Virtual leaks

You have already checked your vacuum setup for real leaks with a helium leak test, can rule out permeation due to the use of suitable seals, and have cleaned and baked out your system sufficiently to rule out outgassing from the surface, and yet the pressure unexpectedly rises sharply?

Then it is very likely that you have a so-called virtual leak.

What are virtual leaks?

According to DIN-EN-ISO-20484, virtual leaks (QVL) are “apparent (not real) leaks resulting from slow release of sorbed or occluded gases” from trapped volumes.

QVL [mbar*l s-1]

But virtual leaks are so much more than just a standardized definition!

Virtual leaks are the reason why the process pressure of your vacuum application is not achieved as desired. Virtual leaks are the reason why your process times increase or why you have to postpump more often than planned.

Vacuum-compatible design and installation can eliminate various causes of virtual leaks in advance. However, it is not always possible to design certain components in such a way that the enclosed volumes, usually in blind holes, are avoided.


VOC – Vacuum Optimizing Components provides you with a vacuum optimizing, cost effective and highly available solution to eliminate virtual leaks efficiently and reliably in your vacuum application.

Where do virtual leaks exist in a vacuum system?

Virtual leaks can be caused by a variety of initial situations. One scenario is that internal welds are not continuously welded and thus trapped gas flows back into the vacuum and contaminates it.

Another scenario is the wrong choice of parameters during welding. This results in so-called voids, i.e. cavities inside the weld seam. In addition, gases trapped in incorrectly designed flange joints can act as virtual leaks.

The most common cause, however, lies in the design of threaded connections within the vacuum system and its components. In theory, all cavities (e.g. blind threads or thread outlets) that cannot be avoided by design should be equipped with venting channels. In practical implementation, however, this is not feasible in terms of cost and production technology. Therefore, the use of so-called venting screws is indispensable. A venting channel incorporated in the fastener makes it easier to pump out the trapped gases, usually air components, since assembly is carried out at atmospheric pressure, but also particles and other sources of contamination.

Vented Screw

Learn more about VOC’s vacuum-optimizing fasteners. Our standard portfolio is permanently in stock and available at low cost. Effectively avoid virtual leaks in your systems!

What are the effects of virtual leaks?

The smaller the volume of the chamber and the higher the number of of blind holes, the greater the effect of the virtual leak(s).

The decisive factor for the function of the venting channels is the conductivity of the channel that connects the cavity with the vacuum environment. In the case of screw connections with standard screws the gap between the external thread of the screw and the internal thread of the mating counterpart serves as a venting channel.

In the viscous flow area, the gas creeps over the thread and prolongs the required pump-down time. Through the use of vacuum screws, the trapped gases are constantly pumped out, and a typical exponential a typical exponential pump-down curve is produced (see green graph in diagram1).

Since the gap diameter in the molecular flow range with
If factor 3 is included, the conductance deteriorates here compared to one
vacuum-optimizing screw by a factor of approx. 1,000,000,000,000 (assuming gap diameter 1 µm). As a result, when standard screws are used, the trapped gas flows out continuously in small quantities over a very long period of time, limiting the ultimate pressure that can be achieved in ultrahigh vacuum (UHV) / extreme ultrahigh vacuum (XHV).

Preventing virtual leaks with VOC - vacuum screws

With the help of our vacuum-optimizing fasteners, blind holes and through holes can be vented. For this purpose, we insert venting channels into a wide variety of fasteners. These venting channels run centrally through the center axis of the screw. If such a vacuum screw is installed in a vacuum system, virtual leaks are reduced to a minimum. Through the venting channel, the gases located in the blind hole are pumped out during the evacuation process of the recipient.

Your advantages by using VOC vacuum screws

Achieve your process pressure faster and with less energy consumption with VOC vacuum screws!

Effectively eliminate virtual leaks in your systems by using vacuum-optimizing fasteners!

Keep your process pressure constant over a long period of time with our vented screws!