With the advancement of the times, humidity sensors are increasingly needed in industrial sectors such as scientific research, agriculture, HVAC, textiles, computer rooms, aerospace, and power generation. The requirements for product quality are becoming increasingly stringent. Controlling ambient temperature and humidity, as well as monitoring and analyzing the moisture content of industrial materials, have become common technical requirements. Humidity sensor products and humidity measurement are an industry that emerged in the 1990s. However, how to effectively use humidity sensors and how to evaluate their performance remain complex technical issues for the average user.

1. Classification and Humidity Sensing Characteristics of Humidity Sensors
Humidity sensors are categorized as resistive and capacitive. Both types essentially consist of a substrate coated with a humidity-sensing material to form a film. When water vapor in the air adsorbs onto the humidity-sensing material, the impedance and dielectric constant of the element undergo significant changes, resulting in a humidity-sensitive element.

Humidity sensors from various manufacturers, both domestically and internationally, vary widely in quality and price. Choosing the ideal product with the best price-performance ratio presents a challenge, requiring in-depth understanding. Humidity sensors have the following characteristics:
1)Accuracy and Long-Term Stability
The accuracy of a humidity sensor should reach ±2% to ±5%RH. If this level is not achieved, it is difficult to use as a measuring instrument. Achieving an accuracy of ±2% to ±3%RH is even more challenging. The characteristics listed in product data are typically measured at room temperature (20°C ±10°C) in clean air. In actual use, due to the effects of dust, oil, and harmful gases, humidity sensors will age over time, causing their accuracy to decrease. The accuracy of a humidity sensor should be considered in conjunction with its long-term stability. Generally speaking, long-term stability and service life are the primary factors affecting humidity sensor quality. Few products have an annual drift within 1%RH; it is generally around ±2% or even higher.

2)Temperature Coefficient of Humidity Sensors
In addition to being sensitive to ambient humidity, humidity sensors are also very sensitive to temperature. Their temperature coefficient is generally in the range of 0.2% to 0.8%RH/°C, and some humidity sensors exhibit different temperature coefficients at different relative humidities. Temperature drift is nonlinear, requiring temperature compensation in the circuit. Humidity sensors using microcontroller software compensation or without temperature compensation cannot guarantee accuracy across the entire temperature range. The linearization of the humidity sensor's temperature drift curve directly affects the effectiveness of compensation. Nonlinear temperature drift often results in ineffective compensation. Only hardware temperature-responsive compensation can achieve true compensation. The operating temperature range of the humidity sensor is also a critical parameter. Most humidity sensors struggle to operate properly above 40°C.

3) Humidity Sensor Power Supply
Applying DC voltage to humidity-sensitive materials such as metal oxide ceramics, polymers, and lithium chloride can cause performance changes or even failure. Therefore, these humidity sensors cannot be powered by DC voltage or AC voltage with a DC component. They must be powered by AC.

4)Interchangeability
Currently, humidity sensors generally suffer from poor interchangeability. Sensors of the same model are not interchangeable, seriously impacting performance and increasing the difficulty of maintenance and commissioning. Some manufacturers have made various efforts in this area and, although interchangeability remains poor, have achieved better results.

5)Humidity Calibration
Humidity calibration is much more difficult than temperature calibration. Temperature calibration can often be done with a standard thermometer, but humidity calibration is more difficult to achieve. Dry-bulb and wet-bulb thermometers and some common pointer hygrometers cannot be used for calibration, and the accuracy cannot be guaranteed because they require very strict environmental conditions. Generally, (preferably under conditions where the humidity environment is suitable) in the absence of complete calibration equipment, a simple saturated salt solution calibration method is usually used to measure its temperature.

2. Several Methods for Preliminary Evaluation of Humidity Sensor Performance
If actual humidity sensor calibration is difficult, several simple methods can be used to evaluate and inspect humidity sensor performance.

1) Determine consistency: It is best to purchase at least two humidity sensors of the same type and manufacturer at a time; the more the better. Power them on and compare their output values. Observe consistency under relatively stable conditions. For further testing, record data at intervals over a 24-hour period. A typical day typically includes high, medium, and low humidity and temperature conditions. This allows for a more comprehensive assessment of the product's consistency and stability, including its temperature compensation characteristics.
2) Humidify the sensor with your mouth or other humidification methods, and observe its sensitivity, repeatability, dehumidification performance, resolution, and maximum range.
3) Test the product with the package open and closed. Compare consistency and observe any thermal effects.
4) Test the product at high and low temperatures (according to the specifications in the manual), then return it to normal conditions and compare the test results with the pre-test records to examine the product's temperature adaptability and observe its consistency.

The product's performance ultimately depends on the quality inspection department's formal and comprehensive testing methods. Calibration can be performed using a saturated salt solution, or a comparison test using a reputable brand. The product should also undergo long-term calibration during long-term use to fully assess the quality of the humidity sensor.

3. Analysis of Humidity Sensor Products on the Market
There are many humidity sensor products available in China and abroad, with capacitive humidity sensors being the most common. The main sensing materials used are polymers, lithium chloride, and metal oxides.

The advantages of capacitive humidity sensors include fast response speed, small size, good linearity, and stability. Some foreign products also offer high-temperature performance. However, products that achieve these performance characteristics are mostly foreign brands and are relatively expensive. Some low-priced capacitive humidity sensors available on the market often fail to meet these standards, exhibiting suboptimal linearity, consistency, and repeatability. The sensing range exhibits significant deformation below 30% RH and above 80% RH. Some products utilize microcontroller compensation, which results in "step" humidity jumps, reducing accuracy and leading to poor consistency and linearity. Both high-end and low-end capacitive humidity sensors exhibit suboptimal long-term stability, with most experiencing significant drift over time. The capacitance of the humidity sensor capacitor fluctuates in the pF range, with a 1% RH change of less than 0.5 pF. This capacitance drift often results in errors of tens of RH%. Most capacitive humidity sensors are not designed to operate at temperatures above 40°C, often leading to failure and damage.

Capacitive humidity sensors also lack corrosion resistance and often require high environmental cleanliness. Some products also suffer from failure due to light exposure and static electricity. Metal oxide ceramic humidity sensors offer the same advantages as humidity-sensing capacitors, but in dusty environments, the ceramic pores become blocked, causing the components to fail. Electrical dust removal is often used, but this is less than ideal and cannot be used in flammable or explosive environments. Alumina humidity sensors cannot overcome the "natural aging" weakness of their surface structure, resulting in unstable impedance. Metal oxide ceramic humidity sensors also suffer from poor long-term stability.

The most prominent advantage of lithium chloride humidity sensors is their extremely high long-term stability. Through rigorous manufacturing processes, instruments and sensors made from them can achieve high accuracy. This high stability ensures excellent linearity, precision, and consistency, providing a reliable guarantee for a long service life. The long-term stability of lithium chloride humidity sensors is unmatched by other humidity sensing materials.