Humidity Sensor: Classification, Package and Application

Catalog

I Classification

There are mainly two types of humidity sensors: resistive and capacitive, though modern developments have introduced additional technologies including optical and thermal conductivity sensors.

• Resistive Humidity Sensors

The substrate is covered with a film made of a humidity-sensitive material. When water vapor in the air is adsorbed on the humidity-sensitive film, the resistivity and resistance value of the element change. Using this characteristic, we can measure humidity.

There are many types of resistive humidity sensors, including metal oxide resistive humidity sensors, silicon resistive humidity sensors, and ceramic resistive humidity sensors. The main advantages of resistive humidity sensors are high sensitivity, good product interchangeability, fast response speed, small hysteresis of humidity, ease of manufacturing, and suitability for miniaturization and integration. However, their accuracy is generally lower than that of capacitive humidity sensors, and they may experience drift over time.

Figure 1. Schematic View of Resistive Humidity Sensor

• Capacitive Humidity Sensors

Capacitive humidity sensors are generally made of polymer film capacitors, with commonly used polymer materials including polystyrene, polyimide, and cellulose acetate. When environmental humidity changes, the dielectric constant of the capacitive humidity sensor changes, and its capacitance also changes proportionally to the relative humidity.

Capacitive humidity sensors offer high sensitivity, excellent product interchangeability, fast response speed, and small hysteresis. They generally provide better accuracy than resistive humidity sensors (typically ±2-3% RH). Additionally, they are easy to manufacture and suitable for miniaturization and integration. Modern capacitive sensors can operate effectively in a wide temperature range (-40°C to +125°C) and are commonly used in HVAC systems, weather stations, and consumer electronics.

Figure 2. Structure of Capacitive Humidity Sensor

Generally, humidity sensors have limitations in linearity and anti-pollution properties. Since humidity sensors must be exposed to the environment being tested for extended periods, they are susceptible to contamination, which affects measurement accuracy and long-term stability. Regular calibration and maintenance are essential for optimal performance. The following sections provide detailed introductions to various humidity sensor types.

1. Lithium Chloride Humidity Sensor

(1) Resistive Lithium Chloride Hygrometer

The first lithium chloride humidity sensor based on the principle of resistance-humidity characteristics was developed by F.W. Dunmore of the U.S. National Bureau of Standards in the 1930s. This type of element has high accuracy, simple structure, and low cost, making it suitable for measurement and control at normal temperature and humidity conditions.

The measurement range of the lithium chloride element is related to the lithium chloride concentration and other components of the humidity-sensitive layer. The effective humidity sensing range of a single element is generally within 20%RH. For example, a concentration of 0.05% corresponds to approximately (80-100)%RH, while a concentration of 0.2% corresponds to (60-80)%RH.

Therefore, when measuring a wide humidity range, components of different concentrations must be used. A hygrometer combination for full-range measurement typically uses 5 components, with a measurement range of (15-100)%RH. Some modern products claim measurement ranges as low as (2-100)%RH.

(2) Dew Point Type Lithium Chloride Hygrometer

The dew point type lithium chloride hygrometer was first developed by the Foxboro Company in the United States. This type of hygrometer is similar to the resistive lithium chloride hygrometer in construction but operates on a completely different principle, utilizing the characteristic that the water vapor pressure of a saturated lithium chloride solution varies with temperature.

2. Carbon Humidity Sensor

The carbon humidity sensor was first proposed by E.K. Carver and C.W. Breasefield in the United States in 1942. Compared with lithium chloride humidity sensors, carbon humidity sensors offer fast response, good repeatability, no corrosion effects, and narrow hysteresis characteristics.

Figure 3. Carbon Humidity Sensor

3. Alumina Hygrometer

The outstanding advantages of aluminum oxide sensors include their small volume (humidity sensors used for radiosondes are only 90μm thick and weigh 12mg), high sensitivity (with a lower measurement limit of -110°C dew point), and fast response speed (typically 0.3s to 3s). Additionally, measurement signals are directly output as electrical parameters, greatly simplifying data processing procedures. They are also suitable for measuring moisture in liquids.

These characteristics make aluminum oxide sensors particularly suitable for high-altitude atmospheric exploration and industrial applications requiring precise low-humidity measurements. However, despite continuous improvement efforts by researchers worldwide, challenges remain in producing sensors with consistently stable quality and performance across all operating conditions.

