Sensor System for Contactless and Online Moisture Measurements

This paper presents a sensor system based on a novel coaxial resonator that performs contactless measurements of moisture on surfaces of thin materials such as webs. The system comprises a laterally opened coaxial resonator, driver and signal processing circuitry, and a personal computer for storing the acquired measurement results. The design of the resonator is focused on the formation of maximal stray field to maximize sensitivity. As opposed to other sensor systems of this kind, the scalar transmission coefficient at a given fixed frequency is measured, whereas the resonance frequency and the quality factor are not considered. This makes the execution of measurements faster and the circuit potentially more economical. Measurements have shown that moisture quantities within an estimated range of 0-22 mL/m² can be measured with a measurement uncertainty of approximately 0.1 mL/m². The measurement range can be extended at the expense of equally degraded measurement uncertainty. This makes this sensor system particularly interesting for moisture monitoring applications of webs in areas such as paper, foil, and textile processing industries.