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