An acoustic gas monitor has a measurement chamber into which a reference gas of known composition is received and is provided with {HEAD ~ a wall section} for {TAIL ~ 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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An acoustic gas monitor has a measurement chamber into which a reference gas of known composition is received and is provided with { HEAD ~ a wall section } for { TAIL ~ 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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