Non-Contact Measurement of Electrolyte Solution With a Passive Coil Placed in Inductive Coupling System

A non-contact electrolyte solution measurement system is developed based on inductive coupling coils fabricated on printed circuit boards (PCBs). The basic sensing system is made up of a readout coil and the electrolyte solution under test. A passive coil is composed by a coil printed on the PCB substrate, and a capacitance element to make an $LC$ resonator. When it is placed in the basic sensing system, the measurement performance is enhanced because of the strong coupling around its resonant frequency. Sodium chloride solution with 0%-5% massed fraction concentration in a plastic container is adopted as the target to be detected. Impedance spectrums are experimentally analyzed for the cases of that the passive coil is placed in front and at the back of the target solutions, respectively. The results indicate that, when the target is located at the same distance, a measurement system with a passive coil performs a higher sensitivity to solution concentration than that without one. Furthermore, when the target solution is located beyond the sensing range of the basic sensing system, the solution concentration can also be effectively measured with the aid of a passive coil. Such an enhancement effect of the passive coil can efficiently promote the performances of non-contact measurement systems for low conductivity targets, such as electrolyte solutions or biological tissues. This is especially true when the passive coil is fabricated on a flexible circuit substrate, since the flexible coil can be attached to the non-planar surface of the target to ensure a stronger magnetic coupling.