2017 Volume 53 Issue 10 Pages 547-556
This paper describes a new type of level switch which detects the existence of light, hot particulates such as ash in a tank. Here we employed an acoustic tube method. A port of the tube was driven by a piezo-sounder at less than its resonance frequency and the length L of the tube was selected to have the same resonance frequency, i.e., L=n×(half of the wave length). If the other port is covered by particulates, the acoustic impedance at the resonance frequency increases, which yields increases of the electric impedance of the piezo device. The sound velocity increases in proportion to the temperature, which may deteriorate the optimal measurement condition, i.e., the length of tube is not optimal at the altered temperature. However theoretical studies showed that covering a detected port by particulates mostly influenced the acoustic viscosity which is less influenced by the temperature change. As a result of theoretical studies, we showed that the proposed method can work in a certain temperature range. We verified the theoretical results using experiments which showed that the proposed method worked in the temperature range from 5°C to 80°C with a piezo sounder with 760 resonance frequency and an acoustic tube with half the wave length. Moreover, it detected particles with low bulk density such as those with 13kg/m3. We also conducted an experiment to judge the existence of gravel heated to 450°C in at a room temperature of 23°C which was successful.
