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