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Study of the Long-Term Drift of Active Book Humidity Sensors at Constant Relative Humidity
Active Book humidity sensors (ABHS), designed for measurements inside the encapsulated electronics of small implanted devices, are capacitance-sensing circuits that rely on changes in dielectric permittivity for humidity measurement. They were designed to possess a relaxation oscillator whose frequency is inversely proportional to capacitance and thus to relative humidity. In order to fully characterise these new sensors and study their reliability, their long-term drift, if any, in output frequency at constant relative humidity is being investigated.
The project has included the design of a chamber from which to obtain and record frequency measurements from ABHS at constant relative humidity for long periods of time. The Saturated Salts Solution Method was used to ensure constant humidity inside four different chambers. Tasks have included the design of a PCB on to which ceramic plates containing ABHS were connected in order to ensure a signal from each was obtained. Studying the logic of an external circuit from which to control recording times of each chamber’s different outputs has also been necessary. Lastly, we will use a computer package to program the conversion between several analogue inputs and digital outputs for the long-term recording of frequency measurements. This will lead to the complete characterisation of the long-term drift of ABHS.