High-resolution characterisation of the surface moisture sensor

Accurate characterisation of the surface moisture content at sub-millimetre scales is crucial in various industries, particularly in the safety-critical production of electric motors for the automotive industry, in packaging and in electronics manufacturing. Apart from moisture resolution, existing sensors often lack the necessary spatial resolution, making precise control of adhesion quality difficult. To solve this problem, we present a novel experimental setup that generates controlled micrometre-scale moisture gradients in thin polymer samples, enabling high-resolution calibration of surface-sensitive moisture sensors. Since direct measurement of the moisture profile at this scale is difficult, we validated the generated moisture gradients using computational fluid dynamics simulations and confirmed their linearity and stability. We then used this setup to determine the effective detection depth of a previously developed surface moisture sensor based on diffuse reflectance infrared Fourier transform spectroscopy. Our results show that using this experimental setup alone improves the spatial resolution of the sensor by a factor of 14 – from about 0.7 mm to effectively 48 µm. This approach fills a critical gap by providing a practical means of calibrating and characterising moisture sensors that respond to surface moisture gradients, significantly improving quality control in high-precision manufacturing processes. • Moisture measurement below the surface of a sub-millimetre-thin wall of a polymer part. • Development of an experimental setup that allows increased resolution of the surface moisture measurement by a factor of ten. • CFD numerical modelling and the in-depth study of the response of the sensors to moisture profiles was carried out to define and measure the detection depth of the surface moisture sensor.