As the temperature is raised to a level at which the heating elements would be damaged, {HEAD ~ a substantially neutral atomsphere} is maintained in {TAIL ~ the chamber} 
is maintained in
As the temperature is raised to a level at which the heating elements would be damaged , { HEAD ~ a substantially neutral atomsphere } is maintained in { TAIL ~ the chamber }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL