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