Apparatus 2, further comprising {HEAD ~ a membrane} inserted in a wall of said reference cell, said adjusting means comprising {TAIL ~ means} for exerting a force on said membrane, and means for adjusting said force 
Apparatus 2 , further comprising { HEAD ~ a membrane } inserted in a wall of said reference cell , said adjusting means comprising { TAIL ~ means } for exerting a force on said membrane , and means for adjusting said force
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH