Hydrogen partial pressures are recommended when processing some alloys (like copper) and vacuum brazing with some filler metals (ie. silver or copper). A hydrogen partial pressure also improves the cleaning action during a vacuum furnace bake-out cycle. When properly design and constructed, a hydrogen partial pressure system is perfectly safe. The VAC AERO hydrogen partial pressure system is designed to meet NFPA standards that limit the concentration of hydrogen in the furnace to prevent explosive reactions with any residual air. Three levels of safeguards are employed. The system is designed so that a hydrogen partial pressure cannot be introduced unless the furnace has first been evacuated to a pressure less than 1 micron (to remove almost all of the air) and reached a temperature higher than 1400°F (above the temperature range where hydrogen and air will react explosively). In addition, the system will automatically revert from hydrogen partial pressure to an inert gas partial pressure if chamber pressure rises above 15 torr. This limits the maximum possible concentration of hydrogen in air to less than 2%, half of the lower explosive limit.