4 ms·

This has been a pet peeve of mine for a long time, am I correct to assume that deep vacuum is only hard to maintain because very very small impurities in seals

This has been a pet peeve of mine for a long time, am I correct to assume that deep vacuum is only hard to maintain because very very small impurities in seals etc will cause a relatively large amount of off-gassing?

TylerE 4y ago
Not only that, but at those levels of vacuum you’re getting all kinds of outgassing from the object in the chamber.
s0rce 4y ago
Helium and hydrogen can also fairly rapidly diffuse through materials or grain boundaries and contaminate the vacuum.
- usrusr 4y ago
  Should not be a problem here as the vacuum (if it's needed at all) would only be temporary to "make room" for high pressure helium. That would only be required if the helium had to be of particularly high purity and gp wasn't at all implying that this is the case, only that it could be a challenge if it was. From my uninformed perspective, it's just as likely that impurities would condense at the bottom and rest there happily ever after as they would be thrown into an permanent phase change cycle that rims efficiency even in trace amounts. The best case scenario I'd like to read is "actually we don't really need helium but it gives a slight performance edge over other gases and we decided to launch with the setup that gives us the best numbers". No idea if that's close to or very far from reality.
  - s0rce 4y ago
    Yah, I don't think its a problem here, just a general response to the comment about acheiving and maintaining ultra high vacuum.
jcims 4y ago
It also just becomes difficult to motivate the gas towards the intake of the pump. IIRC turbomolecular pumps, which are used for deep vacuum applications, have a large intake area and work by basically spinning a huge array of blades at very high speed to bat the individual gas particles out of the working volume into progressively higher pressure areas, ultimately pumped out by a conventional pump.
- _trampeltier 4y ago
  And then are ion pumps. They have no moving parts at all. An ion pump (also referred to as a sputter ion pump) is a type of vacuum pump which operates by sputtering a metal getter. Under ideal conditions, ion pumps are capable of reaching pressures as low as 10−11 mbar.[1] An ion pump first ionizes gas within the vessel it is attached to and employs a strong electrical potential, typically 3–7 kV, which accelerates the ions into a solid electrode. Small bits of the electrode are sputtered into the chamber. Gasses are trapped by a combination of chemical reactions with the surface of the highly-reactive sputtered material, and being physically trapped underneath that material. https://en.m.wikipedia.org/wiki/Ion_pump_(physics) https://en.m.wikipedia.org/wiki/Ion_pump_(physics)
  - dr_dshiv 4y ago
    Will we ever get those structural vacuum balloons that enable more floaty things, like the mites in Neal Stephenson’s “Diamond Age?” Or like the giant floating geodesic ball proposed by buckminster fuller? I was reading about early Royal Society proposals for flying machines that were based on vacuums in copper balloons. Can any material at any size support a vacuum that can displace the weight of the material? (ie, and thus support lighter than air flight via vacuum)
    - TylerE 4y ago
      No, even carbon nanotubes aren’t strong enough. Hydrogen is already nearly a vacuum from a lift prospective (it’s only about 5% as dense as air at the same pressure).
    - 4y ago
lazide 4y ago
You’ll also get gaseous diffusion through most materials from the atmosphere, offgassing as you note, plastic and other materials decomposing, etc. Nature really abhors a vacuum.
colechristensen 4y ago
It’s not difficult for a pressure vessel to withstand a hard vacuum, it’s difficult to achieve a very low vacuum. Lots of things offgas, lots of things are gas permeable enough to ruin your low vacuum, just getting equipment that can achieve a certain amount of vacuum is hard and expensive. The question is: does maintaining this require a very high vacuum to replace/maintain the working fluid. Its one of those scaling problems where you have to spend ten times as much to get to the next ten times less gas in a vessel (or thereabouts) and an absolute complete vacuum is more or less impossible.

TylerE 4y ago

Not only that, but at those levels of vacuum you’re getting all kinds of outgassing from the object in the chamber.

s0rce 4y ago

Helium and hydrogen can also fairly rapidly diffuse through materials or grain boundaries and contaminate the vacuum.

usrusr 4y ago
Should not be a problem here as the vacuum (if it's needed at all) would only be temporary to "make room" for high pressure helium. That would only be required if the helium had to be of particularly high purity and gp wasn't at all implying that this is the case, only that it could be a challenge if it was. From my uninformed perspective, it's just as likely that impurities would condense at the bottom and rest there happily ever after as they would be thrown into an permanent phase change cycle that rims efficiency even in trace amounts. The best case scenario I'd like to read is "actually we don't really need helium but it gives a slight performance edge over other gases and we decided to launch with the setup that gives us the best numbers". No idea if that's close to or very far from reality.
- s0rce 4y ago
  Yah, I don't think its a problem here, just a general response to the comment about acheiving and maintaining ultra high vacuum.

jcims 4y ago

It also just becomes difficult to motivate the gas towards the intake of the pump. IIRC turbomolecular pumps, which are used for deep vacuum applications, have a large intake area and work by basically spinning a huge array of blades at very high speed to bat the individual gas particles out of the working volume into progressively higher pressure areas, ultimately pumped out by a conventional pump.

_trampeltier 4y ago
And then are ion pumps. They have no moving parts at all. An ion pump (also referred to as a sputter ion pump) is a type of vacuum pump which operates by sputtering a metal getter. Under ideal conditions, ion pumps are capable of reaching pressures as low as 10−11 mbar.[1] An ion pump first ionizes gas within the vessel it is attached to and employs a strong electrical potential, typically 3–7 kV, which accelerates the ions into a solid electrode. Small bits of the electrode are sputtered into the chamber. Gasses are trapped by a combination of chemical reactions with the surface of the highly-reactive sputtered material, and being physically trapped underneath that material. https://en.m.wikipedia.org/wiki/Ion_pump_(physics) https://en.m.wikipedia.org/wiki/Ion_pump_(physics)
- dr_dshiv 4y ago
  Will we ever get those structural vacuum balloons that enable more floaty things, like the mites in Neal Stephenson’s “Diamond Age?” Or like the giant floating geodesic ball proposed by buckminster fuller? I was reading about early Royal Society proposals for flying machines that were based on vacuums in copper balloons. Can any material at any size support a vacuum that can displace the weight of the material? (ie, and thus support lighter than air flight via vacuum)
  - TylerE 4y ago
    No, even carbon nanotubes aren’t strong enough. Hydrogen is already nearly a vacuum from a lift prospective (it’s only about 5% as dense as air at the same pressure).
  - 4y ago

lazide 4y ago

You’ll also get gaseous diffusion through most materials from the atmosphere, offgassing as you note, plastic and other materials decomposing, etc. Nature really abhors a vacuum.

colechristensen 4y ago

It’s not difficult for a pressure vessel to withstand a hard vacuum, it’s difficult to achieve a very low vacuum. Lots of things offgas, lots of things are gas permeable enough to ruin your low vacuum, just getting equipment that can achieve a certain amount of vacuum is hard and expensive. The question is: does maintaining this require a very high vacuum to replace/maintain the working fluid. Its one of those scaling problems where you have to spend ten times as much to get to the next ten times less gas in a vessel (or thereabouts) and an absolute complete vacuum is more or less impossible.