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