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