The unit may also have a tubing to allow {HEAD ~ oxygen} or {TAIL ~ another treated gas} to come in 
The unit may also have a tubing to allow { HEAD ~ oxygen } or { TAIL ~ another treated gas } to come in
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH