What are Humidity Sensors and Their Uses?

Do you know space stations, smartphones, data centers, art galleries, weather stations, and jet engines use humidity sensors? Measuring humidity in the environment is vital for monitoring the health of electronics or metallic equipment. Similarly, humidity sensors are essential for weather prediction and human comfort. The technology of sensing the water content in the air has come a long way. Initially, it was analog with low accuracy, and now, with precision electronic, the accuracy of modern affordable instruments is ±3% RH (Relative Humidity).

Modern humidity sensors detect the presence of water in the air using capacitive and resistive sensing techniques. Some sensors also use thermal conductivity methods to measure absolute humidity. Whether it is HVAC, agricultural, medical, meteorological, food, or electronics processing facilities, monitoring humidity can help protect and preserve processes and equipment.

This article aims to explain humidity sensors, their uses, detection strategies, measurement, and applications of humidity detection. Let's begin by delving into the science of humidity sensors.

What is a Humidity Sensor?

Humidity sensors measure the amount of moisture in the air using various techniques. They are essential in industrial, environmental, and agricultural applications. Depending on their user's intent, humidity sensors can vary in shape, size, and design. For example, the world's most miniature humidity sensor is just 1.3 mm x 0.7 mm x 0.5 mm in size.

▍Types of Humidity Sensors

Humidity sensors can be categorized based on size and design. In this section, we will define the types of humidity sensors based on their working mechanisms.

1. Capacitive Humidity Sensors

The capacitive humidity sensor is like a sandwich cookie. That has a hygroscopic dielectric material in the middle that absorbs the moisture in the surrounding air. Two conductive plates sit on the top and bottom of the hygroscopic dielectric material. The material increases capacitance as it absorbs more moisture. These are the most common types of humidity sensors.

2. Resistive Humidity Sensors

It uses a material that changes its conductivity or resistance when exposed to high humidity. The increase or decrease in the material conductivity represents the humidity in the air. Manufacturers calibrate these sensors against a controlled environment to ensure operational accuracy.

3. Thermal Humidity Sensors

A thermal humidity sensor has two parts: dry and moist air exposure. The sensor heats the wet part moistened by the surrounding air and records the heat dissipation rate. Higher moisture in the surroundings means the faster heat dissipates. It provides absolute humidity compared to relative humidity by capacitive and resistive sensors.

▍How Humidity Sensors Differ from Other Environmental Sensors?

Other environmental sensors, like temperature, pressure, light, wind speed, etc., provide the air's overall characteristics. They operate using different principles like mechanical strain, light scattering, thermoresistive material, or chemical reactions. At the same time, a humidity sensor works with sensitive material to detect the moisture in the air.

How Do Humidity Sensors Work?

Humidity sensors need the ability to absorb the moisture in the air for detectability. It uses various materials like hygroscopic salts, polymers, and metal oxides to achieve this. In this section, we will dive into how they absorb moisture.

♦ The Science Behind Water Absorption in Humidity Sensors

• Hygroscopic Salts

Salt (e.g., lithium chloride or sodium chloride) can absorb moisture in the air. Its chemical structure allows it to attract and retain water. Earlier designs, such as wet-bulb psychrometers, use hygroscopic salts to provide humidity values. A hygroscopic salt-based humidity sensor typically lasts 6 months to a few years. Hygroscopic salt is commonly used in industrial applications for both resistive and capacitive types of humidity sensors.

• Polymers

Polymers are used in longer-lasting humidity sensors. These sensors do not require frequent recalibration and remain stable against most air contaminants. They respond faster and provide accuracy across a wide range of 0-100% RH. They can last between 5 and 10 years.

• Metal Oxides

Metal oxide-based humidity sensors are standard capacitive types. A thin layer of metal oxide (e.g., Al₂O₃, TiO₂, ZnO) sits between two electrodes to form a capacitance. These sensors are stable in high-temperature conditions and provide ideal chemical resistance. They can last as long as 15 years.

