Attenuation in a gas results from a combination of classical attenuation, attenuation from diffusion, and attenuation due to molecular relaxation. In previous papers [J. Acoust. Soc. Am. 109, 1955 (2001); 110, 2974 (2001)] a model is described that predicts the attenuation from vibrational relaxation in gas mixtures. In order to validate this model, the attenuation was measured using a pulse technique with four transducer pairs, each with a different resonant frequency. The attenuation calculated using the model was compared to the measured values for a variety of gases including: air, oxygen, methane, hydrogen, and mixtures of oxygen/nitrogen, methane/nitrogen, carbon dioxide/nitrogen, and hydrogen/nitrogen. After the measured data is corrected for diffraction, the model matches the trends in the measured attenuation spectrum for this extensive set of gas mixtures.

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A chemical gas sensor based on a silicon carbide field effect transistor with a catalytic gate metal has been under development for a number of years. The choice of silicon carbide as the semiconductor material allows the sensor to operate at high temperatures, for more than 6 months in flue gases at 300°C and for at least three days at 700°C. The chemical inertness of silicon carbide and a buried gate design makes it a suitable sensor technology for applications in corrosive environments such as exhaust gases and flue gases from boilers. The selectivity of the sensor devices is established through the choice of type and structure of the gate metal as well as the operation temperature. In this way NH 3 sensors with low cross sensitivity to NO x have been demonstrated as potential sensors for control of selective catalytic reduction (SCR) of NO x by urea injection into diesel exhausts. Here we show that sensors with a porous platinum or iridium gate show different temperature ranges for NH 3 detection. The hardness of the silicon carbide makes it for example more resistant to water splash at cold start of a petrol engine than existing technologies, and a sensor which can control the air to fuel ratio, before the exhaust gases are heated, has been demonstrated. Silicon carbide sensors are also tested in flue gases from boilers. Efficient regulation of the combustion in a boiler will decrease fuel consumption and reduce emissions.

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Acoustic attenuation in a gas mixture results from the combined effects of molecular relaxation and the classical mechanisms of viscosity and heat conduction. Acoustic attenuation was measured in methane–nitrogen mixtures for a wide range of concentrations in a large vessel using a pulsed technique. The signals from four pairs of transducers, each operating at a different frequency, were measured as a function of the distance between the transducers. The pressure in the vessel was varied to cover a wide range of f/p. A correction for diffraction for each transducer pair was applied to the data resulting in overlap between the data from different transducers at the same f/p. The pure methane measurements are in good agreement with published results. Measurements for mixtures of methane and nitrogen agree well with calculations that include both relaxational and classical attenuation [J. Acoust. Soc. Am. 109, 1955 (2001)]. Similar measurements were completed for mixtures of carbon dioxide and nitrogen. Calculations to predict the attenuation for these mixtures are ongoing. [This work was funded in part by the State of Oklahoma.]