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