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Hydrogen is a hazardous gas that becomes explosive above a concentration of 4% in air. As a result, many applications where hydrogen is either used or produced require hydrogen sensors to ensurethat this limit is never reached. These applications include radioactive waste monitoring, clean energy production or more generally industrial gas monitoring. Most existing sensing devices currently on the market are based on the use of a specifically engineered chemical component or film, which in most cases is not stable over time, lasting usually a few months. This has led to the development of methods that rely on physical sensing mechanisms rather than on chemical ones for the detection of hydrogen: these methods have the advantage of being stable over much longer periods of time. In the context of radioactive

waste monitoring, previous work has shown that uncoated (no chemical film) microcantilevers are capable of detecting hydrogen gas up to 0.01% by measuring their change in resonant frequency. This work extends this study by adding the ability to distinguish hydrogen from potential interfering gases. In the case of microcantilevers, gas discrimination is achieved by measuring not only the density of the gas but

also its viscosity with the measurement of both the resonant frequency and the quality factor. Capacitive micromachined ultrasonic transducers (CMUTs) have also been used as an alternative to microcantilevers.

With the CMUTs, both sound velocity and acoustic attenuation have been measured by a time of flight setup instead of device measurements at resonance. In the case of attenuation measurement, a method for obtaining good selectivity of hydrogen measurement against interfering gases such as carbon dioxide and methane has been developed.