The mass flow controllers 46 and 50 control {TAIL ~ the flow rates} of {HEAD ~ the oxygen} 
The mass flow controllers 46 and 50 control { TAIL ~ the flow rates } of { HEAD ~ the oxygen }
OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD