by macona » Sun Sep 05, 2010 10:17 am
Helium will migrate through the seals, it does not care about pressure differential. At low pressures gasses act totally different at low pressures. Once it wanders out of the seal its never coming back. This can be seen is old soft seal helium neon lasers, almost none made in their heyday work today due to helium loss. Its also a reason that it is not recommended to sit on a bunch of these chinese glass tubes as they do have a shelf life.
I use helium for finding leaks in vacuum systems with my quadrupole mass spectrometer. It is incredibly sensitive, it will detect leaks down to 10^-13 atm/cc/sec pretty easily. It might be interesting to put a tube in a container and see how much it does leak.
Vacuum systems is another hobby of mine. I have one system with a 70 l/s turbo pump with 200 AMU rga and pressure reducer system. My other system I am building is for thin film deposition, 18" bell jar, 2.5kw 450khz RF Power supply, 6" Diffusion pump, crystal monitor, ferrofluid feedthoughs, the works. Will be able to do evaporative coatings, sputtering, PECVD, CVD.