salt solution is used for calibration, or the product can be compared and tested. Long-term calibration during the long-term use of the product is also necessary to more comprehensively judge the quality of the humidity sensor.

Analysis of several humidity sensor products on the market: Many domestic and foreign humidity sensor products have emerged on the market, with capacitance-type moisture-sensitive elements being more common. The types of sensing materials mainly include polymers, lithium chloride, and metal oxides.

The advantages of capacitance-type moisture-sensitive elements are fast response speed, small size, and good linearity. They are relatively stable. Some foreign products also have high-temperature operating performance. However, high-performance products of this type are mostly from abroad and are relatively expensive. Some low-cost products on the market often fail to meet the above standards, with poor linearity, consistency, and reproducibility. The variation in the lower and upper humidity ranges (below 30% RH and above 80% RH) is significant. Some products use single-chip microcomputers for compensation and correction, which reduces the accuracy and introduces the shortcomings of large deviations and poor linearity. Regardless of high or low-end capacitance-type moisture-sensitive elements, long-term stability is not ideal. After long-term use, drift is often severe, and the variation in moisture-sensitive capacitance values is at the pF level. A 1% RH change is less than 0.5 pF, and the drift of capacitance values often causes errors of tens of RH%. Most capacitance-type moisture-sensitive elements do not have the performance to work at temperatures above 40 ℃, and they often fail or are damaged.

Capacitive moisture-sensitive elements also have some shortcomings in terms of corrosion resistance. They often require a high level of cleanliness in the environment. Some products are also prone to failure such as light failure and static failure. Metal oxide ceramic humidity sensors have the same advantages as capacitive humidity sensors, but dust plugging of the ceramic pores can cause component failure. Often, the method of powering on to remove dust is used, but the effect is not ideal, and it cannot be used in flammable and explosive environments. Alumina sensing materials cannot overcome the weakness of “natural aging” of the surface structure, and the impedance is unstable. Metal oxide ceramic humidity sensors also have the disadvantage of poor long-term stability.

Lithium chloride humidity sensors have the most prominent advantage of excellent long-term stability. Through strict process production, the manufactured instruments and sensors can achieve high accuracy, good stability, and linearity, ensuring reliable long-term service life. Lithium chloride humidity sensors cannot be replaced by other sensing materials in terms of long-term stability.