Virtual leaks.


You have already checked your vacuum system for real leaks or pore leaks using a helium leak test, can rule out permeation using suitable seals and have cleaned and baked out your system sufficiently to rule out outgassing from the surface and yet the pressure rises unexpectedly? Then there is very probably a so-called virtual leak.

What are
virtual leaks?

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

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 you have to pump more frequently than planned.

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

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

Where do
Virtual leaks in a vacuum system ?

Virtual leaks can arise from a variety of initial situations. One scenario is that internal weld seams are not continuously welded, allowing trapped gas to flow back into the vacuum and contaminate it.

Another scenario is incorrect parameter selection during welding. This creates so-called blowholes, i.e. cavities inside the weld seam. In addition, trapped gases in incorrectly designed flange connections can act as virtual leaks.

However, the most common cause lies in the design of screw connections within the vacuum system and its components. In theory, all cavities (e.g. blind threads or threaded outlets) that cannot be avoided by design should be fitted with ventilation ducts. In practice, this is not feasible in terms of cost and production technology. The use of so-called bleed screws is therefore essential. A venting channel in the connecting element makes it easier to pump out the trapped gases, usually air components as assembly is carried out at atmospheric pressure, but also particles and other sources of contamination.

Find out more about the vacuum-optimizing fasteners from VOC. Our standard portfolio is permanently in stock and available at low cost. Effectively avoid virtual leaks in your systems!

What effects do
virtual leaks?

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

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

In the viscous flow range, the gas creeps over the threads and extends the required pump-down time. By using vacuum screws, the trapped gases are constantly pumped out, resulting in a typical exponential pump-down curve (see green graph).

Since the gap diameter in the molecular flow range is included by a factor of 3, the conductance deteriorates here compared to the molecular flow range. of a vacuum-optimizing screw by a factor of approx. 1,000,000,000,000 (assuming a gap diameter of 1µm). As a result, the trapped gas flows out continuously in small quantities over a very long period of time when using standard screws and limits the achievable ultimate pressure in the ultra-high vacuum (UHV) / extreme ultra-high vacuum (XHV).

System optimization

Prevent virtual leaks with VOC vacuum screws.

Prevent virtual leaks with VOC
Vacuum screws

Blind holes and through-holes can be vented with the help of our vacuum-optimizing connecting elements. To do this, we install ventilation ducts in a wide variety of connecting elements. These ventilation ducts 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. The gases in the blind hole are pumped out through the venting channel during the recipient’s evacuation process.

Your benefits from 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 connection elements!


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