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A contactless conductivity sensor based on capacitively coupled principle

The existing capacitively coupled contactless conductivity detection (C4D) sensors are mainly used to detect the conductivity/ion concentration in capillary tube. A new sensor for conductivity measurement in large-scale pipes is developed based on the C4D technique. A quartz crystal is introduced to eliminate the influence of the coupling capacitances. Meanwhile, a quartz crystal oscillator is designed to simplify the sensor structure and provide the excitation signal with highly stable and accurate frequency. Experimental results indicate that this sensor obtains satisfactory accuracy when measuring conductivity in large-scale pipe (5.5 mm i.d. and 7.5 mm o.d.). In the range of 1–200 mS/cm, the maximum relative measurement error is less than 2%. The developed sensor provides useful reference for contactless conductivity measurement in industrial field.