♦ Key Components of a Humidity Sensor

A typical capacitive humidity sensor has three major components: the upper electrode, thin-film polymer, lower electron, and glass substrate, which hold the assembly together. If it's a resistive type, it will feature a hygroscopic (moisture-absorbing) material that changes resistance to moisture absorption.

What Do Humidity Sensors Detect?

As mentioned earlier, humidity sensors can detect relative and absolute humidity. However, their type can be different.

/ Relative Humidity vs. Absolute Humidity

It is essential to understand that air cannot hold water beyond a specific limit at a particular temperature. Cold air cannot have as much water as hot air.

• Relative Humidity:Relative humidity provides the air's water content. However, it gives the value in percentages. 100% RH means the air has the maximum amount of water it can hold for a particular temperature. Meanwhile, 50% means the air has a water content that is 50% of the amount the air can maintain for that temperature.
• Absolute Humidity:Absolute humidity measures the total amount of water in the air. Its unit is water vapor per cubic meter of air (g/m³). It does not depend on the temperature, as it is an absolute value.

/ Dew Point and Why It Important

In weather forecasting, the dew point is a crucial parameter for predicting the probability of dew, fog, or frost. The dew point is the temperature at which the air completely saturates with water, causing condensation. For example, if the air temperature is 30°C and the dew point is 21°C, any surface at or below 21°C will start to collect moisture, just like your cold glass of water. It is a vital parameter for agriculture, HVAC, aviation, and weather forecasting.

How Humidity Sensors Measure Moisture in the Air?

We know the basic principle of humidity sensors, which detect moisture in the air. However, to fully understand how these sensors work, we need to see how they convert these physical parameters to measurable entities like current or signal.

★ Electrical Properties and Humidity Measurement

• Resistive Type:The hygroscopic material changes its resistance when it absorbs water. The sensors detect these changes and convert them into an analog voltage (e.g., 0-5V, 0-10V).
• Capacitive Type:The capacitance of the sensor changes when it is exposed to moisture absorption from the air. The capacitance is then converted into a measurable analog output (e.g., 0-5V, 0-10V, or 4-20mA)

★ Sensor Output Signals and Data Interpretation

The data coming directly from the humidity sensors is usually from electrical signals. Users can further process the output signals by converting them into digital signals using a microcontroller. These can be RS485 (MODBUS), I2C, or SDI-12. The output is then detectable by digital computational devices to provide data record and display. The same data acquisition can act as control parameters for HVAC and other humidity control applications.

★ Factors Affecting Measurement Accuracy

Due to their design, some humidity sensors are inherently prone to drift and material degradation. However, environmental contaminants and installation issues can also cause measurement inaccuracies. Ensuring the humidity sensor is operated within its design limits and calibrated according to the manufacturer's guidelines is vital. If it is aging, it may also need replacement.

Applications and Uses of Humidity Sensors

● Agriculture and Greenhouse Management

Humidity sensors, which form part of climate control systems to boost biological growth, benefit greenhouse management. These devices enable the assessment of both solid and atmospheric humidity levels. Farmers use water and ventilation system adjustments to enhance crop production.

Examples of Agricultural Humidity Sensors:

✔ RK330-01 Atmospheric Temperature, Humidity & Pressure Sensor

✔ RK330-02 Wall-Mounted Ambient Temperature & Humidity Sensor

✔ RK300-04 Leaf Wetness Sensor (for humidity-related plant health monitoring)

● HVAC Systems and Building Automation

HVAC systems use humidity sensors to maintain comfort zones for the inhabitants. Whether preserving wildlife indoors or in human spaces, keeping the humidity levels within the comfort zone is vital. The comfort zone for most humans is 22 °C to 27 °C and a relative humidity of 40% to 60%. Based on feedback from these sensors, the HVAC systems increase or decrease the moisture.

