Feasibility of measuring moisture content of green sand by a low frequency multiprobe detector based on dielectric characteristics

Abstract

Green sand is a mixture of silica sand, bentonite, water and coal powder, and other additives. Moisture content is an important index to characterize the properties of green sand. Based on the dielectric characteristics of green sand and transmission line theory, a method for rapidly measuring the moisture content of green sand by means of a low frequency multiprobe detector was proposed. A system was constructed, where six detectors with different arrangements and probes were designed. The experimental results showed that the voltage difference of transmission line increases with the increasing frequency before 29 MHz while decreases after 35 MHz. A voltage difference platform occurs in the range of 29–35 MHz, which is suitable for measuring the moisture content due to its insensitivity to frequency. The electric field intensity gradually decreases with the increase of the probe depth, and the intensity of central probe is always greater than that of the edge probe. When the distance of the probe away from the sand sample surface is 80 mm, the electric field intensity of the edge probe is found to be very weak. The optimal excitation frequency for measuring the moisture content of green sand is 29–33 MHz. The optimal detector is the one with one center probe and three edge probes, and their lengths are 80 mm and 60 mm, respectively. The distance between the center and edge probes is 25 mm, and the diameter of probes is 5 mm. Taking the voltage difference of transmission line, bentonite content, coal powder content and compactability as parameters of the input layer, and the moisture content as a parameter of the output layer, a three-layer BP artificial neural network model for predicting the moisture content of green sand was constructed according to the experimental results at 33 MHz. The prediction error of the model is not higher than 3.3% when the moisture content of green sand is within the range of 3wt.%–7wt.%.