A coil-coupled sensor for electrolyte solution conductivity measurement

A coil-coupled sensing system for measuring electrolyte solution concentration is proposed by means of testing resistance between two immersed electrodes. The sensing electrodes, together with resistor and capacitance elements, are connected with a coil to construct a passive sensor. The coil is coupled with the readout coil and the equivalent impedance is analyzed. The results show that, for cases of different coil-coupling positions, lumped parameter values of inductance and resistance at the same angular frequency can be connected by a straight line in complex impedance plane. The slope constant of the straight line, which encompasses a linear relationship with resistance in the passive sensor, is not sensitive to the relative position between the two coupled coils. When the slope constant is selected as measured parameter, the electrolyte solution concentration can be readout without critically aligning the coupled coils positions.