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Counting Down to the New Ampere (2016)

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Re: Counting Down to the New Ampere (2016)

#11

Post by oasisbob »

wohlergehen wrote: post #10

Does anyone know why these apparatuses are usually "inverted", i.e. hang from the ceiling. I've seen the same thing for quantum computers. Is it related to the way they are cooled? Or is it easier to work with somehow?

Swinging wild-ass guess - it might because it makes it easier to damp the equipment against vibrations.

The STM in a surface chemistry lab I worked in for a while was inside a box, and hanging from the roof of the box. By choosing appropriate springs to hang the sensitive bits from and sound proofing the box, it could adequately isolate the scanning table from its environment.

Re: Counting Down to the New Ampere (2016)

#12

Post by pohl »

wohlergehen wrote: post #10

Does anyone know why these apparatuses are usually "inverted", i.e. hang from the ceiling. I've seen the same thing for quantum computers. Is it related to the way they are cooled? Or is it easier to work with somehow?

I'm sharing this prior to reading it:

https://en.m.wikipedia.org/wiki/Dilution_refrigerator

Re: Counting Down to the New Ampere (2016)

#13

Post by marcosdumay »

wohlergehen wrote: post #10

Does anyone know why these apparatuses are usually "inverted", i.e. hang from the ceiling. I've seen the same thing for quantum computers. Is it related to the way they are cooled? Or is it easier to work with somehow?

One thing that I know is that it is easier to dump vibrations by hanging your device with springs than by supporting it above springs.

But I don't think this is relevant here.

EDIT: Yeah, pohl answered it. Vibrations are not relevant.

Re: Counting Down to the New Ampere (2016)

#14

Post by VLM »

pohl wrote: post #12
wohlergehen wrote: post #10

Does anyone know why these apparatuses are usually "inverted", i.e. hang from the ceiling. I've seen the same thing for quantum computers. Is it related to the way they are cooled? Or is it easier to work with somehow?

I'm sharing this prior to reading it: https://en.m.wikipedia.org/wiki/Dilution_refrigerator

The Germans working on the same problem are running at 100 mK aka 0.1 K, so pretty chilly yes

http://iopscience.iop.org/article/10.1088/1681-7575/54/1/S1/...

Re: Counting Down to the New Ampere (2016)

#16

Post by SolarNet »

marcosdumay wrote: post #13
wohlergehen wrote: post #10

Does anyone know why these apparatuses are usually "inverted", i.e. hang from the ceiling. I've seen the same thing for quantum computers. Is it related to the way they are cooled? Or is it easier to work with somehow?

One thing that I know is that it is easier to dump vibrations by hanging your device with springs than by supporting it above springs. But I don't think this is relevant here. EDIT: Yeah, pohl answered it. Vibrations are not relevant.

If you read the article he posted the cooling mechanism is very susceptible to vibrations, which is why they suspend it.

Re: Counting Down to the New Ampere (2016)

#17

Post by klodolph »

perlgeek wrote: post #6

A mostly unrelated question, in case any metrology geeks are around: why is the Kelvin an SI unit? Naively, there seem to be multiple approaches to derive temperature from other, more fundamental units. Like using the thermodynamic definition, 1/T = dS/dE, or using Boltzmann's law to approach temperature from the mean kinetic energy of gas particles. Are none of them suitable for precise measurement?

Maybe we will someday, the kilogram and ampere are a bit higher on the priority list right now. Also remember that S is not a quantity that we can directly measure, so 1/T = dS/dE isn't so useful for measuring T.

The effort to redefine the Kelvin would probably use an acoustic thermometer to measure the speed of sound in a gas. This would fix the Boltzmann constant. It seems a bit silly to do this when the definition of the kilogram is expected to change, though.

In the meantime, triple-point cells are pretty convenient, much more convenient than the international prototype kilogram, or the infinite length of wire used to define the ampere. Also remember that laboratory temperature measurements tend to be much less accurate than voltage or mass measurements. ITS-90 is apparently good to within 100ppm, but 1ppm or better is no problem when you're measuring voltage, mass, or length.

Here's a presentation on thermometry, the acoustic thermometer described is what's used to accurately measure the Boltzmann constant: https://www.youtube.com/watch?v=Irr8fOLtiWc

Re: Counting Down to the New Ampere (2016)

#18

Post by tacon »

wohlergehen wrote: post #10

Does anyone know why these apparatuses are usually "inverted", i.e. hang from the ceiling. I've seen the same thing for quantum computers. Is it related to the way they are cooled? Or is it easier to work with somehow?

The entire apparatus sits in a cryostat (a fancy Thermos bottle) which is filled with liquid helium. In general, one supports things in liquid baths from above.

Re: Counting Down to the New Ampere (2016)

#19

Post by miduil »

pohl wrote: post #12
wohlergehen wrote: post #10

Does anyone know why these apparatuses are usually "inverted", i.e. hang from the ceiling. I've seen the same thing for quantum computers. Is it related to the way they are cooled? Or is it easier to work with somehow?

I'm sharing this prior to reading it: https://en.m.wikipedia.org/wiki/Dilution_refrigerator

Thank you for sharing.

I've started searching for "dilution refrigerator" and found this video, explaining a little bit how ³He–⁴He mixture cooling works and what the applications are. I think the video also helps putting the Wikipedia article in a perspective and vice versa.

Quantum Cooling to (Near) Absolute Zero (2013): https://www.youtube.com/watch?v=7jT5rbE69ho

Re: Counting Down to the New Ampere (2016)

#20

Post by gaze »

wohlergehen wrote: post #10

Does anyone know why these apparatuses are usually "inverted", i.e. hang from the ceiling. I've seen the same thing for quantum computers. Is it related to the way they are cooled? Or is it easier to work with somehow?

It's because in a dilution refrigerator, the bottom of the dilution unit gets cold. So its easier to put the coldest part at the bottom.
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