Anyone who has used a vacuum system will know the frustration when a leak develops. This can result in poor base pressure or a steadily rising pressure during a process. Such issues can result in a batch of material being scrapped or needing to be re-worked, with significant cost implications.
Assuming that pumps and gauges are routinely maintained and performing correctly, a detailed leak test of the system will be required.
There are many ways of leak detecting, such as bubble test or pressure rise, but none can compete with the sensitivity and ability to pinpoint a leak that a helium leak checker can offer. Additionally, such an instrument allows both quantitative and qualitative measurements
A helium, or more precisely, a Helium 4 leak detector is essentially a tuned mass spectrometer which detects this gas and has the further option of identifying Helium 3 (used in cryogenics) and hydrogen. The unit has appropriate pumps supplied, making it portable, with only mains electricity required. The unit is extremely sensitive to helium. For high vacuum applications a leak rate below 10-6 mbar l/s is needed, and Helium leak checkers have an ultimate sensitivity of 10 mbar l/s.
Related: Don't let your research be at risk of a leaky vacuum chamber. Watch the webinar in which our in-house leak detection expert, Mike Ridenour, shares what he's learn in his 40 years' experience in vacuum and leak detection, and how you can avoid common errors when leak testing.
Why we use Helium as the search gas
There are several key benefits of using Helium as the search gas:
- It is non-toxic
- Relatively low cost
- Only 5 ppm occurs naturally in air
- It is one of the smallest atoms, resulting in the ability to investigate very small leaks
- It is a low-density gas which easily disperses
Related: Vacuum Science World shared a great article on Helium leak detection methods. Find the blog post here, Four Ways of Finding Vacuum Leaks Using Helium.
Ways to use a Helium leak detector
Generally the detector is employed in what is known in qualitative or “local” mode, where the user is trying to find the source or sources of leaks. In this configuration helium, as illustrated, is sprayed carefully and slowly around the test object, which is often a vacuum processing chamber employed in a number of sectors including heat treatment, metallurgy, and optical coating.
A typical testing arrangement is shown below:
Any leak present will allow Helium to pass through and be detected by the instrument. There are some important practical rules to be followed to allow accurate and reliable leak detection:
- Helium should be sprayed slowly across the test piece
- The flow rate of the gas should be small: I bubble per second in water is a good rule of thumb
- Leak check from top to bottom to allow the gas to disperse vertically
The leak detector generally has a small backing pump ranging from 2.5 m3h-1 to 20 m3h-1, and for large vacuum chamber applications this pump would be incapable of pumping the item down to a level of 10-3 mbar needed to allow the unit to reach its most sensitive level. As in the illustration, the most effective method is to attach the leak checker to the backing line of the system above a forevacuum pump. This inevitably will have a higher pump speed than the leak checker, and the unit will not detect the complete He flow. To give a true leak rate, the observed leak rate needs to be factored by the pumping ratio of the competing pumps.
In other instances, such as hermetically sealed electronic packages, high vacuum valves require the total leak rate to be determined to ensure that the expected lifetime or specification of the component is met. The diagram below demonstrates how the unit operates in a quantitative manner. The component is placed in a suitable vacuum container which, depending on its volume, might require additional pumps to allow the correct inlet pressure to be achieved.
Again, a pumping ratio would be needed to achieve the actual leak rate, but the value determined will be the total leakage of the entire component. Depending on its value, the device could be retested to find the actual leak(s) by the sniffer method and then reworked to meet specification.
More on Leak Detection
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