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 The one-inductor design has been replaced by a two-inductor design.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.