Therefore, aluminum oxide hygrometers are typically used under specific conditions and within limited ranges, particularly in applications requiring very low humidity measurements or high-temperature environments.

4. Ceramic Humidity Sensor

In the field of humidity measurement, measuring low humidity and high humidity under extreme temperatures remains challenging. Humidity measurement technology under high temperatures has been particularly underdeveloped historically.

Previously, the ventilated wet and dry bulb hygrometer was almost the only method available for high-temperature applications, which presented various practical challenges. However, with technological advances, there is increasing demand for humidity measurement at high temperatures in industrial processes such as cement production, metal smelting, and food processing.

Since the 1960s, many countries have developed humidity sensors suitable for high-temperature measurements. Considering operational requirements, researchers have focused on inorganic substances that are both water-absorbent and high-temperature resistant.

Ceramic components are not only humidity-sensitive but can also function as temperature-sensitive and gas-sensitive elements. These characteristics make them promising candidates for multi-functional sensors. Modern ceramic humidity sensors can operate at temperatures up to 300°C and are increasingly used in harsh industrial environments.

Figure 4. Optical Micrograph of Ceramic Humidity Sensor

II Selection of Humidity Sensors

1. Measuring Range

As with weight and temperature measurements, the measurement range must first be determined when selecting a humidity sensor. Except for meteorological and scientific research applications, temperature and humidity measurement and control generally do not require full humidity range (0-100%RH) measurement.

In today's information age, sensor technology is closely integrated with computer technology and automatic control systems. For users who do not require measurement and control systems, a general-purpose hygrometer is sufficient. However, for industrial applications, IoT devices, and smart building systems, integrated temperature-humidity sensors with digital output (I²C, SPI, or analog voltage) are increasingly preferred.

2. Measurement Accuracy

Measurement accuracy is a critical indicator for sensor selection. Achieving different precision levels involves varying manufacturing costs and prices. For example, low-cost humidity sensors may cost only a few dollars, while high-precision full-range humidity sensors for calibration purposes can cost hundreds of dollars. Users must select based on their specific requirements and budget.

Manufacturers typically specify humidity sensor accuracy in segments. For example, mid to low humidity ranges (0-80%RH) may have ±2%RH accuracy, while high humidity ranges (80-100%RH) may have ±4%RH accuracy. These accuracy values are specified at a particular temperature (such as 25°C). When using humidity sensors at different temperatures, temperature drift effects must be considered.

Time Drift & Temperature Drift

Almost all sensors experience time drift and temperature drift. Since humidity sensors must contact atmospheric moisture and cannot be sealed, their stability and lifespan are limited. Manufacturers typically specify calibration validity periods of 1-2 years and provide recalibration services upon expiration.

When selecting humidity sensors, verify that the sensor can operate normally at specified temperatures and that temperature drift remains within design specifications. The temperature coefficient α of capacitive humidity sensors is variable, changing with temperature and humidity range. This is because the permittivity of water and polymer materials changes asynchronously with temperature, and the temperature coefficient α primarily depends on the permittivity of water and humidity-sensitive materials.

Capacitive humidity sensors have the smallest temperature coefficient at normal temperature and mid-humidity levels. When operating between 5-25°C, temperature drift at mid-low humidity is negligible. However, in high temperature/high humidity or sub-zero temperature/high humidity environments, temperature drift effects must be considered and appropriate compensation or correction applied.

Figure 5. Effect of Chemical Exposure on Humidity Sensor Readings

Relative humidity is a function of temperature, and temperature significantly affects relative humidity in a given space. A temperature change of 0.1°C can result in a 0.5%RH humidity change. If maintaining constant temperature is difficult, demanding high humidity measurement accuracy becomes impractical. As humidity varies with temperature, the practical significance of humidity measurement accuracy diminishes. Therefore, temperature control must be prioritized, which explains why temperature-humidity combination sensors are more commonly used than standalone humidity sensors.

In most cases, without precise temperature control methods or in unsealed measured spaces, ±5%RH accuracy is sufficient. For local spaces requiring precise constant temperature and humidity control, or where tracking and recording humidity changes is necessary, accuracy better than ±3%RH should be selected. Modern digital humidity sensors often provide ±2%RH accuracy or better with integrated temperature compensation.

