My university has a sputtering machine which is this crazy expensive piece of equipment that has to have a really strong vacuum pump and wacky copper seals and if it loses power for even a minute it has to spend 16 hours pumping it’s vacuum back down.
I know people talk about how a perfect vacuum is like near impossible, but why? We can pressurize things really easily, like air soft co2 canisters or compressed air, which is way above 1 atmosphere in pressure, so why is going below 1 atmosphere so hard? I feel dumb asking this as a senior mechanical engineering student but like I have no clue lol.
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I used to be a vacuum engineer, I spent like 8 years mostly designing vacuum chambers lol. One issue is the pumping, yes. You are trying to move air molecules from a space where there are basically none to a space that’s at 1 ATM. There’s really no way to do that absolutely perfectly, there will always be some atoms left behind. Imagine you have a steel container half filled with motor oil (I’m using oil because it won’t just evaporate over time lol). You want to get 100% of the oil on one side of the container and absolutely none on the other side. How do you do that? Maybe you can put a divider in and start scooping oil over to one side. Ok, but eventually you’ll be left with a surface that’s still coated with oil and no way really to completely remove every single atom of it, plus your divider is not a perfect seal. That’s kind of how vacuum is, it’s easy pumping the air out at first but as you get down to lower pressures it becomes almost impossible.
So that’s one issue, another issue is leaks. All seals leak, the only question is how much. Copper seals (conflats) are basically the best we have and while they do work very well they still leak a little bit. Even if you have a magic pump that can create a perfect vacuum your seals will leak anyway.
Other major issue is outgassing. Materials, even metals, will outgas, meaning when placed under vacuum they will give off gasses that are embedded in the materials which will contaminate the vacuum. You can reduce this by using vacuum compatible materials but it’s usually not perfect. Another issue is off gassing of dirt and residue that’s on your materials. Things like grease, oil, water, etc will simply vaporize under vacuum. Even fingerprints will contaminate a vacuum. If you’re trying to do things as good as possible you would clean all your parts multiple ways (acetone, IPA, ultrasonic) and then assemble them in a cleanroom. Then you bake it at several hundred degrees, under vacuum, for several hours to try and drive off as much residue as you can. That’s not perfect either though, there’s always going to be some residue left behind.
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