Hard Vacuum Leak Test

The Basics of a Hard Vacuum Helium Leak Test

Helium Leak Detection is very niche. If you’ve never been involved with it, you’ve probably never heard of it. Luckily there are companies out there, although very few of them, that have very good helium knowledge and can provide you with a plethora of solutions to find your tiny leaks. Whether this is in a production setting where you have to test every part you manufacture, or whether you have 1 fitting that is causing you nightmares and you just need someone to pinpoint where the weld fail is –

VES have manufactured helium leak detection systems for some of the world’s largest manufacturing groups, for over 20 years, and are more than happy to impart a little knowledge now and again.

So what is helium leak detection?

Helium leak detection is a non-destructive test methodology that can be employed in various ways to find leaks that less sensitive methods cannot. It involves either filling your part to be tested with, or bathing your part to be tested in…. helium, and measuring said helium which leaks at a molecular level via mass spectrometry. This article focuses on the most complete method.

Hard Vacuum (Chamber) Helium Leak Detection

So how is this done? We’ve broken this down into 5 key steps –

Step 1:

Clean up your helium background. There is 5ppm of helium in atmospheric air and to ensure you only send particles of helium that are leaking from/to our components to the mass spec during the test, it is very important to remove the residual helium from the test environment (the chamber) before the helium backfill. This is done by evacuating the test piece and the chamber to approximately 1 mbar to remove that partial pressure of helium from the environment. This pump down procedure is slightly elongated to ensure removal of all helium and once this background read is stable, it’s time for leak testing!

Step 2:

Create a pressure differential. Whether you have a dripping tap or dunking your bicycle inner tube in water and looking for bubbles – you have a pressure differential. Your leak path will always follow the very basic physics principles and higher pressure will try to equalise with lower pressure around it, thus you must drive your leak path in the correct direction by pressurising the outside or the inside with helium (P1V1=P2V2).

In a hard vacuum chamber machine this is done by following step 1 with a helium backfill to the relevant test pressure – approx 100 mbar for a fuel tank, up to 500 bar for a fuel rail. You should always create your pressure differential during your leak test to mimic conditions your part will see in operation, this way leak paths are stressed in the same way they are in the field, where your part must be guaranteed.

Step 3:

Take the helium to the mass spectrometer. Typical vacuum pumping systems seen on helium leak detection systems lose speed at lower pressures so it is very important to ensure you have an appropriate response time to ensure any leakage from your test piece is actually picked up by the mass spec. This area is where so many mistakes are made by companies who have little vacuum or helium knowledge bases.

Vacuum conductance combined with a lower pumping speed and large chambers often lead to missed leaks. Calibration of your system can give you confidence that your response time is well configured. Don’t let a fancy-looking, automated system get away with conducting a poor leak test. It will come back to haunt you.

Step 4:

Know your leak rate threshold. Our favourite saying at VES is that everything leaks. All you can do is guarantee your product meets a certain threshold. So now you know your mass spec is reading helium leaking, from or into your product, you need to know how much it leaks. EN 1779 rates hard vacuum leak detection to be able to detect leakage to 10-9 Pa⋅m3/s. Depending on your industry, you might have to relate you leak rate to a whole size (automotive), to a weight loss per year (HVAC), or the size of a virus (medical device). If your test piece leakage is less than your acceptable limit, it’s a pass!

Step 5:

The vent. Many thin-walled products are susceptible to damage during the vent. Any mismanagement of the pressure differential and your product may implode or explode. This is carefully managed by a PLC program and often monitored by deflection sensors in real time. Once this vent is complete, your product is good to go!

If you would like to learn more on this subject, please Contact Us today. Our VES consultants operate worldwide.