The pressure in {HEAD ~ the tube} is then reduced to {TAIL ~ below 500 millitorr} but not less than 150 millitorr 
The pressure in { HEAD ~ the tube } is then reduced to { TAIL ~ below 500 millitorr } but not less than 150 millitorr
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH