Virtual leaks: formation, detection, and avoidance
A virtual leak is an apparent, unreal leak caused by the slow release of sorbed or occluded gases from the surfaces or bulk of material or from volumes partially trapped within the system. The key component of the definition above is the slow release of gases. This clearly identifies the effect of virtual leaks as gas sources that slow down a pumping process or significantly prolong the time to reach the desired base pressure.
The gas flow rate from the virtual leak is determined by the dimensions of the leak channel between the trapped gas reservoir and the free volume of the vacuum chamber. If the channel is very small, the trapped gases are released very slowly due to the high flow resistance of the channel. The pump-down time for the chamber can be extended significantly. If the vacuum system is repeatedly vented, the gas reservoir can replenish itself over and over again. This effect is repeated every time pumping occurs. The pump itself does not influence the degassing rate. The leakage rate (in mbar·l/s) from the cavity is in equilibrium with the gas extracted by the pump, i.e. the pumping speed (l/s) multiplied by the actual pressure (mbar).
Formation of virtual leaks through enclosed gas volumes
Enclosed gas volumes may, for instance, occur in joining processes during the production of a vacuum chamber. If, for example, a wall of a vacuum container is to be butt welded onto a bottom plate, inexperienced designers often specify a double fillet weld. This runs the risk that gas becomes trapped between the two welds.
Double fillet weld. The vacuum side is indicated by the dashed line.
If the welding seam on the vacuum side is not tight, the trapped gas will spread into the interior of the vacuum chamber.
This will happen slowly due to the high flow resistance of the small pores between the trapped gas volume and inner volume of the chamber. If, at the same time, the outer welding seal is tight, there is no way to localize the leak using helium leak detection. The welds that are accessible from the atmospheric side are therefore not continuous but either interrupted or subsequently spot-drilled.
Read more about this state of the art technology in the attached application report.
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