III Performance Judgment

When actual calibration of humidity sensors is difficult, sensor performance can be judged and verified using several simple methods:

1. Consistency Judgment

It is best to purchase multiple humidity sensor products of the same type and manufacturer simultaneously. Connect them together and compare output values under relatively stable conditions to test consistency. For further testing, record readings at 24-hour intervals. Generally, there are three humidity and temperature states—high, medium, and low—within a day. This allows more comprehensive observation of product consistency and stability, including temperature compensation characteristics.

2. Response Testing

Use breath or other humidification methods to humidify the sensor and observe its sensitivity, repeatability, humidification and dehumidification performance, resolution, and maximum range of the product.

3. Thermal Effect Testing

Test the product in both open and closed box configurations. Observe whether thermal effects are consistent between configurations.

4. Temperature Cycle Testing

Test the product in high-temperature and low-temperature states (according to specification standards), then test again when returned to normal state. Compare recordings to check consistency and temperature adaptability of the product.

Product performance should ultimately be verified using formal and complete testing methods from quality inspection departments. Saturated salt solutions are used for calibration, and brand-name products can be used for comparative detection. Long-term calibration is necessary to comprehensively judge humidity sensor quality. Modern testing facilities may also use climate chambers with precise humidity control for more accurate verification.

IV Packaging Methods

Due to working principle limitations, humidity sensors cannot be sealed. Instead, packaging must include contact holes or contact windows connecting to the outside environment, allowing the humidity-sensitive part to interact with atmospheric moisture.

Simultaneously, to prevent contamination by dust or airborne impurities, protective measures are necessary. Current methods primarily use metal dust covers or polymer porous membranes for protection. The following sections introduce different packaging types for humidity sensors.

1. Transistor Outline (TO) Package

TO-type packaging technology is currently a relatively common method for packaging humidity sensors. There are two types: metal packaging and plastic packaging.

Figure 6. TO Package

In metal packaging:

• First, the humidity sensor is fixed to the center of the housing base with epoxy resin

• Then Au wire or other metals are connected using a thermal welding machine or ultrasonic welding machine in the sensor's welding area

• Finally, the tube cap is placed on the flange around the base, and the cap and base edge are firmly welded by resistance welding or circular parallel welding. A small hole or window is opened on the top or side of the metal tube cap, allowing the humidity sensor to contact outside air

According to different humidity sensor performance requirements, adding a metal dust cover can extend service life.

2. Single In-Line Package (SIP)

Single in-line package (SIP) is also commonly used for humidity sensors. Generally, humidity sensors have only several output pins, so I/O pins on the substrate are led to one side. Ni, Ag, or Pb-Sn plated leads are fixed on the I/O welding area of the substrate and immersed in molten Pb-Sn for reflow soldering to firmly weld the joints.

Assembled substrates are coated for protection. The simplest method is impregnating with epoxy resin and then curing. Finally, plastic packaging is applied, burrs are removed, and packaging is complete.

Figure 7. SIP

Single in-line package sockets occupy minimal substrate area, and insertion is straightforward. The SIP process is simple and easy to use, suitable for diverse varieties and small batch production, and facilitates replacement and repair of individual leads.

3. Small Outline Package (SOP)

Small outline package (SOP) is another packaging method for humidity sensors. SOP evolved from dual in-line package (DIP) by bending DIP's in-line pins to 90° to create a package suitable for surface mount technology (SMT).

SOP is primarily plastic packaging, with the following packaging process:

• First, the humidity sensor is attached to the lead frame with conductive adhesive or epoxy resin

• The resin is cured to fix the humidity sensor, then the soldering area on the humidity sensor is connected to the lead frame pin bonding area by wire bonding

• It is then placed in a plastic mold for film-plastic packaging. After mold removal, burrs are removed and the frame's outer leads are bent. A small window for air contact is opened on the plastic casing surface, and an air filter film is attached to block impurities such as dust, protecting the humidity sensor

Compared with TO and SIP packages, SOP packaging is much smaller and lighter. Humidity sensors in SOP packages have good long-term stability, minimal drift, low cost, and ease of use, making them suitable for SMT applications. This packaging type is widely used in modern electronics and IoT devices.

