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
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.
Latest Answers