Flexible Circuits for Moisture Measurement in Cylindrical Timber of Wood

This work describes the development of flexible sensor to monitor moisture level in pieces of wood, where the sensor is stuck. The manufactured sensor consists of a rectangular flat two-wire coil whose operation is based on resonance frequency ( f o). The sensor capacitance ( C ) depends on the electric permittivity of the wood, therefore, moisture. The approach is useful for planar or cylindrical objects. However, bending the sensor to fit it around a cylindrical object changes the magnetic links and thus its self-inductance L (and f o as well). Regarding this issue, the paper describes the behavior of L and C in function of wood moisture level and sensor flexion. The flexible sensor is built based on a pattern able copper-coated flexible polyimide substrate, as will be explained forward. An impedance analyzer (4294A) readouts the electrical parameters ( L , C , and f o) of sensor. For calibration, moisture in wood is determined by a precision scale and by DUC instrument.