The main drawback to the UV technique is that it only deals with surface or desorbed gas, and has little effect upon absorbed contaminants or virtual leak
The ball valve has never been considered a good choice because of inherent virtual leak
possibilities, stem leaks, and gas entrapment.
Improvements in pumpdown time following replacing the backfilling gas with inert gases or [N.sub.2] instead of air is more a function of pre-filling the virtual leak
voids with water-free gas.
To prevent the possibility of a virtual leak
, A&N drills a small hole in the ball in a direction that is perpendicular to the main conductance opening.
2 Virtual Leaks
. A virtual leak
is caused by entrapped air within the vacuum system, commonly between two mating metal surfaces that are not welded or brazed.
Thermal effects create virtual leaks
that must be compensated for by test methods to achieve the failsafe leak test standard.
are one of the most common sources of unwanted gas loads.
O-ring grooves often provide dead volumes that can trap gases and become virtual leaks
. This is especially true where dove-tailed grooves are used to retain the O-ring in large vertical flanges such as doors that are opened frequently.
Pockets of gas trapped within the confines of the vacuum chamber are usually referred to as virtual leaks
, but this term is used only because they can easily be mistaken for real leaks.
Internal virtual leaks
also provide a useful example in that pockets of active gas can result in gas-process reactions that are never detected by the gauges.
Backfilling the chamber with a dry gas will accomplish much the same results as found with virtual leaks
. The O-ring's surface will absorb the dry gas and slow the absorption of water vapor.
Although this problem is avoided with UV energy, which will flood the whole chamber, the UV energy will not penetrate into the trapped volume of virtual leaks
or reach the surfaces of O-rings recessed between flanges.