Helium Leak Testing
A highly sensitive test method for detecting minute leaks using helium as a test gas and a mass spectrometer as a detector, primarily applied in high-vacuum and pressure vessel construction.
Helium is the smallest molecule we use for leak testing. With it, you can find even the finest leaks that wouldn't show up with compressed air or water. The mass spectrometer sucks in the air and shows you whether helium is escaping through the weld seam.
When you do a leak test after welding, you must make sure the seam is clean and free of slag. Otherwise, the device will report a leak that isn't there. It's best to do the test at room temperature, because otherwise the material may distort and skew the results.
For the workshop, that means: prepare the weld seam, let it cool down, then apply the helium and slowly run the probe over the seam. If you find a leak, mark the spot and weld it again. Then re-check the spot.