An improved capacitively coupled contactless conductivity detection sensor for industrial applications

To bring the potential power of the capacitively coupled contactless conductivity detection (C4D) technique to full play and extend the application fields of C4D, our previous work has developed a C4D 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 C4D sensor can overcome the influence of coupling capacitances on conductivity measurement. By a specific design of shield configuration, the C4D sensor can overcome the influence of stray capacitances and the environment interference in industrial fields. Although the measurement performance of the C4D sensor is satisfactory and it is suitable for the conductivity detection in industrial fields, the input–output characteristic of the C4D sensor is non-monotonic, which may limit the practical applications of the C4D sensor. To overcome this drawback, an improved C4D 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 C4D sensor are the same as those of the C4D sensor in our previous work. However, the measurement circuit of the detection path of the improved C4D 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 The one-inductor design has been replaced by a two-inductor design.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 C4D 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 C4D sensor is successful. In comparison with the previous C4D sensor, the measurement accuracy of the improved C4D sensor is comparable. But, the improved C4D 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 C4D sensor is nonlinear and the sensitivity of the improved C4D sensor decreases with the increase of conductivity