Examples of HVAC Humidity Sensors:

✔ RK330-04 HVAC Temperature & Humidity Sensor

✔ RK330-02 Wall-Mounted Ambient Temperature & Humidity Sensor

● Medical and Pharmaceutical Storage

Monitoring the drug storage areas is vital in medical applications. If the drugs' quality changes, they can become ineffective or even harmful to the user. Similarly, controlling the humidity during the processing of these drugs is equally vital to ensure the quality control of the product.

Examples of Medical Humidity Sensors:

✔ RK330-02 Wall-Mounted Ambient Temperature & Humidity Sensor

✔ RK330-01 Atmospheric Temperature, Humidity & Pressure Sensor

● Meteorology and Weather Forecasting

In weather and meteorological applications, humidity sensors can help forecast weather conditions. Remote weather stations can provide in-depth analysis of changing weather parameters, such as fog, precipitation, and dew point, to improve weather prediction and conditions.

Examples of Meteorological Humidity Sensors:

✔ RK330-01 Atmospheric Temperature, Humidity & Pressure Sensor

✔ RK310-01 Ambient Temperature Sensor

● Electronics Manufacturing and Data Centers

In electronics, humidity can enhance the oxidation process and damage components. Excessive moisture can lead to corrosion, short circuits, and malfunctioning of sensitive electronic parts.

Examples of Manufacturing and Data Centers Humidity Sensors:

✔ RK330-02 Wall-Mounted Ambient Temperature & Humidity Sensor

✔ RK330-01 Atmospheric Temperature, Humidity & Pressure Sensor

● Food Processing and Storage

Maintaining product quality and prevention of spoilage is directly interlinked with humidity levels. Some food may require higher humidity levels, while others stay fresh in low humidity conditions. For example, leafy greens and vegetables thrive in high-humidity environments to prevent wilting and dehydration. Conversely, dry goods like grains and cereals need low humidity to avoid mold growth and spoilage.

Examples of Food Processing and Storage Humidity Sensors:

✔ RK330-02 Wall-Mounted Ambient Temperature & Humidity Sensor

✔ RK330-01 Atmospheric Temperature, Humidity & Pressure Sensor

Conclusion

The importance of humidity sensors in industrialization and agriculture is growing. Humidity sensors can be capacitive, resistive, or thermal, but they all monitor the water content in the air. Maintaining humidity ensures quality, sustainability, and productivity for humans, machines, wildlife, and crops.

As technology improves rapidly, humidity sensors have already been miniaturized to a grain of salt size. Rising accuracy, longer lifetimes, and the incorporation of humidity sensors in IoT devices are rapidly modernizing humidity sensors.

If you are looking for high-precision sensors with the latest output capabilities, consider Rika Sensors. We can provide ±3% RH humidity accuracy, ±0.5°C temperature accuracy, and ±1 hPa pressure accuracy. They are ideal for multiple meteorological, agricultural, and industrial applications. Their outputs include (4-20mA, 0-5V, 0-10V, RS485 MODBUS) with robust designs lasting long-term (5-10 years lifespan). Be sure to visit our website for the ultimate collection of humidity sensors for all applications.

Frequently Asked Questions (FAQs)

1. Is 70% Humidity Considered High?

The comfortable range for an indoor atmosphere is 40% to 60% RH. However, this may not be considered high for specific plants in greenhouses or food preservation applications. 70% humidity is considered high for the human environment as it can lead to condensation on windows and change the “feel-like” temperature.

2. How Often Should Users Calibrate a Humidity Sensor?

Different humidity sensors need calibration according to their specific type. Capacitive sensors need annual calibration procedures, while resistive humidity sensors need checks at least once every six to twelve months. The calibration process for thermal conductivity sensors needs to be performed annually. Following the sensor manufacturer's guidelines is essential to finding the correct calibration window.

3. Can Humidity Sensors Work in Extreme Temperatures?

Yes, humidity sensors can work in conditions up to 200°C. Capacitive humidity sensors, which use polymer as a hygroscopic material, offer the highest operating range. They can work in extreme temperatures while maintaining decent accuracy for control and measurement.