As mentioned in last months article, all vacuum furnaces will leak air into their vacuum chambers over time, causing the pressure in the vacuum chamber to go back up. Thus the term “leak-up rate” is used to determine just how fast that vacuum chamber is allowing air back in.
Of course, air represents the presence of oxygen entering the furnace, which is not good for brazing. The air will also contain moisture, which then becomes another source of oxygen. Moisture itself can become an adherent problem on the metal surfaces being brazed, with adsorbed moisture often being very difficult to drive off the metal surface (can be quite a problem in aluminum vacuum brazing).