measuring the volume of {HEAD ~ discharged fluid} entering {TAIL ~ the cavity} until the cavity reaches the known volume; and 
entering
measuring the volume of { HEAD ~ discharged fluid } entering { TAIL ~ the cavity } until the cavity reaches the known volume ; and
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH