Redefining the Kilogram

https://www.scientificamerican.com/article/redefining-the-kilogram/

by bushido • 8 years ago

61 46 8 years ago

petepete• 8 years ago

For anyone interested there's an excellent BBC4 documentary on this very subject by Professor Marcus du Sautoy. It's called Precision: The Measure of All Things, unfortunately isn't on iPlayer at the moment but it can be found elsewhere, including DailyMotion.

https://www.dailymotion.com/video/x2hy5l5

guitarbill• 8 years ago

It really is shockingly good. Despite the slightly mundane title, it's interesting and understandable all the way throught. Highly recommended.

jihadjihad• 8 years ago

I think this is good. Not a physicist but a lot of the SI definitions seem contrived and arbitrary [1] rather than based on physical fundamentals.

[1] https://en.wikipedia.org/wiki/SI_base_unit

icebraining• 8 years ago

This will still have some arbitrary constant, much like the meter and the second.

analog31• 8 years ago

Indeed, it's not the arbitrariness that's going away, but the need for special artifacts such as the kg standard.

Patient0• 8 years ago

Fun fact: the metre was was originally defined to be 1/10 millionth the distance from the pole to the equator (so that the circumference of the Earth would be 40 million metres).

dpcx• 8 years ago

Sadly that list is based on data from the 2005 standard. Several of these have since been redefined.

jobigoud• 8 years ago

Can you point to one in particular? We can update the article if it needs to.

JdeBP3• 8 years ago

It's inline with the 8th edition of the SI Brochure, published in 2014. It's not inline with the draft 9th edition.

Double_a_922• 8 years ago

It will be always be arbitrary anyway. Say we used the wavelength of some special light as unit for length... Why did we choose that light, and not something else?

The point with the kilogram redifinition is that it's currently tied to some piece of metal stored in Paris somewhere. We can physically lose the definition.

pintxo• 8 years ago

And we actually do:

„Though the IPK, the current primary artefact, and its replicas are stored in carefully controlled laboratory conditions, their masses have been subject to fluctuation as a result of poorly understood factors, possibly including handling, cleaning and contamination. The IPK has diverged from its replicas by 50 μg since their manufacture late in the 19th century.“

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

krastanov• 8 years ago

We do use resonant frequency of certain atoms to define units, but it is not arbitrary, rather it was a transition chosen for it's extremely small width and ease of measurement. It is quite a bit less arbitrary than using the size of our planet or of an artifact we have made.

And we are getting even better, with definitions based on fundamental constants now, independent of a particular measurement experiment. This is as non-arbitrary as it can get (we will continue using a conversion factor to make the unit human scaled, but that is all).

nraynaud• 8 years ago

Yeah, but there are interesting criteria: an abstract definition for theoretical re-construction of reference artifacts(I don’t know if anyone has ever actually re-built a reference without comparing it to a previously existing reference during development), using integers in the definition where it makes sense, etc.

moftz• 8 years ago

They were created well before the natural units or atomic units were discovered so they are going to based off of arbitrary measurements that were repeatable by scientists of the past. SI is just as arbitrary as imperial but at least it has the advantage of base-10 for the prefixes. The problem is that you can't just change the kilogram to whatever nice natural unit you want overnight so even moving to nice universal constants would still give you some weird coefficients.

jedimastert• 8 years ago

There's also an excellent video from Veritasium, a science and education YouTube channel, about what's being done to define the kilogram

https://www.youtube.com/watch?v=Oo0jm1PPRuo

aequitas• 8 years ago

Also interesting 'Practical Engineering' building a DIY watt scale: https://www.youtube.com/watch?v=ewQkE8t0xgQ

logicallee3• 8 years ago

How were these working copies made?

The article makes it seem like there's some "copy" operation you can apply to the reference artifact, but that seems absurd...

I Googled "working standard kilogram" (no quotes), in case I'd find some article, and I did find only this:

https://www.technology.matthey.com/article/17/2/66-68/

I don't really get a sense after reading, of how these things are used to make more of them.

bo0tzz• 8 years ago

I would assume you just make an object that will be the working copy, and align it's weight with that of the standard.

logicallee3• 8 years ago

How do they align its weight?

anarcat• 8 years ago

For those actually curious of the details of all those changes, Wikipedia has a good overview:

https://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_ba...

nj65537• 8 years ago

NIST has also just released its own article about the redefinition: https://www.nist.gov/si-redefinition/kilogram-kibble-balance

It has quite a few details that give a sense of the engineering challenges involved.

- The Kibble balance has a weighing mode, and a calibration mode; the two are used to cancel out a significant portion of the systematic error introduced by the mechanical system

- Measurements are made in a vacuum to eliminate effects of air buoyancy, which are significant at the desired level of precision.

- One advantage of the wheel configuration is that it allows the supporting cables to move the coil exactly vertically, thereby avoiding the undesirable sideways motion that a coil suspended from a beam balance can experience.

I could go on, but this gives an idea and you can go read it yourself ;)

zvrba• 8 years ago

Can somebody explain to me why we need both kilogram and mole (https://en.wikipedia.org/wiki/Mole_(unit))? One can be defined in terms of the other.

Koshkin5• 8 years ago

Kilogram of any substance weighs the same, whereas the moles of different substances have different weights (numerically equal to the molecular weight of the substance).

Doxin2• 8 years ago

Sure, but can't the kilogram then be easily defined in e.g. carbon-moles?

lanius• 8 years ago

it is roughly five parts in 100 million heavier than all the working standards, which have been leaving behind a few atoms of metal every time they are put on scales

Are the working copies really only shedding such a small number of atoms per use? Just seems like it should be a higher number.

pedrosorio• 8 years ago

Platinum has molar mass ~195g, Iridium ~192g. This means the working copies have between ~5.128 and ~5.208 mol atoms in a kg. Let's round it to 5.2 mol, that's ~3.13 * 10^24 atoms.

Five parts in 100 million is ~1.565 * 10^18 atoms. Sound like a pretty large number to me.

rjp0008• 8 years ago

Well some back of the napkin math, atomic mass of platinum, the heavier constituent of the standard is 195. So more than 5 mols of these atoms are in the block, because 1 mol of pt is 195 grams. So there’s around 5610^23 atoms. So 5 parts per 100 million adds up to a lot. Maybe 1.5*10^15 atoms? It’s been a while since I had chemistry...

Koshkin5• 8 years ago

Unit conversion no longer being an issue, I wish everyone went back to the imperial units. (Computers are not 10-based anyway.)

amanaplanacanal• 8 years ago

I'm trying to imagine what the benefit would be. Could you explain?

ggm• 8 years ago

Parties of two, three, four, eight, and twelve kids can now look forward to simple divisions of cake.

You can "spend a penny" again. Although I haven't seen a pay toilet in over 30 years. (I don't mean the ones you put money on a plate at the entry, I mean one you put money INTO the door)

Koshkin5• 8 years ago

E.g., the largest economy in the world uses them.

Redefining the Kilogram

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