Abstract. We present an equivalent circuit model for a titanium dioxide-based humidity sensor which enables discrimination of three separate contributions to the sensor impedance. The first contribution, the electronic conductance, consists of a temperature-dependent ohmic resistance. The second contribution arises from the ionic pathway, which forms depending on the relative humidity on the sensor surface. It is modeled by a constant-phase element (CPE) in parallel with an ohmic resistance. The third contribution is the capacitance of the double layer which forms at the blocking electrodes and is modeled by a second CPE in series to the first CPE. This model was fitted to experimental data between 1 mHz and 1 MHz recorded at different sensor temperatures (between room temperature and 320 ∘C) and different humidity levels. The electronic conductance becomes negligible at low sensor temperatures, whereas the double-layer capacitance becomes negligible at high sensor temperatures in the investigated frequency range. Both the contribution from the ionic pathway and from the double-layer capacitance strongly depend on the relative humidity and are, therefore, suitable sensor signals. The findings define the parameters for the development of a dedicated Fourier-based impedance spectroscope with much faster acquisition times, paving a way for impedance-based high-temperature humidity sensor systems.

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To meet the requirements for high quality data capturing of metal oxide semiconductor (MOS) gas sensors, a platform is presented that can handle a large variety of sensors. Besides manufacturer modes, the platform is able to run sensors in temperature cycled operation (TCO). A system concept is presented that provides hardware modules to adapt to different MOS types like analog/digital or thin film/thick film. For analog sensors, an analog frontend with two different power variants handles the different power demands of thick film and thin film sensors. The concept and realization of the analog frontend is shown. For digital sensors, a generalized design with digital signal input sections was developed to receive data via I2C, SPI, or UART. A single microcontroller model constitutes the core of each board variant and it is flashed with a unified firmware which manages the module specific tasks. For continuity and usability, a graphical user interface (GUI) is presented that allows the user to modify measurement parameters and monitor the measurement. GUI and firmware are tuned to one another and exchange data and information to perform user commands. The validation of the accuracy of the analog temperature control is discussed and the successful usage of the system in various applications like indoor air quality (IAQ) is shown.

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