A gas source inside the system rather than a path from outside: a blind tapped hole under a screw, a double weld with a sealed gap, a space behind a gasket or a porous part. The trapped gas escapes through a tiny path and keeps the pressure up for hours or days, yet helium sprayed on the outside never finds it. Designers prevent virtual leaks with vented screws, single continuous welds on the vacuum side and through-holes instead of blind holes.
Leak checker shows nothing and the rate of rise rolls over slowly, I'm betting on a virtual leak in those new blind holes.
Vented screws, with a hole drilled down the shank, cost almost nothing and remove the most common virtual leak in home-built chambers.
Do not mix it up with
- Outgassing: Outgassing is gas released from surfaces and bulk material, while a virtual leak is free gas trapped in a pocket; both give no helium signal.
Related terms
Rate of rise
Test that isolates a pumped chamber and tracks how fast the pressure climbs, to estimate leaks and outgassing.
Outgassing
Slow release of gas from the surfaces and bulk of materials inside a vacuum system.
Chase a leak
To hunt down a leak of unknown location joint by joint, often over hours, until the source is pinned down.
Gas load
Total gas flow a pumping system must remove, summed from process gas, leaks, outgassing and permeation.