Humidity Sensor Measurement Principles

eyc-tech Humidity Sensing Technology Overview

Capacitive Humidity Sensor Technology

In capacitive humidity sensors, the sensing element typically contains a layer of polymer sensing material. When the polymer absorbs moisture from the air, its dielectric constant changes, which causes a change in the capacitance value.

By measuring the variation in capacitance, the sensor can determine the relative humidity (RH) of the surrounding environment. For this reason, capacitive humidity sensors are widely used in industrial measurement, environmental monitoring, and HVAC systems.

Common Humidity Measurement Methods

Common methods used for humidity measurement and calibration include:

• Psychrometric Method (Wet- and Dry-Bulb Method)
• Saturated Salt Method
• Two-Temperature / Two-Pressure Method
• Humidity Mixing Method

Psychrometric Method

During the calibration of hygrometers, the instrument must be placed in an environment with a standard relative humidity value. The measured value of the hygrometer is then compared with the standard value to establish a calibration curve.

Therefore, the accuracy of the device used to generate the standard humidity environment directly affects the calibration accuracy of the humidity sensor.

Saturated Salt Method

The saturated salt method dissolves chemical salts in pure water within a sealed container until the solution reaches a saturated crystalline state. At this point, the air inside the enclosure reaches equilibrium with the saturated salt solution and forms a specific relative humidity value.

Different saturated salts produce different relative humidity levels. Because of its low cost and simple operation, this method is widely used by researchers for humidity sensor calibration.

Two-Temperature / Two-Pressure Method

Two-Temperature Method:
Air is first passed through a saturated water bath at temperature T1, allowing the air to reach 100% humidity. The air is then introduced into another chamber at a higher temperature T2.

As temperature increases, the relative humidity decreases. By adjusting T1 and T2, different relative humidity environments can be generated.

Two-Pressure Method:
Saturated airflow at a constant temperature passes through an expansion valve into another chamber with the same temperature. After expansion, the pressure decreases, and the partial pressure of water vapor drops, resulting in different relative humidity levels.

The National Bureau of Standards (NBS) in the United States uses this method as the basis for its standard humidity generator.

Humidity Mixing Method

Under a constant temperature environment, two airflows with different humidity levels are mixed:

• Completely dry air
• Fully saturated air

By adjusting the flow ratio of these two air streams, different relative humidity levels can be generated.

This method can rapidly produce a wide range of humidity conditions, but precise control of the airflow ratio can be difficult.

FAQ

How do humidity sensors work?

Humidity sensors typically measure humidity by detecting changes in the electrical properties of sensing materials after they absorb moisture. For example, capacitive humidity sensors detect humidity by measuring changes in capacitance caused by variations in the dielectric constant.

What is Relative Humidity (RH)?

Relative humidity is the ratio of the actual amount of water vapor in the air to the maximum amount of water vapor the air can hold at the same temperature, usually expressed as a percentage.

Do humidity sensors require calibration?

To maintain measurement accuracy, humidity sensors require periodic calibration, especially in high-precision environmental monitoring and industrial applications.

Download Technical Document