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.
In: 3143
I think you’re looking at this from the wrong direction. Compressing gasses is easy up to a point, because you’re just saying hey, let’s put more of this stuff in here. If in the process some leaks out, no biggie, just add some more. You could use a super sloppy piston with no seal and still manage to smoosh some gas in.
If you want to take all the gas OUT of the canister though, how can you do that? If we use the analogy of say ping pong balls in a jar, it seems easy. you just grab the balls and remove them one at a time until they are all gone. Ok… how do you grab a single molecule of gas? They aren’t exactly standing still. So you can’t use tiny little tweezers to grab them. Let’s walk through a simple example of a displacement pump, the piston. All displacement pumps work on a similar principle of pushing gas molecules by pressing one piece of metal or other material next to another.
1) piston. requires valves. Piston goes down, intake valve opens, pulls some of the gas out of your canister. Valve shuts. Piston goes up, a different exhaust valve opens, and you push out the gas into the air. works pretty good at first. But you can never pull all the gas out of the canister right? even if the piston created a perfect vacuum, at best you can only pull out a fraction of the molecules from the canister right? Unless you had an infinitely large piston, ~half of the gas stays in the canister and half goes into the piston. And as you get close to a vacuum, you run into a new problem. You are pushing the piston up to get rid of those last few molecules right? but as soon as the exhaust valve opens, air from outside comes rushing into the piston. Which is no big deal, you still push out the few molecules you grabbed from the canister right? But wait, that means your piston must be able to push right up against the top of the cylinder. no gaps. how do you make a cylinder head so perfectly mated to the piston that there is no gap for air molecules to hide in? that includes the valve seat itself, it must be perfectly mated to the piston head. In practice, piston pumps can get a good vacuum, close to 29 in (vs 29.92 being a perfect one) or 20 mbar. You can add a second stage to improve the vacuum, but again, there’s always those last few air molecules hiding at the top of the piston. Plus in the real world seals and valves do leak a little even with oil seals.
2) Turbo molecular pump. This is a better way of getting gas molecules to do what you want, using a rapidly rotating collection of blades, like a turbine in a jet engine in reverse. Even down to a near perfect vacuum, any stray molecule of gas can strike the turbine blades and get boosted upwards to the next blade, and so on until they are ejected from the pump. This kind of pump works well at extremely low pressures , as even a single molecule of gas could in theory be swatted by the blades and kicked out. But even then, you have the chance of a lucky gas molecule leaking back in. You can get pressure as low as 10^-10 mbar. We’re talking trillionths of an atmosphere. That seems good, but that’s still billions of molecules in your canister!
3) Exotic pumps can get pressure down further by for example liquefying the gases so you can actually just scoop them up, or the an Ion Sputter pumps which can can drive the vacuum to even lower levels, i think i read the record was 1 x 10^-12 mbar (1 x 10^10-15 atm), but that still means you have hundreds or thousands of molecules in your canister. But hey, that’s the pressure on the moon. I’d call that a pretty good vacuum. At such low pressures, the molecules in the canister stop acting like a gas because they are so far apart that they rarely bump into each other. There isn’t a practical benefit of getting too much lower. In theory better, near perfect vacuums could be achieved in the lab.
If you’re not impressed with these low pressure records, then remember that the laws of physics themselves don’t really allow for zero of anything. And while lab vacuums are always going to have to deal with leaking and outgassing from the pump materials themselves, even in deep space there are atoms here and there, and stray particles. Nature abhors a vacuum.
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