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
Let’s do a 2d anolgy.
You have a hockey rink with 10,000 hockey pucks. You bump one, you’ll likely make all of them move, eventually. If everything is slick enough, they’ll never stop moving, sliding and bumping each other. So consider them all in constant motion from bumping into each other.
Now you want to empty the hockey rink of hockey pucks. You make a hole in the ice, but it takes a lot of energy to get keep that hole open so that hockey pucks fall out and don’t come back in.
Because the pucks are in the constant motion from colliding with each other other, at first, hocky pucks fall down the hole at a high rate, but as the pucks drop out, there are fewer and fewer pucks on the ice. They don’t bump into each other as often. They move slower and slower because there are fewer and fewer collisions.
Eventually you’re sitting there watching the last ten or so pucks meandering around the ice, sometimes bumping the wall or each other, but they’re going slowly and its just stupid luck when they even get close to the the hole (which still requires a lot of energy to keep open and to keep pucks from coming back in).
That’s why. There’s no real force moving the last molecules out of the container as it gets more and more empty. If they get close enough to the exit, they’ll leave, but unlike the start, when there are a lot of collisions moving things around, its just a blind luck that they’ll get close enough to the pump to get the boot.
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.
Beyond a certain level of vacuum, the oils used in the vacuum pump itself become a problem because they start evaporating and contributing to the gases inside the area you’re trying to evacuate.
Also, as you evacuate a volume, you eventually get to the point where the only way for any remaining gases to leave is for the molecules to physically bounce their way out of the outlet hole. It ends up like those old video games where you’re trying to break bricks by bouncing balls around; the remaining molecules are so sparse that there isn’t really pressure pushing them out, they just bounce around inside the chamber, and need to bounce into the outlet hole.
The hardest vacuums that we can conventionally produce require cryogenics, because these will condense the gases that remain when they impact the super cold surface. The other way of making an extremely hard vacuum is to use a [Sprengel pump](https://en.wikipedia.org/wiki/Sprengel_pump). Sprengel pumps use drops of mercury to entrap gases in a little volume, carrying the gases away because mercury’s density is too high for the atmosphere to overcome, especially when you have a column in a glass tube with many droplets descending it. Mercury itself can vaporize, but if it is decently cool, because the atoms are so heavy, it typically doesn’t vaporize much in this kind of pump, at least not enough to break the vacuum.
Due to mercury being hazardous in various ways, very few people use mercury based vacuum pumps anymore.
Let’s ignore technical details like when you compress the air, there is room full of air you can take molecules from while pumping for vacuum is like fishing for individual molecules in empty room hoping that you catch one. Let’s look at the numbers instead.
If your airsoft tank drops from 200 atmospheres to 199.9999999, you won’t notice that. If someone tries to create vacuum at **0.000000000001**atm and leak causes it to raise to **0.000000001**001atm, that’s thousand times worse than intended.
There’s no such thing as *sucking*.
A pressure differential *pushes* fluids from one place to another.
If there is almost nothing inside of the container you want to make into a vacuum, there’s almost nothing to push the molecules (or atoms?) of the material out of the container.
Say you’ve got a box that has 20 molecules of Oxygen in it, and you want no molecules of Oxygen in it. Those molecules have to push one another out of the container, but they are so few they basically never interact. There’s no other physical force that will move them out of the container.
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