A virtual leak is a pocket of gas physically trapped within the weld with only a small, very low conductance, path from which to evacuate.
The trapped volume of gas within a vacuum chamber weld cannot be easily pumped out due to restrictions in the path. A cracked or imperfect weld is a common example.
Common examples of virtual leaks caused by poor design are:
Trapped volumes caused by unvented double o-ring seal designs.
Trapped volumes caused by unvented single or double dovetail o-ring groove designs.
Trapped volume caused by incorrect weld design.
Common examples of virtual leaks caused by workmanship are:
Virtual leaks are easily avoided if good vacuum practice is followed in the design and fabrication of your chamber. Future articles will discuss these practices.
This gas load is called a “virtual leak”.
To obtain a vacuum sealed joint, only one welding run has to be done on the inside(vacuum side). Structural runs have to be done outside with short skip runs that cover 30-45% of the joint.