An inductive salt solution concentration sensor using a planar coil based on a PQCR-L circuit

•An inductive salt solution concentration sensor utilizing self-inductance effect with frequency signal output was proposed.•Tiny changes in inductance of the sensing coil due to solution concentration variation was successfully converted to obviously frequency changes.•The sensor exhibited good linearity, high stability, fast response, and good reproducibility.•This sensor had the advantages of low cost, noncontact measurement, and easy manufacture.

An inductive salt solution concentration sensor is proposed in this paper. A planar sensing coil with a thin insulation covering layer, which was set at the bottom of the testing container for sensing purposes, was used as the sensor probe. The operating principle underlying this work is based on detection of tiny changes in inductance of the sensing coil due to solution concentration variation. This variation was detected and converted to a frequency signal by a well-designed inductance-to-frequency switching circuit. A sensor prototype was developed to further demonstrate the feasibility of this approach. Tests for cupric sulfate, sodium chloride, and potassium sulfate aqueous solutions with various concentrations were performed. The sensor showed a responding sensitivity of 50 Hz/mM for cupric sulfate solution test. Input-output characteristics revealed that the sensor has a good linearity. Due to its high quality factor, the sensor showed a good stability of the output frequency. In addition, rapid response and good reproducibility were observed in the experiments. The proposed sensor had the advantages of low cost, noncontact measurement, and easy manufacture.