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