{HEAD ~ An acoustic gas monitor} has a measurement chamber into which a reference gas of {TAIL ~ known composition} is received and is provided with a wall section for the selective transmission of a gaseous substance to be monitored between the reference gas internal the chamber and a host gas externally of the chamber 
{ HEAD ~ An acoustic gas monitor } has a measurement chamber into which a reference gas of { TAIL ~ known composition } is received and is provided with a wall section for the selective transmission of a gaseous substance to be monitored between the reference gas internal the chamber and a host gas externally of the chamber
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