Timber moisture content evaluation using spiral capacitive structures

In this work a transducer based on spiral planar inter-digital capacitive structure is designed and implemented for timber moisture content measurement. The sensor capacitance measured at frequency 100 KHz is found to depend strongly on the material dielectric properties, which is found to increase with increasing moisture contents (MC). The moisture contents in the range from 0% to 82% for 5 different varieties of timber specimens are measured and the results presented. The measured capacitances increase with moisture contents 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 with the specimen as well as its moisture content. The best sensitivity obtained is 0.01 pF/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.