An improved capacitively coupled contactless conductivity detection sensor for industrial applications

摘要

To bring the potential power of the capacitively coupled contactless conductivity detection ((CD)-D-4) technique to full play and extend the application fields of (CD)-D-4, our previous work has developed a (CD)-D-4 sensor which is suitable for conductivity detection of electrolyte solution in industrial fields (Sensors and Actuators A 213 (2014) 1-8). With the introduction of series resonance principle, the (CD)-D-4 sensor can overcome the influence of coupling capacitances on conductivity measurement. By a specific design of shield configuration, the (CD)-D-4 sensor can overcome the influence of stray capacitances and the environment interference in industrial fields. Although the measurement performance of the (CD)-D-4 sensor is satisfactory and it is suitable for the conductivity detection in industrial fields, the input-output characteristic of the (CD)-D-4 sensor is non-monotonic, which may limit the practical applications of the (CD)-D-4 sensor. To overcome this drawback, an improved (CD)-D-4 sensor for conductivity detection in industrial fields has been developed. The basic conductivity measurement method (the series resonance principle) and the shield configuration of the improved (CD)-D-4 sensor are the same as those of the (CD)-D-4 sensor in our previous work. However, the measurement circuit of the detection path of the improved (CD)-D-4 sensor has been optimized. The one-inductor design has been replaced by a two-inductor design. With this optimization, the input-output characteristic of the improved (CD)-D-4 sensor is theoretically monotonic. Conductivity measurement experiments were carried out on four pipes with inner diameters of 1.8 mm, 3.3 mm, 5.0 mm and 7.6 mm, respectively. The experimental results indicate that the development of the improved (CD)-D-4 sensor is successful. In comparison with the previous (CD)-D-4 sensor, the measurement accuracy of the improved (CD)-D-4 sensor is comparable. But, the improved (CD)-D-4 sensor has the advantages of the monotonic input-output characteristic and the convenience for practical conductivity detection in industrial fields. Meanwhile, it is found that the input-output characteristic of the improved (CD)-D-4 sensor is nonlinear and the sensitivity of the improved (CD)-D-4 sensor decreases with the increase of conductivity. (C) 2015 Elsevier B.V. All rights reserved.