The selective transmission may be achieved by providing a wall section consisting of a selectively permeable material, chosen to permit the transfer of {HEAD ~ only the gaseous substance} to be monitored between the interior and {TAIL ~ the exterior} of the chamber 
The selective transmission may be achieved by providing a wall section consisting of a selectively permeable material , chosen to permit the transfer of { HEAD ~ only the gaseous substance } to be monitored between the interior and { TAIL ~ the exterior } of 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 HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL REL OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH