An inductive telemetric measurement system for humidity sensing

In some specific applications, the measuring environment can have unsuitable characteristics for a correct electronic functioning; it is not possible to connect a sensitive element to the conditioning electronics by standard cables or by a radiofrequency link. A possible solution could be a contactless link with a passive sensor inside the measuring environment and a readout outside. This paper discusses the telemetric system consisting of two contactless coupled planar inductors. The two inductors form a coupled transformer in air, with the primary one connected to the measurement electronics and the secondary one working as the sensitive element. A polymer deposited on the secondary one is sensible to the humidity, and changes its dielectric permittivity causing a variation of the inductor parasitic capacitance. A conditioning electronics measures frequency resonances, while the system extracts the corresponding capacitance values, compensating the distance variation as well. The whole system has been tested in the laboratory, and several results have been reported. These show about 200 fF variation over 1.7 pF for a change of the RH between 60% and 90% at a constant temperature of 22 ◦C. Moreover, the measured data are compensated on distance variation changing from 15 mm up to 30 mm. The complete explanation of the whole measurement system is described here. The low cost of the sensors and conditioning electronics implies a high diffusion in many application fields, from food packages to automotive.