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