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Copper in vacuum thermos

Posted: Mon Jan 08, 2018 4:23 am UTC
by Eebster the Great
For Christmas, my employer gave me a vacuum-insulated stainless steel tumbler that says the inner wall is copper-plated. What is the role of the copper in keeping drinks hot or cold?

Re: Copper in vacuum thermos

Posted: Mon Jan 08, 2018 10:31 am UTC
by p1t1o
Most likely pure marketing IMO, maybe something to do with taste/flavour?

Re: Copper in vacuum thermos

Posted: Mon Jan 08, 2018 10:41 am UTC
by Xenomortis
Copper has antimicrobial properties - maybe that's the point?

Re: Copper in vacuum thermos

Posted: Mon Jan 08, 2018 11:16 am UTC
by Soupspoon
Stainless steel (generally with 10%ish chromium) is already fairly good at doing everything it should. Include some moly, and it's improved against certain acids.

Copper reacts against environmental acids (verdigris) and may do for the copper what the chromium oxide does for Stainless (and has famous and historical repute of antimicrobial and antifungal effects *edit: Ninjaed!*), but any visible discoloration of a 'wetted' surface would put me off. I suppose the outer being patinaed (pre-purchased, especially) might be ok.

Other than that, I suppose it depends on whether you think it'll deal with your arthritis/whatever. But it sounds better (a pure metal, especially if plated with deposition methods) unless you know that an 18/10 chro-nickel austenitic stainless is typically good for use in culinary situations.

(Electrically, stainless tends to be vastly out-conducted by copper. This may mean creating a galvanic corrosion of the copper, even as the stainless stays (and always would stay) oxide-protected. I'd have to check this. I suppose you could sell copper loss as "this is why the copper's there - to be safely lost and protect the stainless", but I suspect it'd be a bit of a porky… And it's been a while since I had to know about this kind of stuff, so don't take my word for any of this.)

Re: Copper in vacuum thermos

Posted: Mon Jan 08, 2018 12:31 pm UTC
by p1t1o
Im 100% convinced its just a sales gimmick, but to give it the XKCD treatment - would copper have any advantage over steel in terms of IR reflection/absorption?

Googling the use of copper in terms of vacuum thermal vessels, I can only find very esoteric uses in industrial cryogenics...like ultra high vacuum gaskets and cryogenic ambient-heat shields.

Re: Copper in vacuum thermos

Posted: Mon Jan 08, 2018 10:59 pm UTC
by Zamfir
It appears to be this:

https://www.google.com/patents/US4427123

Idea is to prevent outgassing from the S-S, which would reduce the vacuum. Of course, a patent doesn't mean it's not hokum.

The outgassing is a real thing, and so is plating to prevent it.But I have no clue if it's big enough to matter.

Some website says that untreated SS has an outgassing rate of 10-7 "torr-liter per second per cm2". Never encountered that unit before.
And apparently thermos bottles have a pressure of 10-3 torr. No clue at which point it stops working well.

If I understand this right, then the math is:

Volume of bottle vacuum: 0.1 liter?
Surface area to vacuum: 10-3 cm2
Unwanted rise in torr: 10-2 torr (tenfold)
Time needed: 10s

That suggests, to my that unknowing eye, that special surface treatments are indeed needed for a good vacuum in a thermos bottle, so the copper might not be hokum.

Re: Copper in vacuum thermos

Posted: Mon Jan 08, 2018 11:21 pm UTC
by morriswalters
Is the copper inside the bottle or in the vacuum chamber?

Re: Copper in vacuum thermos

Posted: Tue Jan 09, 2018 4:39 am UTC
by Eebster the Great
p1t1o wrote:Most likely pure marketing IMO, maybe something to do with taste/flavour?
Xenomortis wrote:Copper has antimicrobial properties - maybe that's the point?
Soupspoon wrote:Copper reacts against environmental acids (verdigris) and may do for the copper what the chromium oxide does for Stainless (and has famous and historical repute of antimicrobial and antifungal effects *edit: Ninjaed!*), but any visible discoloration of a 'wetted' surface would put me off.

Marketing gimmick was my first thought, but as to the other answers given, I should have clarified:
morriswalters wrote:Is the copper inside the bottle or in the vacuum chamber?

It's in the vacuum chamber.

Zamfir wrote:It appears to be this:

https://www.google.com/patents/US4427123

Idea is to prevent outgassing from the S-S, which would reduce the vacuum. Of course, a patent doesn't mean it's not hokum.

The outgassing is a real thing, and so is plating to prevent it.But I have no clue if it's big enough to matter.

This is interesting. No clue if it matters, but it follows the principle of "gold plating improves conductivity, so gold-plated cables are better."

Zamfir wrote:Some website says that untreated SS has an outgassing rate of 10-7 "torr-liter per second per cm2". Never encountered that unit before.
And apparently thermos bottles have a pressure of 10-3 torr. No clue at which point it stops working well.

If I understand this right, then the math is:

Volume of bottle vacuum: 0.1 liter?
Surface area to vacuum: 10-3 cm2
Unwanted rise in torr: 10-2 torr (tenfold)
Time needed: 10s

That suggests, to my that unknowing eye, that special surface treatments are indeed needed for a good vacuum in a thermos bottle, so the copper might not be hokum.

Those calculations yield a suspicious result, but even if its the difference between a well-maintained thermos holding a good insulating vacuum for, say, three years vs two, that is a legitimate reason to prefer the copper insulation.

Re: Copper in vacuum thermos

Posted: Tue Jan 09, 2018 5:50 am UTC
by Zamfir
The patent describes experiments. They say a bottle with plating (nickel and silver in their case) drops from 90 C to 55 C in 24 hours, and a bottle without plating to 47 C. That significant, though not revolutionary.

Not the most reliable source, but the patent doesn't smell like woo, at least. Many do.

I don't think the outgassing last indefinitely, it's just that a short time scale implies that the vacuum can be significantly diminished before the outgassing is done.

Edit: come to think of it, a pressure of 10-3 Torr (0.1 Pa) really is the sweet spot for a thermos bottle. In a fluid, thermal conductivity is roughly constant with pressure, because a higher density is compensated by shorter path length. The mean free path at 1 Pa is about 10mm. So roughly below that point the vacuum stops behaving as a fluid (in a thermos bottle wall), and becomes molecules bouncing from wall to wall. Only then can you lower heat transfer by lowering pressure. But once you reduce that by a factor 10 or 100, other transfer mechanisms will dominate.