A humidity sensor is capable of sensing the water vapor content in the atmosphere and converting it into usable output signals. There are various types of humidity sensors, including those that utilize porous ceramics and polymer films to absorb moisture, causing changes in resistance or capacitance, as well as those that rely on the weight change of a resonator's moisture-absorbing substance to alter its resonant frequency. Several types of humidity sensors have been developed, such as dry-wet bulb hygrometers, hair hygrometers, lithium chloride humidity sensors, electrolytic humidity sensors (P2O5 humidity sensors), polymer film humidity sensors, quartz oscillator humidity sensors, alumina humidity sensors, ceramic humidity sensors, thermistor humidity sensors, microwave humidity sensors, dew point sensors, condensation sensors, as well as humidity sensors integrated with humidity-sensitive devices and signal processing circuits.
Principles of Humidity Sensors
Humidity-sensitive devices are the simplest type of humidity sensor, mainly divided into two categories: resistive and capacitive.
Resistive humidity sensors are characterized by a film made of a moisture-sensitive material covering a substrate. When water vapor in the air is adsorbed onto the moisture-sensitive film, both the resistivity and resistance value of the element change. This property is utilized to measure humidity.
Capacitive humidity sensors are typically made of polymer thin-film capacitors, with commonly used polymer materials including polystyrene, polyimide, and cellulose acetate. When the environmental humidity changes, the dielectric constant of the capacitive humidity sensor changes, resulting in a corresponding change in capacitance. The magnitude of this capacitance change is proportional to the relative humidity.
The accuracy of electronic humidity sensors can reach 2-3% RH, higher than that of dry-wet bulb humidity measurement. However, humidity-sensitive devices have poor linearity and resistance to pollution. When detecting environmental humidity, humidity-sensitive devices must be exposed to the measured environment for a long time, making them susceptible to contamination and affecting their measurement accuracy and long-term stability.
Applications of Humidity Sensors
Climate Monitoring: Weather measurement and forecasting are essential for various sectors such as industry, agriculture, military, and scientific experiments. Therefore, humidity sensors are indispensable measuring devices, with resin diffusion-type humidity sensors already used in meteorological balloon humidity measuring instruments.
Greenhouse Cultivation: Modern agriculture, forestry, and animal husbandry industries have a considerable number of greenhouses. Controlling humidity in greenhouses is equally important as temperature control. Maintaining humidity within the suitable range for crops, trees, livestock, and poultry is one of the conditions for reducing pests and diseases and increasing yields.
Industrial Production: In sectors such as textiles, electronics, precision machinery, and ceramics, air humidity directly affects the quality and yield of products, necessitating effective monitoring and control.
Item Storage: Various items have certain adaptabilities to the environment. Excessive or insufficient humidity can cause items to lose their original performance. For example, in high-humidity areas, electronic products in warehouses are severely damaged, non-metallic components may mold and deteriorate, and metal components may corrode and rust.
Protection of Precision Instruments: Many precision instruments and equipment require high environmental requirements. Humidity must be controlled within a certain range to ensure their normal operation, improve work efficiency, and reliability. For example, the working humidity of telephone program-controlled switches is better at 55% ±10%. High temperatures can affect insulation performance, while low temperatures can generate static electricity, affecting normal operation.
Characteristics of Humidity Sensors
Temperature Coefficient of Humidity Sensors: In addition to being sensitive to environmental humidity, humidity-sensitive devices are also highly sensitive to temperature, with temperature coefficients typically ranging from 0.2% RH/°C to 0.8% RH/°C. Some humidity-sensitive devices may have different temperature coefficients at different relative humidity levels. Temperature drift is nonlinear, requiring temperature compensation in the circuit. Using microcontroller software compensation or humidity sensors without temperature compensation cannot guarantee accuracy across the full temperature range. The linearity of the humidity sensor's temperature drift curve directly affects the compensation effect. Nonlinear temperature drift often results in poor compensation effects, and only hardware temperature-following compensation can achieve real compensation effects. The operating temperature range of humidity sensors is also an important parameter. Most humidity-sensitive devices are difficult to work normally at temperatures above 40°C.
