Discrimination of Binary Gas Mixture Using CMUT Based Sound Attenuation Spectrum Gas Sensor

Introduction In order to overcome the long term stability issues caused by functionalized films in gas sensing [1], uncoated sensors have become increasingly attractive for applications where the selectivity is not a major concern such as industrial gas monitoring. Despite their poor selectivity, discrimination can be achieved by measuring different properties of the gas mixture [2]. The sound attenuation of a gas depends on several of its physical properties such as mass density, viscosity and sound velocity among several others [3]. Its value depends on the frequency in a non-linear manner which makes measuring large parts of its spectrum interesting for gas discrimination. In this abstract, an uncoated sensor capable of measuring the attenuation spectrum continuously over a frequency range is presented. Measurements on binary mixtures such as nitrogen (N 2 ) with either hydrogen (H 2 ), carbon dioxide (CO 2 ) or methane (CH 4 ) are presented. Then, a simple method based on the construction of a preliminary mixture signature allowing to distinguish each type of mixture demonstrates the potential of this sensor to be used in smart gas sensors as a perspective to future work. Although attenuation sensors can be found in the literature [3], to the best of our knowledge, it is the first of this kind with easy integration thanks to the use of capacitive micromachined ultrasonic transducers (CMUTs) and allowing discrimination of binary mixtures. Setup The manufacturing process of the CMUT arrays is similar to the one used in reference [4] and their characteristics are reported in Table 1. Schematics of the experimental setup are shown in Figure 1. An electrical signal (1) is sent to an emitter CMUT array (2) which generates a continuous ultrasonic wave at a given frequency f . The wave travels a distance d through the gas and is attenuated by an amount that depends on the gas composition, through the attenuation coefficient α , before reaching the receiver (3) which is connected to a charge amplifier (4). Both the emitter and receiver signals are fed to a network analyzer (5) in order to measure the total transfer function of the setup | H | as a function of frequency. Far from the resonant frequency of the CMUT array ( f r = 9.6MHz), | H | is given by Equation 1, where | H e | is the transfer function of the setup which is independent of the gas. Thus, by measuring first | H | under pure N 2 , | H N2 |, it is possible to know the shift in attenuation Δα accor