Powder and Bulk Level Switch Using an Acoustic Tube Method

This paper describes a novel acoustic-tube level switch that detects hot powder and finely gained bulk. The tube length L is selected as L = n × (half wavelength) and the input port of the tube is acoustically driven by a piezo sounder. When the output detection port is covered by powder and bulk, the electric impedance of the sounder increases with the increase of the acoustic impedance of the tube. The wavelength increases proportionally to the temperature, which causes a deviation from the optimal length L above. However, theoretical studies showed that covering the detection port mostly influences the acoustic viscosity and robust to the temperature. As a result, the method worked well in a wide temperature range. The method operated consistently in the range ±50 °C from the reference and detected powder and bulk with 13-kg/m
³
bulk density. In addition, the technique reliably detected the level of gravel heated to 450 °C; in short, the tube functioned as a heat radiator. Furthermore, the sound at the detection port could blow out dust accumulated in the tube. A dust filter located at the quarter-wavelength position had little influence on the normal measurement function, even for a dust layer of 26.5-mm thickness.