An acoustic gas monitor 6 wherein said wall section consists of {HEAD ~ material} which is selectively permeable to {TAIL ~ said gaseous substance} 
is selectively permeable to
An acoustic gas monitor 6 wherein said wall section consists of { HEAD ~ material } which is selectively permeable to { TAIL ~ said gaseous substance }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL