Direct-Ink-Written Interdigital Capacitive Sensor on Beech Wood for Surface Moisture Monitoring: Electrostatic FEM Design and Proof-of-Concept Characterization

Moisture limits timber durability, and wetting begins at the surface. An interdigital capacitor (IDC) was printed by direct ink writing (DIW) of a silver paste onto beech wood, the substrate and sensing dielectric. The electrodes were designed within the smallest printable line width (210 μm) with an electrostatic finite element method (FEM) model; 90% of the sensitivity lies within 0.2 mm of the surface. One 24-finger sensor (width and spacing 300 μm) was characterized at 1–100 kHz over a 15-day sorption cycle (30–90% relative humidity (RH), 23 °C). Reference capacitors revealed a meter gain error of 22% at 100 kHz; after calibration, an identically printed glass reference agreed with the two-dimensional model within 3% (same session). The calibrated beech capacitance at 100 kHz rose from 7.0 to 13.7 pF, by 0.11 pF per percentage point of RH (short-term, noise-limited 3σ resolution 0.6–3.5% RH). Referred to moisture content (MC) assigned from tabulated equilibrium values, this is 0.47 pF per percentage point, close to the predicted 0.48 pF; the absolute values lie 5–17% below the model. The step response is two-phase. No gravimetric calibration was performed; the RH response is the primary result of this single-sensor study.