Sciencemadness Discussion Board

Kilogram Standard

Morgan - 16-11-2018 at 21:00

The world just redefined the kilogram
https://www.vox.com/science-and-health/2018/11/14/18072368/k...

j_sum1 - 16-11-2018 at 21:17

Damn. I was just about to do some cooking. Now I need to change all of my recipes.


Seriously though, it is a significant move and pretty cool. Veritassium has the best simple explanation I have come across.

Dibs on Le Grand K. I would kinda like a big chunk of platinum.

Ubya - 17-11-2018 at 01:22

Quote: Originally posted by j_sum1  
.

Dibs on Le Grand K. I would kinda like a big chunk of platinum.


probably it's going to placed in a museum, still can't have your chunk of platinum, but you will be able to see it in person (and steal it:cool:)

12thealchemist - 17-11-2018 at 01:55

One wonders if they will continue measuring it over the next few decades to see how it changes mass. With enough data, they could extrapolate backwards, and figure out how much 1 kg used to be... and therefore how "wrong" we are.

Edit: formatting

[Edited on 17-11-2018 by 12thealchemist]

CharlieA - 17-11-2018 at 17:20

Nuts!!! I just calibrated my balances with the (student, I think) set of brass weights I bought at an antique store for about $20. :D

fusso - 17-11-2018 at 17:47

Quote: Originally posted by 12thealchemist  
One wonders if they will continue measuring it over the next few decades to see how it changes mass. With enough data, they could extrapolate backwards, and figure out how much 1 kg used to be... and therefore how "wrong" we are.

Edit: formatting

[Edited on 17-11-2018 by 12thealchemist]
If this is true then it wont be moved to a museum. A museum's conditions arent constant enough for the weight.
btw will a PtIr alloy sublime in ambient/such conditions?

unionised - 18-11-2018 at 01:59

They may choose to do long term measurements on the other , secondary, standard kilos dotted round the world, but a back extrapolation would be subject to huge uncertainty.

The Pt/Ir alloy will, in principle, sublime but far too slowly for it to be measurable.
Even the radioactive decay of the Pt will be more "significant".
(0.01 % or so is an isotope with half life of nearly a trillion years)

Tsjerk - 18-11-2018 at 05:03

I think absorption of hydrogen is most significant. Possibly some hydrogen was absorbed in the process of casting the Kilo, which has been released over time.

unionised - 18-11-2018 at 05:22

Do you realise they already do a lot of work on this sort of question?
http://iopscience.iop.org/article/10.1088/0026-1394/48/3/012...

http://iopscience.iop.org/article/10.1088/0026-1394/50/1/27/...

They think that dirt is a major contributor.

Tsjerk - 18-11-2018 at 05:45

You mean it became lighter because they cleaned it too well?

unionised - 18-11-2018 at 05:51

It's impossible to tell.
We do not know if Le Grand Kilo has gained or lost weight.

Tsjerk - 18-11-2018 at 05:59

I know, either the K became lighter or the other kilo's became heavier, but I think they were cleaned the same way.

woelen - 19-11-2018 at 02:59

There are other issues as well with a physical object, which represents a kg. Gravity is not the same on all places on earth. Under a certain area, there may be somewhat more dense rocks, or some place on top of a hill or mountain may have a little less gravity. The effect will only be relevant at sub-ppm levels, but with this kind of precise things it may be relevant.

j_sum1 - 19-11-2018 at 03:18

Actually I believe the effect of gravity fluctuations is an order of magnitude greater than the deviation of all the Grand K copies.

kulep - 19-11-2018 at 10:17

An actual balance really measures the difference in mass to another object so you could use it even on the moon's surface.

An electronic or spring scale measures force, you assume kgf = kg. It is really a newton meter or as known in other languages dynamometer (fr. Dynamomètre, it. Dinamometro. from the greek dýnamis, force, strength, power and, métron, measurement).

Anyways g variations don't really play a role as you will measure using the same equipment in the same place and (almost) at the same time, being one of the measured objects exactly 1kg

unionised - 19-11-2018 at 11:35

Quote: Originally posted by j_sum1  
Actually I believe the effect of gravity fluctuations is an order of magnitude greater than the deviation of all the Grand K copies.

The change in weightwith altitude and latitude etc is much bigger than the disparity between the master and the copies.

A good analytical balance can detect the change in gravity between the top + bottom of a tall building.

None of them affects the mass which is what they are standards of.

j_sum1 - 19-11-2018 at 13:47

Sorry if I was unclear. The various physical kilograms have actually been diverging in mass. I forget the mechanism involved. This effect is small but one of the reasons the change in standard is necessary.

Tje point I was trying to make is that the imprecision caused by variations in the gravitational field (if one is using gravity and weight force to determine mass), these fluctations are far greater effect than the observed discrepancies in mass. Woelen's post was a bit unclear on this.

nezza - 19-11-2018 at 14:25

Yup. Part of a drive to get all KGS standards linked to universal physical constants as the various metal weights/measures change slightly with time.

fusso - 19-11-2018 at 15:07

So that means total # of atoms in the weights still remain unchanged? (neglecting handling,transferring and cleaning processes)

j_sum1 - 19-11-2018 at 16:12

Veritassium has really accessible videos on this whole matter, fusso. I recommend.
You also get to look at some of the artifacts and equipment close up.

In no particular order:
The kg is dead, long live the kg
How we are redefining the kg
Is America actually metric?
World's heaviest weight
Spinning sphere of molten sodium

unionised - 20-11-2018 at 11:39

Quote: Originally posted by j_sum1  
Sorry if I was unclear. The various physical kilograms have actually been diverging in mass. I forget the mechanism involved.


How very unfortunate, nobody else ever knew what the mechanism was, and you have for gotten it.

MrHomeScientist - 20-11-2018 at 12:47

Damn you j_sum, linking to videos that start at the product placement at the end! Veritasium is a good channel, though.


Side rant: I HATE the direction YouTube has taken. Thumbnails and titles are more and more clickbait-y; ad banners before, during and after the video; ads built into the video itself so you can't skip them. People making YouTube their main source of income makes it AWFUL for the viewers, imo.

And **** Audible in particular. I swear everybody is peddling them nowadays. I'm here for science, not audio books. Damn sellouts.

Morgan - 20-11-2018 at 12:53

Quote: Originally posted by unionised  
Quote: Originally posted by j_sum1  
Sorry if I was unclear. The various physical kilograms have actually been diverging in mass. I forget the mechanism involved.


How very unfortunate, nobody else ever knew what the mechanism was, and you have for gotten it.


They don't seem to know what's going on according to this article.
https://phys.org/news/2011-04-scientists-kilo-weight.html

j_sum1 - 20-11-2018 at 14:32

Ok. That's why I have forgoten it -- I never actually knew. I probably recall someone atempting an explanation.