Figure 8. Small Outline Package

4. Mixed Package

In many cases, humidity sensors are not packaged separately but integrated with other sensors such as temperature sensors, wind speed sensors, or pressure sensors, along with backend processing circuits to meet specific functional requirements. The packaging process is as follows:

• First, the humidity sensor is bonded to the substrate with conductive adhesive or epoxy resin, and the resin is cured to fix the humidity sensor

• Then, the welding area on the humidity sensor and the substrate bonding area are connected by wire bonding

• Next, the case is covered (covering material can be crystalline polymer). A small window for air contact is opened on the casing surface, allowing the humidity sensor and temperature sensor to fully contact air while isolating other parts. An air filter film is attached to the small window to prevent impurities

This integrated approach is increasingly common in modern sensor modules, particularly in smart home devices, HVAC systems, and environmental monitoring stations.

5. Other Packaging Methods

External support frames are formed by polymer compounds and cast with pre-designed molds. The design fully considers spatial structure to ensure humidity sensors can fully contact air.

• First, the humidity sensor is inserted directly into the outer frame along a slide

• The outer lead is inserted from the other end of the outer frame and connected to the humidity sensor's soldering area (it can also be suspended), then the humidity sensor and outer lead are connected with conductive adhesive

• Finally, air filter membranes are attached to both sides of the outer frame. The filter membrane is a porous membrane made of polytetrafluoroethylene (PTFE), which allows air to penetrate to the sensor while blocking dust and water droplets

This humidity sensor packaging method differs from traditional approaches. It does not use conventional wire bonding to connect outer leads and humidity sensors, but directly connects the humidity sensor's outer leads, avoiding failures caused by internal leads. Simultaneously, its small volume and stable sensor performance enable long-term operation. However, it has higher requirements for external frame manufacturing, and the process is relatively complex.

V Humidity Sensor Applications

Work in any industry is inseparable from air, and air humidity is directly related to work, life, and production, making humidity monitoring and control increasingly important. Humidity sensor applications mainly include the following aspects:

1. Climate Monitoring

Weather monitoring and forecasting are significant to industrial and agricultural production, military operations, people's lives, and scientific experiments. Therefore, humidity sensors are indispensable humidity measuring devices. For example, polymer humidity sensors have been used in meteorological balloon humidity measuring instruments and modern weather stations worldwide.

2. Greenhouse Farming

Greenhouses are widely applied in modern agriculture, forestry, and animal husbandry industries. Humidity control in greenhouses is as important as temperature control. Controlling humidity within suitable ranges for crop, tree, livestock, and poultry growth helps reduce pests and diseases and increase production. Modern smart greenhouse systems use networked humidity sensors for automated climate control.

Figure 9. Humidity Sensor in Greenhouse

3. Industrial Production

In textile, electronics, precision machinery, ceramic, pharmaceutical, and food processing industries, air humidity directly affects product quality and output, requiring effective monitoring and regulation. For example, semiconductor manufacturing requires extremely precise humidity control (typically 40-45%RH) to prevent static discharge and ensure product quality.

4. Goods Storage

Different goods have different environmental adaptability. If humidity is too high or too low, goods will lose their original performance. For example, in high humidity areas, electronic products in warehouses suffer serious damage. Non-metallic parts will mold and deteriorate, while metal parts will corrode and rust. Modern warehouse management systems use humidity sensors for continuous monitoring and automated dehumidification control.

5. Protection for Precision Instruments

Many precision instruments and equipment have high requirements for working environments, and environmental humidity must be controlled within certain ranges to ensure normal operation and improve work efficiency and reliability. For example, program-controlled telephone switchboards work best at 55% ± 10% humidity. If humidity is too high, insulation performance is affected. If too low, static electricity is easily generated, affecting normal operation. Data centers and server rooms typically maintain 40-60%RH for optimal equipment performance and longevity.

6. Smart Home and IoT Applications

In recent years, humidity sensors have become integral components of smart home systems and IoT devices. They are used in smart thermostats, air purifiers, dehumidifiers, and HVAC systems to maintain comfortable indoor environments and optimize energy efficiency. Modern humidity sensors often feature wireless connectivity (Wi-Fi, Zigbee, Bluetooth) enabling remote monitoring and control through smartphone applications. These sensors contribute to improved indoor air quality, energy savings, and enhanced comfort in residential and commercial buildings.

VI Matters Needing Attention

1. Environmental Considerations

Humidity sensors are not sealed. To protect measurement accuracy and stability, they should not be used in atmospheres containing acids, alkalis, organic solvents, or dusty environments. To correctly reflect space humidity, sensors should not be placed in dead corners too close to walls or in poorly ventilated areas. If the room under test is too large, multiple sensors should be used for accurate measurement.

