A two chamber system with fill gas in one chamber and vacuum in the other provides a means of burning in one or more vacuum tubes while avoiding contamination from environmental gases. Vacuum tubes are often burned in after being sealed. Some processes burn-in the tubes before sealing them. The burn in process can take days and provide ample opportunity for environmental gases to contaminate the vacuum tube. The vacuum tube's fill tube passes through the vacuum chamber and into the fill gas chamber. Environmental gases leaking past the fill tube are evacuated by the vacuum. Similarly, fill gas leaking past the fill tube is also evacuated to vacuum. As such, the environmental gases are drawn away before contaminating the vacuum tube.