An acoustic velocity meter is arranged to supply to an analyzer a signal indicative of an acoustic velocity within the mixture of reference gas and {HEAD ~ gaseous substance} in {TAIL ~ the chamber} 
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An acoustic velocity meter is arranged to supply to an analyzer a signal indicative of an acoustic velocity within the mixture of reference gas and { HEAD ~ gaseous substance } in { TAIL ~ the chamber }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL