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