A third vacuum pump, connected to the processing chamber, is operated when {TAIL ~ the vacuum} in the processing chamber reaches {HEAD ~ a predetermined value} 
A third vacuum pump , connected to the processing chamber , is operated when { TAIL ~ the vacuum } in the processing chamber reaches { HEAD ~ a predetermined value }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD