Acoustic on-line grain moisture meter

A prototype of an acoustic on-line grain moisture meter was designed and developed. It consists of a funnel to produce a continuous grain stream with about 72 cm3 s−1 flow rate falling onto a 30° inclined sensor surface with 150 mm diameter. A microphone, which is placed underneath the surface, receives sound waves generated by kernel impact and converts them to an electronic signal. The sensor was insulated to eliminate the ambient noise effects. Through the experiments the sound pressure level (SPL) variations of the impact sound were measured as output voltage and their relationship with grain moisture content was determined. In the first experiment, effects of grain drop height and types of impact surfaces (glass, wood and metal) on SPL were evaluated. Results showed that a drop height of 10 cm with the glass sensor surface had the best calibration equation (R2 = 0.94) with the highest accuracy and sensitivity for wheat moisture measurement. In the second experiment, effects of wheat varieties (Mahooty, Karaj-3 and C-73-18) with the selected impact surface (glass) and grain drop height (10 cm) on the output voltage related to SPL, were studied. Relationships between the output voltage and kernel moisture content for three wheat varieties were developed as calibration equations. The maximum instrumental error in 8–20% moisture content range was 1.25%.