How Do Helium Leak Detectors Work?

[tot]

I am trying to figure out how Helium Leak Detectors work.
If a leak detector is pumping on an enclosure that is at a low pressure, the detector will detect helium leaking into the system instantly as far as my human senses can tell.
The helium will propagate 4 meters in a time so small I can't even sense.
How is this possible?
There can't be very much airflow through the system because the leak is so small.

does helium propagate this quickly through air?
or is simply because the pressure is low in the enclosure?
why?

 

[f95toli]

Leak detectors aren't that fast, it will take a few seconds for it to react even if you are pumping on a small enclosure and when trying to find a small leak (say 10^ 9 - 10 ^-8 m|barl/s) in a large enclosure it can easily take half a minute or more beforethe helium reaches the mass spectrometer in the leak detector.

 

[tot]

from my experience I could not even notice any time.
Perhaps my enclosures are small in comparison to yours.
I would say perhaps 3 cubic meters on the inside of a 6 in pipe.
Does it propagate faster because of the vacuum?

Because it does not seem like the pumping would be making much of an air current due to the very small size of the leaks.

 

[bjacoby]

from my experience I could not even notice any time.
Perhaps my enclosures are small in comparison to yours.
I would say perhaps 3 cubic meters on the inside of a 6 in pipe.
Does it propagate faster because of the vacuum?

Because it does not seem like the pumping would be making much of an air current due to the very small size of the leaks.

Of course it all depends. Depends on how good the vacuum is and how big the leak is and how efficient the pumping is etc. With a good vacuum system (corners tend to slow down pumping) and a sizeable leak you can have a pretty decent flow to the pump. But of course there is more to it than that. In a gas molecules are bouncing about with pretty decent velocity. So if you pull a good vacuum you can have pretty long mean free paths which means that the leak molecules also diffuse quite rapidly down the system even if the pump is not creating much of a pressure differential to "pump" them.

 

[mgb_phys]

It also depends what's in the vacuum. If it's a cryogenic system with many layers of superinsulation it can take a lot longer for the helium to work it's way through.