As applications using vacuum technologies evolve and become ever more precise, the vacuum systems and pumps used have even less margin for error than ever before. In every operation that uses vacuum – particularly where incredibly precise instruments are involved (R&D, mass spectrometry) – the preservation of the vacuum is of the utmost importance.
The reality is that unforeseen problems in a vacuum can be catastrophic for the process, the environment and the operator, and one which can prove tricky is a vacuum leak.
What is a vacuum leak?
A vacuum leak is a small hole in one or several parts of the system that allows the uncontrolled entry or exit of gas. The leak rate is dependent on several factors, including the size of the hole, gas type, and the pressure differential (between the inside and the outside of the system).
Why is avoiding leaks important?
If a vacuum system is not “tight” or rather, tight enough, then not only is effort wasted trying to create and maintain ideal conditions (as an open system could never maintain the ideal vacuum anyway), operators and products are needlessly put at risk.
- toxic gases/fluids could leak out of the vacuum,
- vacuum processes deliver faulty products due to impurities entering though the leak
- air could enter the system and potentially contribute to the formation of an explosive mixture,
- the vacuum system could be damaged,
- and the surrounding environment could be affected.
A vacuum system doesn't need to be vacuum-tight – it just needs to be manageable or low enough so that the operating pressure, gas balance and the ability to reach and maintain the final/ultimate pressure are not majorly influenced.
It’s also worth noting that leaks at lower vacuum would be unacceptable and possibly dangerous at higher vacuum levels.
Want to find out more about vacuum system maintenance and safety? Check out our guide.
How do you identify vacuum leaks?
There are four ways to identify vacuum leaks: the bubble test, the pressure decay test, the pressure rise test and the helium sniffer mode/helium vacuum test. The tests – in order of mention – correspond to low to high vacuum respectively.
The main vacuum leak tests:
- The bubble test
This involves putting a punctured bicycle tube underwater and marking where the bubbles come from or placing washing up liquid around the joint of an active water/gas pipe to see where the bubbles/froth forms.
- The pressure decay test
This test is conducted by evacuating a closed vacuum vessel until a certain pressure is reached. From there, the pump’s inlet valve is closed and then opened again after some time. This process is repeated several times; if the time to return the vacuum to its original level remains constant, then a leak is present. If this time decreases, gas is most likely being removed from the inside of the system – but this doesn’t exclude the possibility of a leak.
- The pressure rise test
This test is essentially the opposite of the pressure decay test. It involves marking the vacuum level against the time taken to achieve that level. After isolating the system, if a leak is present the curve will become a straight line.
Monitoring the pressure after evacuation, to a certain pressure, with the pump then valved off can be useful too. If pressure rises constantly there could be a leak. Strong degassing from liquids or the vessel walls rise the pressure too, so it is not an exact proof of a leak.
- Helium tests
The only way to detect a leak smaller than 1x10-6 mbar *l/s (i.e. which – for example – could be viruses leaking into the system) is with a helium leak detector.
Helium is typically used not just because of its ability to measure small vacuum leaks, but also because it’s very mobile (low mass), completely inert/non-reactive (and therefore harmless) and is available at a relatively low cost.
So, how do you fix a vacuum leak?
Creating a totally leak-tight system is – in practice – impossible, and not even necessary. What needs to happen is that the leak rate must be small enough to allow the required pressure level to be reached. With this in mind, you must specify a “minimum” acceptable leak rate for your system.
Rather than “fixing” a vacuum system leak, the idea is to minimise the leak rate as much as possible to attain your desired level of vacuum. Leak testing and measurement, therefore, are just to identify the position of a leak and the rate of leakage.
Once the position and rate of leakage are recorded, steps can be taken to minimise it. Here’s what you can do:
1. Check your seals
Most leaks occur in a vacuum system at joints to components (seals). This includes pump lines, chamber doors and electrical leadthroughs. Please be aware that seals have a limited lifetime – they become brittle after a few years or if they are compressed too often or assembled incorrectly.
If metal seals are leaking, they have either been reused (which is never recommended) or incorrectly compressed. In rare cases, it may be that there is no seal installed!
2. Investigate your view glasses and leadthroughs
View glasses or leadthroughs are either welded or glued, but mechanical force or temperature cycling can crack the junction between these materials and others, causing a vacuum system leak.
Repair is often impossible, leading to the component being exchanged, but in some cases view glasses and leadthroughs can be repaired by “patching” the leak with low degassing resins or Vacseal® (a type of cement).
However, if the leak is in the vacuum vessel body or the welding of a port, patching with resins is not advised as they will not last. Instead, we would advise welding as it’s the best solution to ensure the tightness of your system. In high vacuum vessels, it’s best to weld from the inside to prevent degassing from virtual leaks hidden by the new welding.
For more on leak detection, please check out our essential guide by clicking the button below.