a vacuum source fluidly connected to {TAIL ~ the scavenging tube} for scavenging {HEAD ~ excess gases}; and 
a vacuum source fluidly connected to { TAIL ~ the scavenging tube } for scavenging { HEAD ~ excess gases } ; and
OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH