Plethysmograph apparatus comprising a measuring cell and a reference cell constructed in such manner that exterior pressure variations cause {HEAD ~ equal pressure variations} in {TAIL ~ both cells} 
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Plethysmograph apparatus comprising a measuring cell and a reference cell constructed in such manner that exterior pressure variations cause { HEAD ~ equal pressure variations } in { TAIL ~ both cells }
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 HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL