What are the big uses of a small humidity sensor?

Produced by: Science Popularization China

Author: Xu Yong (University of Chinese Academy of Sciences)

Producer: China Science Expo

Humidity is a physical quantity used to characterize the water content of the air. With the continuous development and progress of society, the detection and control of humidity has become increasingly important. When it comes to humidity sensors, are you unfamiliar with them? In fact, humidity sensors are widely used in our lives. Although humidity sensors are small, they have great uses! Next, let's take a closer look at what humidity sensors are, the classification, working principle and application of humidity sensors!

Humidity sensor classification and working principle

According to the different signal conversion methods, humidity sensors can be divided into acoustic humidity sensors, electrical humidity sensors, optical humidity sensors, etc. Acoustic humidity sensors are made according to the principle that acoustic signals change with changes in humidity. Electrical humidity sensors include resistive humidity sensors and capacitive humidity sensors. They are made according to the principle that capacitance or resistance changes with changes in humidity. Optical humidity sensors are made based on the principle that changes in humidity cause changes in the properties of the medium layer, thereby changing the properties of light propagation. The following takes an electrical sensor as an example to explain its working principle in detail.

The working principle of humidity sensor is to use some materials that are very sensitive to humidity changes (hygroscopic materials), such as ceramics or polymers. When they absorb or release moisture, the conductivity of the hygroscopic materials will change. This change is read by the processor and converted into analog signals or digital signals according to certain rules, and finally the corresponding humidity value is displayed.

At present, most of the hygrometers we use in daily life use electrical sensors, and many of them have integrated temperature sensors. The picture shows the integration of temperature and humidity sensors. Image source: wikipedia

Humidity Sensor Applications

Humidity sensors are widely used in many fields such as aerospace, industrial manufacturing, meteorology, daily life and healthcare.

Humidity sensor applications in aerospace.

When the air humidity is high, the aerodynamic performance of the aircraft wing surface will be greatly reduced, which may cause condensation and blurred vision of the pilot. Therefore, in order to ensure the safety and stability of the aircraft, we often use humidity sensors to detect and control the internal and external humidity of the aircraft. Inside the aircraft, humidity sensors are often installed in the cabin and in areas used by the crew. With the relevant data on humidity, pilots can reasonably make flight plans and deal with unexpected weather conditions.

Excessive humidity often causes ice to form outside the cabin, affecting aircraft performance and pilot vision. Image source: Wikipedia

Thermohygrometers on the outside of aircraft are generally integrated together. The left picture shows the structure of the thermohygrometer, and the right picture shows its installation location. Image source: Berkes F, Neis P, Schultz MG, et al. Two decades of in-situ temperature measurements in the upper troposphere and lowermost stratosphere from IAGOS long-term routine observation[J]. Atmospheric chemistry and physics/Discussions, 2017, 494(FZJ-2017-06050).

Humidity sensors play an important role in industrial manufacturing.

In industrial production, air humidity not only affects the service life of machinery and equipment, but also has a great impact on the quality of products. If the air humidity is high, it will accelerate the aging of equipment. Therefore, proper air humidity protection of production equipment is very important. Electronic products may even short-circuit if they are in a high humidity environment for a long time. Therefore, in industrial manufacturing, the application of humidity sensors is particularly important for humidity detection and control.

(Photo source: Veer Gallery)

In daily life, people also use humidity sensors.

Excessive air humidity may cause problems such as rapid mold and decay of food. When the air humidity is high, some molds that are harmful to people's health will multiply rapidly. Eating food containing harmful molds will affect people's health and even endanger their lives. During food production and transportation, people will use humidity sensors to detect air humidity and take corresponding measures to control humidity.

(Photo source: Veer Gallery)

Humidity sensors are also widely used in meteorology.

Humidity refers to the amount of water vapor in the air. When the temperature and volume are constant, the more water vapor in the air, the more humid the air is and the higher the humidity is; conversely, the less water vapor in the air, the drier the air is and the lower the humidity is.

Weather forecasting plays a key role in agricultural production, scientific experiments, military and daily life. Therefore, weather stations use humidity sensors to detect relative humidity in the air and analyze the data to assess meteorological conditions and air quality.

(Photo source: Veer Gallery)

Humidity sensors also play an important role in medical applications.

Recently, Wang Xiufeng's research group at Xiangtan University invented a hidden sweat humidity sensor that is only the size of a camera memory card, but has significant sensitivity, good air permeability and fast response. It can continuously detect the body's sweat loss, continuously monitor the body's body heat status and skin barrier function, and evaluate the user's psychological activities and environmental comfort.

Top and side views of the invisible sweat sensor. Image source: Liu Y, Li X, Yang H, et al. Skin-Interfaced Superhydrophobic Insensible Sweat Sensors for Evaluating Body Thermoregulation and Skin Barrier Functions[J]. ACS nano, 2023.

Hidden sweat is also a type of sweating. One of the main functions of sweating is to dissipate heat, so we often sweat profusely after strenuous exercise. But in fact, in addition to these visible sweat, the human body can also discharge "hidden sweat" in the form of steam.

This hidden sweat sensor is small and light, and uses a super-hydrophobic fabric. The fabric allows hidden sweat vapor to penetrate, while being unaffected by external water droplets and internal sweat. The sensor provides a feasible way to evaluate nervous system activity, diseases, and psychological activities, and also lays the foundation for the next generation of sweat detection devices.

The sensor is small and light and can be easily installed in various parts of the human body. Image source: Liu Y, Li X, Yang H, et al. Skin-Interfaced Superhydrophobic Insensible Sweat Sensors for Evaluating Body Thermoregulation and Skin Barrier Functions[J]. ACS nano, 2023.