a sterile containment welding device includes a self - monitoring heater unit upstream from the space between the tube receiving channels . the wafer is positioned in the unit and mounted for movement through the unit prior to moving through the space . the self - monitoring heater unit comprises a heater which heats the wafer and a temperature sensor for monitoring the temperature created by the self - monitoring heater in a rapidly alternating sequence so that the wafer is heated to a predetermined temperature before being moved into the space and contacting the plastic tube or tubes .