2. Power Supply Requirements

Some humidity sensors have relatively high power supply requirements; otherwise, measurement accuracy will be affected or sensors may interfere with each other and fail to work properly. Therefore, suitable power supplies should be selected according to technical requirements. Modern digital humidity sensors typically operate on 3.3V or 5V DC supplies and have low power consumption, making them suitable for battery-operated IoT devices.

3. Signal Transmission

When sensors need to perform long-distance signal transmission, signal attenuation may occur. When transmission distance exceeds 200m, humidity sensors with frequency output signals or digital communication protocols (such as RS-485, Modbus) are recommended to maintain signal integrity.

4. Calibration and Maintenance

Because humidity sensors all have a certain degree of dispersion, both imported and domestic sensors need calibration. Most need recalibration after replacement, especially humidity sensors with relatively high measurement accuracy. Regular calibration (typically annually or bi-annually) is essential to maintain accuracy. Saturated salt solutions or reference humidity chambers are commonly used for calibration purposes.

5. Installation and Placement

Proper installation is crucial for accurate measurements. Sensors should be installed away from direct sunlight, heat sources, air conditioning vents, and areas with rapid air movement. Allow adequate time for sensors to stabilize after installation (typically 30 minutes to 1 hour) before taking measurements. In outdoor applications, use weatherproof enclosures with appropriate ventilation to protect sensors while allowing air circulation.

Humidity sensors are now widely used in food protection, environmental detection, industrial processes, and smart building systems. Regardless of the application, attention to the above details before and during use is essential for optimal performance and longevity.

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Key Takeaways:

• Humidity sensors are classified into two main types: resistive and capacitive, with capacitive sensors generally offering better accuracy (±2-3% RH)

• Modern humidity sensors feature various packaging options including TO, SIP, and SOP packages, with SOP being preferred for SMT applications

• Temperature significantly affects humidity measurements - a 0.1°C change can result in 0.5% RH variation

• Regular calibration (annually or bi-annually) is essential to maintain measurement accuracy

• Applications range from climate monitoring and industrial production to smart home systems and IoT devices

• Proper installation away from heat sources, direct sunlight, and air vents is crucial for accurate measurements

Latest Developments (2024-2025):

           Recent advances in humidity sensor technology include the development of MEMS-based sensors with improved miniaturization, wireless humidity sensors with extended battery life for IoT applications, and AI-enhanced sensors with self-calibration capabilities. Additionally, new materials such as graphene oxide and metal-organic frameworks (MOFs) are being explored for next-generation humidity sensors with ultra-fast response times (< 1 second) and enhanced stability.        

Frequently Asked Questions:

Q1: How often should humidity sensors be calibrated?

A: Most manufacturers recommend calibration every 1-2 years for standard applications. However, sensors used in critical applications or harsh environments may require more frequent calibration (every 6 months).

Q2: What is the typical lifespan of a humidity sensor?

A: With proper maintenance and calibration, capacitive humidity sensors typically last 5-10 years. Resistive sensors may have shorter lifespans (3-5 years) depending on environmental conditions.

Q3: Can humidity sensors work in extreme temperatures?

A: Standard humidity sensors typically operate between -40°C to +125°C. For extreme temperatures, specialized sensors such as ceramic humidity sensors can operate up to 300°C, though with reduced accuracy.

Q4: What's the difference between absolute humidity and relative humidity?

A: Absolute humidity measures the actual amount of water vapor in air (g/m³), while relative humidity (RH) measures the percentage of water vapor present compared to the maximum amount the air can hold at that temperature. Most humidity sensors measure relative humidity.

Conclusion:

           Humidity sensors have evolved significantly since their introduction, becoming essential components in numerous applications from industrial processes to consumer electronics. Understanding the different types, packaging methods, and proper selection criteria is crucial for achieving accurate and reliable humidity measurements. As technology advances, modern humidity sensors continue to improve in accuracy, miniaturization, and integration capabilities, particularly with the growth of IoT and smart building applications.        

           When selecting a humidity sensor, consider factors such as measurement range, accuracy requirements, operating environment, power consumption, and communication interface. Proper installation, regular calibration, and appropriate maintenance will ensure optimal performance and longevity of your humidity sensing system.        

           Last Updated: November 2025 | Article originally published: 2020        

           For the latest information on humidity sensors and other electronic components, visit www.utmel.com