Yesterday I found that the Stern-Gerlach apparatus had a vacuum leak. Molecules in the atmosphere travel only about a tenth of a micrometer before they hit another molecule and go off in a different direction. If you want to get a molecule to fly in a straight line for a whole metre then you have to get rid of most of the other molecules (removing roughly 99.9999999% of them should do the trick).
To do this you need a container and a sucking device (the pump). The pump has the job of removing the molecules, and the container has the job of not letting new ones get in. If everything goes well you will have a vacuum container with practically nothing in it.
The trouble with vacuum containers is that they tend to leak. And they follow Murphy's laws of vacuum leaks.
1/ A vacuum leak will never be quite large enough that you can hear the air whistling through it, but will be large enough to screw up your measurement.
2/ A vacuum leak will always occur at the point in a vacuum system furthest away from a pressure gauge.
3/ A vacuum leak will always occur at point closest to the most sensitive device in your system.
Laws 2 and 3 combine to form the vacuum-system exclusion principle: It is impossible to mount a pressure gauge next to your most sensitive device, usually because you have the wrong connector.
Fortunately laws 2 and 3 can also be used to divine the exact position of your leak. If you want to be really sure you can buy yourself a nice Helium leak chasing device
2 comments:
Yes, vacuum pumps suck. :-) And the day microsoft builds something that does not suck will be the day they start manufacturing vacuum pumps (the original joke uses vacuum cleaners).
How did you fix the leak? Chewing gum? Welding? Epoxy? New copper seals?
For the record, the leak was repaired by using a different flange with electrical feedthroughs. (only 7 months late with the reply)
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