Broad bandwidth air-coupled micromachined ultrasonic transducers for gas sensing

Gas discrimination by simultaneous sound velocity and attenuation measurements using uncoated capacitive micromachined ultrasonic transducers

“…In such a setup, can be defined as: where d is the distance between the emitter and the receiver and and are the amplitudes of the acoustic wave at the emission and the reception ends, respectively. This work is the continuation of previous work showing the possibility to use CMUTs to determine the concentration of a binary gas mixture by measuring either 17 , 18 or 15 separately and in which the best selectivity obtained was slightly over 10. Although this value is high enough for some applications, it can be considered very low for others.…”

“…In the case of the humidity, one can express the variations of with respect to the molar fraction of vapor water in 17 , where 20 , 20 , 20 are the mass density, the isobaric heat capacity and the isochoric heat capacity of water vapor, respectively and 20 , kJ/K/kg 20 , 20 the same properties for . Since RH is the mass of water in the gas with respect to the maximum amount of water that can be diluted in , it is given by 20 : with where 20 is the saturated water vapor pressure at and 20 the molar mass of water.…”

Gas Discrimination by Simultaneous Sound Velocity and Attenuation Measurements using Uncoated Capacitive Micromachined Ultrasonic Transducers

“…where d is the distance between the emitter and the receiver and P e and P r are the amplitudes of the acoustic wave at the emission and the reception ends, respectively. This work is the continuation of previous work showing the possibility to use CMUTs to determine the concentration of a binary gas mixture by measuring either τ 17,18 or α 15 separately and in which the best selectivity obtained was 12. Although this value is high enough for some applications, it can be considered very low for others.…”

“…Since S α depends on the excitation frequency, it was measured for the whole frequency range of 1MHz up to 4.5MHz (e) which is limited by the charge amplifier at the lower end and by the strong attenuation above 4.5MHz. Similar to the derivation of the expression of the temperature cross-sensitivity, the expression of S τ can be obtained 17 :…”

“…It should be noted that, apart from the main peakP 1 (at t = τ) and the reflected echoP 2 (at t = 3τ) there is an additional peak at t = 0. This is due to an instantaneous electrical coupling between the emitter and the receiver resulting from the high voltages applied 17 . However, since the objective is to isolate P 1 , the coupling won't influence the final result.…”