Timber moisture content evaluation using spiral capacitive structures

In this study a transducer based on a spiral planar inter-digital capacitive structure is designed and implemented for the measurement of timber moisture content (MC). The sensor capacitance measured at a frequency of 100 kHz is found to depend strongly on the material dielectric properties, and increases with increasing MCs. The MCs in the range from 0 to 82% for five different varieties of timber specimens are measured and the results presented. The measured capacitances increase with MCs with an exponential fitting, with a correlation coefficient better than 0.98 indicating good fit for all the timber varieties. The sensitivity of the technique varies based on the specimen as well as its MC. The best sensitivity obtained is 0.01 pF/MC% (where MC% is percentage MC). The performance of the technique in terms of relevant sensor parameters has been analysed and compared with existing techniques for the same measurement. Although several moisture measurement methods exist for timber, the present inter-digital capacitive technique has many advantages over them in terms of sensitivity, dynamic range, cost effectiveness, non-invasiveness, non-destructiveness and measurement speed.