Category: Educational Resource Center

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:

diagram showing trapped volume tororous leak path through threads

Trapped volumes caused by un-vented or poorly vented blind tapped holes within the vacuum volume.

diagram of vacuum with trapped volume no vent

Trapped volumes caused by unvented double o-ring seal designs.

diagram of vacuum with dove tail on wrong side or unvented

Trapped volumes caused by unvented single or double dovetail o-ring groove designs.

daigram of vacuum with trapped volume and crack in weld leading to virtual leak

Trapped volume caused by incorrect weld design.

Common examples of virtual leaks caused by workmanship are:

diagram of vacuum with trapped volume behind poorly executed full penetration weld and crack in weld leading to virtual leak

Trapped volumes caused by poor weld fit up of full penetration welds and lack of penetration.

diagram of vacuum showing trapped volume due to poor braze workmanship, crack in braze leads to virtual leak

Trapped volumes caused by cracks in interior welds of braze joints.

diagram of vacuum showing trapped volume

Trapped volumes caused by uneven torquing of conflat flanges.

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.


Post Tags

Vacuum Vacuum Chamber virtual leaks Welding