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