Measurement of Weight Fraction in HDPE Bales Using a Capacitive Sensor and a Weighing Device

Weight fraction is a key indicator to classify HDPE (High Density Polyethylene) bales with or without contamination into different quality categories, which determines the values of the bales for reuse purpose. However, the main existing technique, sampling and testing, is still not able to achieve accurate and efficient measurement of weight fraction in HDPE bales. In this study, a new method combining a capacitive sensor and a weighing device is proposed for the first time to measure the weight fractions of HDPE and a single known contaminant in HDPE bales, independent of packing density. The purpose-built capacitive sensor, consisting of two pairs of transceiver units, senses the mixture permittivity of the bale from different directions, while the weighing device provides the information of the mixture density of the bale. A mathematical model is established to derive the weight fractions of HDPE and the contaminant based on the mixture permittivity and density obtained from the capacitive sensor and the weighing device. To verify the proposed measurement method, experimental tests were conducted in the laboratory with small-scale model bales. The HDPE bales were mixed with the contaminant, PVC (polyvinyl chloride), and produced with the HDPE weight fraction ranging from 18.0% to 100% and PVC weight fraction ranging from 33.6% to 100%. Experimental results demonstrate that the relative errors of the measured weight fractions of HDPE and the contaminant are within ±5% under laboratory conditions.