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 am sitting at work monitoring a TVAC (thermal vacuum) test for space-bound hardware right now, and have about 2 decades experience building, operating and maintaining high vacuum, ultra-high-vacuum and Extra-high vacuum systems for use in scientific research, aerospace testing, and nano-materials fabrication (like your sputtering setup, PVD etc).
One of the main concepts that a lot of people don’t grasp that makes reaching deep vacuum levels hard is the difference between what is called viscous flow and molecular flow. At pressures we are used to, gasses are said to be in the viscous flow regime. This means that the molecules of gas are densely packed enough that they interact with (impact and bounce off of) each other a LOOOOT more than they interact with other stuff (surfaces they would exert pressure on). This gives them the property known as viscosity. This also means that if you manage to pull or push on this ‘chunk’ of air over here, it will have an impact on the chunk of air adjacent to it, which will have a slight impact on the chunk of air next to IT, which will have a slight impact on the chunk of air next to… well you get the point. The air is so closely packed that the molecules sort of ‘drag’ each other along when they are pushed or pulled. Kind of like being in a closely packed crowd. If some people start moving one direction, you could get pulled along with them. This makes it relatively easy to evacuate (pump down) a chamber when in the viscous flow regime. You can just use a pump to start pulling air out of the side, and it will drag a lot more other air with it, which will drag other air with it, etc. And you get a steady flow out of the chamber.
However as you remover more and more air the pressure inside reduces more and more, and the molecules get less and less densely packed. At some point they will transition into the ‘molecular flow’ regime (usually somewhere around 1×10-3 Torr). This means that the molecules are more likely to interact with another surface (like the walls of a chamber) than with each other. The technical definition is that the ‘mean free path’ (the average distance a molecule will travel before it hits another molecule) is larger than than the dimensions of the chamber the gas is in). Instead of people in a densely packed crowd, the molecules are now billiard balls bouncing randomly around a poool table the size of a ballroom. They just go around bouncing off the walls, and very rarely hitting each other. Now you no longer have any ‘viscous drag’ to help you out. If you want to get those billiards balls of the tables, it’s really tricky. You can basically make a big door and wait for them to randomly bounce out through the door… but then theres a good chance they bounce off a wall in the hallway and just end up back in the ballroom again. So you make the back wall of the hallway suuuuuper cold (just 10-12 degrees above absolute zero) so that when the billiard balls hit it they freeze and stick to the wall (cryocapture pump array), or maybe you have a sort of big fan blade at the end of the hallway and when the ball reaches it, it gets whacked by the angled backside of the blade and gets knocked into another room (turbomolecular pump) or maybe a machine that shoots a huge waterfall of billiards balls down the hall into a giant pit so your billiard ball gets caught up in them and pulled out with them (diffusion pumps). Either way, it gets REAL tricky to get those last few billiards balls out and keep them out. And you find out pretty quickly that geometry (shape of your chamber, how big the ‘door’ for escaping billiards balls is, etc) becomes a lot more important to how quickly the evacuation goes than how big your capture/pump device is. And theres not much you can do to make it go faster, it basically becomes a matter of statistics (you’re just waiting for those last few molecules of gas to randomly bounce their way down the throat of your pumping system and interact with your UHV/EHV pump)
And then the LAST piece of the puzzle is: EVERYTHING LEAKS. At temperatures above absolute zero, gasses will diffuse through solid metal. Much less any sort of seal. Even UHV seals like the copper ones you describe (Conflat seals) will leak SOME amount. If you’re only pulling down to the 10-6 Torr range? you can get by fine with KF/ISO style elastomer rubber seals. They’ll leak a little, but your pump will be able to keep up to maintain your pressure level. Want to get down into the 10-9 range? Gonna need single-use copper Conflats. They’ll reduce the leaking enough that your same pumping system can get you a little deeper. Wanna go deeper than that? How much money and time you got?
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