the density determination system is operable to determine a first density value and a second density value of {TAIL ~ the fluid} at {HEAD ~ the first time} and the second time, respectively 
the density determination system is operable to determine a first density value and a second density value of { TAIL ~ the fluid } at { HEAD ~ the first time } and the second time , respectively
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH