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Rough and dirty vacuum testing: my path towards the 1 Pascal range


Cremona, 2018-ongoing...
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Larger vacuum vessel


Hi everyone, time to move on the large vessel, finally! The small vacuum reservoir was more of a test, and mostly I can't connect a turbomolecular pump to it, since it's super dirty inside. The walls are internally rusted and covered in some type of crust. So, the new - and final - large reservoir is actually made in stainless steel. It comes from a water fire extinguisher, the large ones that you see mounted on some wheels, and was filled with this soapy and bubblish water. The material indeed is some good stainless steel, which is not magnetic but welds as a charm. Not sure of the precise alloy.
So, this post is mainly about:
  1. Transforming the fire extinguisher into a vacuum vessel, by cutting and welding flanges;
  2. ... And making large gaskets, able to sustain the required vacuum level.

Part 1: cheap flanges

Once you cut open the fire extinguisher (if you are working with a gas bottle, beware that cutting a gas bottle open may make it explode! You won't be able to tell it to friends. Don't cut open a bottle, especially of flammable gas, unless you know very very well what you are doing!), you'll need to weld a couple of flanges to it.
Unfortunately, flanges cost as crazy. Buying them is not an option for me. The good news is: we can fabricate them from sheet metal. As I wanted to stay cheap, I just bought some ordinary steel sheet metal, thickness 1.5 mm. It proved to be very thin, probably too thin, and welding it deforms it a lot, and creates a large number of holes. This is not a problem though, and we'll build pretty thick and soft gaskets that will fix this issue. If you want to do things better, however, consider buying sheet metal with a thickness of at least 2 mm (2.5 should be good, it will be harder to cut though).
The diameter of my fire extinguisher is slightly above 30 cm. So, once you got the dimensions written down, you can build a small tool to draw circles, by drilling some holes in a scrap piece of wood, and inserting a pencil. See the picture below.

sheet metal flange

Part 2: large gaskets for large vessel!

You may guess, gaskets play a pretty important role in vacuum technology... Now, you may think that you need super expensive or specialized gaskets to get to high vacuum (below 0.1 Pa, or 1 mTorr). Well, not quite! Back in the high school days I was making gaskets out of a thick ring of silicone, the one that you use for bathroom repair. They work as a charm. The only problem is getting them to cure (do you say "cure" for silicon?) or solidify properly. Indeed, it may take weeks. There are different options, such as bicomponent silicones, or liquid rubbers, but they are not quite as cheap. I may investigate them one day.

Anyway, the way I make gaskets is this. First, I cut out a shape out of cheap wood, using the same tool that draws circles, that you saw in the pictures above. Then, I assemble the circles as to make a pie-like shape. I melt some candle wax, and use it to clog residual holes and make everything pretty flat (help yourself with a brush). Also, this is pretty important, as it hinders the silicon from attaching to the walls.
Here are some pics. My mold was made such that the final gasket has a lip, that allows to "hang" it to the flanges, and prevents it from falling when you are operating it.

gasket silicone 1

Finally, I put in the silicon, one layer at a time. Put one layer, let it solidify for some days, then move to the next layer. It takes time, but if you do it one-shot, well, it may even take months to solidify. And if you try to remove it from the mold before it's ready, you have to start over.
You can help yourself with a spatula to depose the last layer.
So, once more, do it one layer at a time, or if you find yourself some bicomponent silicone, let me know how it works.

Here are some pictures of the final gasket, ready for operation!

gasket silicone 2

Part 3: a stand for the large vessel

Not much to say here. Just, the vessel needs a cool stand to bring it aroud, with some place to put the turbomolecular and roughing pumps, some instrumentation and electrical connections. Here are some pictures of my stand for your inspiration.

vessel stand

Ok, that's it for today.

Cheers,
Stefano

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