eyc-tech Temperature and Humidity Sensing Principle

The polymer layer in the capacitive sensing element absorbs moisture, causing a change in the dielectric constant, which in turn results in a change in capacitance.

Humidity Measurement Methods

Psychrometric Method (Dry- and Wet-Bulb Method),
Saturated Salt Solution Method,
Dual-Temperature / Dual-Pressure Method,
Dry–Wet Gas Mixing Method

Psychrometric Method (Dry- and Wet-Bulb Method)

When calibrating the performance of a relative humidity meter, the instrument must be placed in an environment with a “standard relative humidity value.” The standard value is then compared with the reading of the humidity meter to establish a calibration curve. Therefore, the accuracy of the equipment used to generate this standard relative humidity directly affects the calibration performance of the humidity meter.

Saturated Salt Solution Method

Chemical salts are dissolved in pure water within a sealed chamber. The amount of salt added must be sufficient to produce crystallization in the solution, allowing the air in the sealed space to reach equilibrium with the saturated salt solution. Different saturated salts generate different relative humidity values. This method is low in cost and simple to operate, and is commonly adopted by researchers for relative humidity meter performance calibration.

Dual-Temperature / Dual-Pressure Method

Dual-Temperature Method:
A gas is passed through a saturated water chamber at an initial temperature (T1) to achieve 100% saturation. The saturated gas is then introduced into another chamber at a higher temperature (T2). As the temperature increases, the relative humidity decreases. By adjusting T1 and T2, different relative humidity values can be obtained. The accuracy of this method mainly depends on the precision with which the two temperatures are maintained.

Dual-Pressure Method:
A saturated gas flow at a constant temperature is passed through an expansion valve into another chamber at the same temperature. According to gas laws, the pressure decreases after expansion. Since the mass of water vapor in the gas remains unchanged, the partial pressure of water vapor decreases, and the gas is no longer in a saturated state. By adjusting the pressures before and after expansion, different relative humidity environments can be generated. The accuracy of this method is influenced by the control of temperature and pressure before and after the expansion process. The U.S. National Bureau of Standards (NBS) adopts this method as its standard relative humidity generator.

Dry–Wet Gas Mixing Method

Under a constant temperature environment, two gas streams with different flow rates—one completely dry and the other fully saturated—are mixed thoroughly to generate a wide range of relative humidity values. The relative humidity can be calculated based on the mass ratio of the two gas streams and the pressures at each stage before and after mixing. The main advantage of this method is the ability to rapidly generate arbitrary humidity values. However, its drawback lies in the difficulty of precisely controlling the ratio of the two gas streams. Commercially available split-flow relative humidity generators based on this method have become increasingly common, but they are relatively expensive.

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