Accuracy and Long-term Stability: The accuracy of humidity sensors should reach ±2%~±5% RH. Failure to meet this level makes it difficult to use them as measuring instruments. Achieving ±2%~±3% RH accuracy for humidity sensors is quite challenging. The characteristics provided in the product data are usually measured in clean gas at room temperature (20°C±10°C). In practical use, due to the influence of dust, oil, and harmful gases, aging occurs over time, resulting in decreased accuracy. The level of accuracy of humidity sensors should be judged in combination with their long-term stability.
Principles of Humidity Sensors
Humidity-sensitive devices are the simplest type of humidity sensor, mainly divided into two categories: resistive and capacitive.
Resistive humidity sensors are characterized by a film made of a moisture-sensitive material covering a substrate. When water vapor in the air is adsorbed onto the moisture-sensitive film, both the resistivity and resistance value of the element change. This property is utilized to measure humidity.
Capacitive humidity sensors are typically made of polymer thin-film capacitors, with commonly used polymer materials including polystyrene, polyimide, and cellulose acetate. When the environmental humidity changes, the dielectric constant of the capacitive humidity sensor changes, resulting in a corresponding change in capacitance. The magnitude of this capacitance change is proportional to the relative humidity.
The accuracy of electronic humidity sensors can reach 2-3% RH, higher than that of dry-wet bulb humidity measurement. However, humidity-sensitive devices have poor linearity and resistance to pollution. When detecting environmental humidity, humidity-sensitive devices must be exposed to the measured environment for a long time, making them susceptible to contamination and affecting their measurement accuracy and long-term stability.
Applications of Humidity Sensors
Climate Monitoring: Weather measurement and forecasting are essential for various sectors such as industry, agriculture, military, and scientific experiments. Therefore, humidity sensors are indispensable measuring devices, with resin diffusion-type humidity sensors already used in meteorological balloon humidity measuring instruments.
Greenhouse Cultivation: Modern agriculture, forestry, and animal husbandry industries have a considerable number of greenhouses. Controlling humidity in greenhouses is equally important as temperature control. Maintaining humidity within the suitable range for crops, trees, livestock, and poultry is one of the conditions for reducing pests and diseases and increasing yields.
Industrial Production: In sectors such as textiles, electronics, precision machinery, and ceramics, air humidity directly affects the quality and yield of products, necessitating effective monitoring and control.
Item Storage: Various items have certain adaptabilities to the environment. Excessive or insufficient humidity can cause items to lose their original performance. For example, in high-humidity areas, electronic products in warehouses are severely damaged, non-metallic components may mold and deteriorate, and metal components may corrode and rust.
Protection of Precision Instruments: Many precision instruments and equipment require high environmental requirements. Humidity must be controlled within a certain range to ensure their normal operation, improve work efficiency, and reliability. For example, the working humidity of telephone program-controlled switches is better at 55% ±10%. High temperatures can affect insulation performance, while low temperatures can generate static electricity, affecting normal operation.
Characteristics of Humidity Sensors
Temperature Coefficient of Humidity Sensors: In addition to being sensitive to environmental humidity, humidity-sensitive devices are also highly sensitive to temperature, with temperature coefficients typically ranging from 0.2% RH/°C to 0.8% RH/°C. Some humidity-sensitive devices may have different temperature coefficients at different relative humidity levels. Temperature drift is nonlinear, requiring temperature compensation in the circuit. Using microcontroller software compensation or humidity sensors without temperature compensation cannot guarantee accuracy across the full temperature range. The linearity of the humidity sensor's temperature drift curve directly affects the compensation effect. Nonlinear temperature drift often results in poor compensation effects, and only hardware temperature-following compensation can achieve real compensation effects. The operating temperature range of humidity sensors is also an important parameter. Most humidity-sensitive devices are difficult to work normally at temperatures above 40°C.
Accuracy and Long-term Stability: The accuracy of humidity sensors should reach ±2%~±5% RH. Failure to meet this level makes it difficult to use them as measuring instruments. Achieving ±2%~±3% RH accuracy for humidity sensors is quite challenging. The characteristics provided in the product data are usually measured in clean gas at room temperature (20°C±10°C). In practical use, due to the influence of dust, oil, and harmful gases, aging occurs over time, resulting in decreased accuracy. The level of accuracy of humidity sensors should be judged in combination with their long-term stability.
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