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