Meaning
Electronic instruments that measure relative humidity by detecting capacitance changes in a polymer or metal oxide layer are widely used in environmental monitoring and weather balloon radiosondes. The operation of a thin film hygrometer depends on the absorption of water vapor into a sub-micron dielectric layer to change the sensor capacitance. This capacitive response is directly proportional to the relative humidity of the surrounding air.
The device is favored for its small size and low power consumption in battery-powered telemetry nodes.
Dielectric Absorption
Water molecules from the ambient air diffuse into the porous polymer film until equilibrium is achieved. In a thin film hygrometer, the high dielectric constant of water compared to the polymer causes a significant shift in capacitance. This capacitance is measured by an onboard oscillator circuit.
The frequency shift is then converted to a humidity percentage.
Response Timing
The thickness of the active sensing layer determines how rapidly the sensor responds to changes in humidity. Implementing a thin film hygrometer with a layer under one micron in thickness enables rapid response times of less than a second, which is critical for high-altitude weather profiling. This speed allows the sensor to capture sharp humidity boundaries as the sonde rises through the atmosphere.
Sensor Degradation
Exposure to chemical pollutants or condensation can alter the properties of the polymer layer over time. This degradation causes a calibration shift in the thin film hygrometer.
