Let’s say you have some liquid that you want to contain without leaks, say, milk for a baby. What do you do?
Well, you put it in something like a baby bottle, the components of which are shown here:
You have a hard plastic bottle, a soft silicone nipple, and a hard plastic ring that screws onto the bottle. When you put it together and screw the cap down tight, it compresses the silicone between the two plastic bits, squeezing it into the small gaps, and plugging any leaks. Done properly, this will ensure that milk doesn’t leak out of the bottle except through the whole in the nipple.
Now, imagine that you want to contain some extremely dilute gas, in an ultra-high-vacuum chamber. What do you do there?
Well, the principle is exactly the same as with the baby bottle: you take two hard pieces, and clamp a soft gasket between them. The only difference between a UHV chamber and a baby bottle is the materials. When you’re working with vacuum hardware, the “bottle” is stainless steel, and the soft gasket is copper:
This picture shows a partially assembled vacuum system, with a fresh copper gasket on a flange, waiting for the top piece to be put on. Below that, you can see a bit of the copper gasket on the flange below– the top flange in this case is a reducing flange to connect a rather large vacuum pump to a smaller tube coming off the main chamber. You can see the copper gasket pinched between the reducing flange and the main chamber flange, around the inside of the tube.
When the two flanges are put in place, the gasket is squeezed between two knife edges, one on each flange. They’re hard to get a good picture of, but you can see their effect by comparing used and new gaskets:
The gasket on the right is a fresh one, prior to being put on the flange. The gasket on the left is the used one that I took out when I took the chamber apart. You can see a small line going around the middle of the copper ring, which is the impression made by the knife edge when the copper was squeezed down.
How does this thing go together? Well, in the picture of the flange, you can see a bunch of holes around the outer edge. Those are through holes for 1/4″-28 bolts. You put a bolt through each hole, with a nut on the top, and tighten them down to crush the copper gasket into place.
Here’s a picture of a just-assembled flange, showing the bolts, nuts, and a tiny gap between the stainless flanges with just a hint of copper showing:
Here’s the same flange, after all the bolts have been tightened down:
You can see that the gap has gone away, and no more copper is visible. That’s the sign of a properly sealed system.
If this sounds like kind of a pain in the ass, you’re right. If you look at the picture of the flange above, you can see that there are 20 bolt holes on the flange, each of which must be cranked down hard enough to cut into the copper. Better yet, you need to make sure that the compression is uniform, which means tightening the bolts in a star pattern– tighten one bolt, then the bolt on the opposite side, then a bolt 90 degrees from the initial bolt, then the bolt opposite that one, and so one. It’s tons of fun. Better still, each bolt (on this particular flange, anyway) is coated with a grey anti-seize grease that is next to impossible to get off your hands.
Fun, fun, fun. And that’s how I spent the day Saturday.