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An old artifact kept in a vault outside Paris is no longer the standard kilogram

news.mit.edu

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Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#131

Post by SilasX »

max76 wrote: post #115
jelder wrote: post #81

The new reference kilogram is an engineering marvel, but I’ve always wondered this: why didn’t they just build a reference gram instead? Seems like that would be at least a thousand times easier to pull off.

The reference kilogram is estimated to have lost 50 micrograms over it's lifetime. I think it's safe to say that a reference gram would have lost a similar amount of mass. This would have caused a larger change in the definition of a gram/kilogram over time because the ratio of mass lost in the reference would have been greater. Not much, but still.

Why wouldn't the mass loss scale with the mass of the object? Why would it always be the same mass loss in absolute terms?

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#132

Post by marcosdumay »

crispyambulance wrote: post #105
wongarsu wrote: post #65

Earlier quoted context omitted.

You were also never going to test your scale with a weight that is stored in a vault in Paris. So far you would test with a replica of a replica of that weight, in the best case (getting a copy directly from the local standards body). With the new definition in principle anybody can create an object that weighs exactly one kg and sell it to you.

It's actually really hard to imagine an actual physical apparatus for producing a "reference" Kg mass using the new definition of Kg. I get the feeling this just changes the definition of "Kg", but not how reference Kg's are actually produced?

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

The standard changed because people came out with a apparatus for using it. If there wasn't one, it wouldn't have changed.

But it's fit for measuring micrograms up to grams. One won't measure actual kg on it, at least not currently.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#133

Post by max76 »

SilasX wrote: post #131
max76 wrote: post #115

Earlier quoted context omitted.

The reference kilogram is estimated to have lost 50 micrograms over it's lifetime. I think it's safe to say that a reference gram would have lost a similar amount of mass. This would have caused a larger change in the definition of a gram/kilogram over time because the ratio of mass lost in the reference would have been greater. Not much, but still.

Why wouldn't the mass loss scale with the mass of the object? Why would it always be the same mass loss in absolute terms?

It's hard to say because the mechanisms for the mass loss are not fully known. In fact, we are just guessing that mass loss actually occurred, and how much did occur. How would we have measured the mass change of the reference kilogram? By definition the reference kilogram was the most stable quantity of mass we could have generated at the time, and the most well protected quantity of mass for the duration of it's service as the reference kilogram.

I think mass loss is probably proportional with either the surface area or the surface area of the handled portion. This is just a hypothesis based on what I suspect the mechanism for mass loss is (friction from air resistance and being touched). Now that we have a more accurate way of measuring mass we could repeat the experiment of trying to preserve a quantity of mass, even doing different variations based on quantity, material, conditions, etc. Such an experiment would be relatively expensive and take a long time for good results, but maybe someone curious with funding will do it.

EDIT: The idea that we could have preserved a gram of material, while measuring it's mass every 40 years, for the last 130 years while it only lost 5 picograms (1/1000th of the estimated mass of the reference kilogram lost) is so crazy I can't believe it.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#134

Post by hexane360 »

Izkata wrote: post #114
lelf wrote: post #55

Definition of mole changes too. One mole contains exactly 6.022 140 76×10²³ elementary entities (as opposed to the number of atoms in 12 grams of ¹²C). Effective 20 May 2019.

Eh? Straight 6.022*10^23 is what I learned in highschool chemistry in 2003..

The difference is more epistemological than practical. Before, 6.022x10^23 was the measured value of the conversion factor between a gram (exactly 1/100 of the Prototype Kilogram) and a dalton (exactly 1/12 the mass of a carbon-12 atom). After the redefinition, a dalton is defined as being 1/6.022x10^23th of a gram, and measured to be approximately equal to 1/12 the mass of a carbon-12 atom. In essence, the dependency chain was reversed, so now it all flows from the kilogram outwards.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#136

Post by anticensor »

anvandare wrote: post #30

For the next revision: remove the mole from the SI. It's completely pointless. A number is not a unit! Instead replace it by the bit, the fundamental unit of information.

A SI byte would comprise of 10 bits.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#137

Post by fl0wenol »

crankylinuxuser wrote: post #68
sanxiyn wrote: post #10

Earlier quoted context omitted.

The term of art is "realisation". The article mentions two realisations but does not elaborate: "In practice, there are currently two known methods for measuring such masses with great precision. These are known as the Kibble balance and the single-crystal silicon sphere". You should be able to google from that.

I never understood why a sphere was even in the discussion of a standardized atomic count. Any rational fractions using pi = pi atoms or molecules. And that's absurd to have r=(3V/4pi)^1/3 . No matter how you mess with it, it's irrational to the extreme! Now.. Face-centered or ody-centered cubic follows a nice quadratic expansion for integer based counting. We don't need no steenkin pi's!

Because it's easiest to polish perfectly and verify the diameter of if there's no preferrer orientation. Edges of any kind are liable to wear.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#138

Post by a1369209993 »

garmaine wrote: post #60
jabl wrote: post #33

Earlier quoted context omitted.

> Aside, are the mega and giga prefixes ever used for anything except bits and bytes? Sure, few examples of the top of my head: - Megatons, as in yield of nuclear weapons. Which is a bit odd unit, should be in J, goddammit! - Similarly, energy consumption/production. Commonly used units MWh, GWh, TWh. Again, the proper unit would be J, not Wh. Or Mtoe, for Million tons of oil equivalent.

> Megatons, as in yield of nuclear weapons. Which is a bit odd unit, should be in J, goddammit! That’s what is odd about it? It should be teragrams.

Actually, it should be grams. 51.3162(...) grams, to be somewhat precise.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#139

Post by a1369209993 »

jschwartzi wrote: post #125
saalweachter wrote: post #123

Earlier quoted context omitted.

As a scientist, use scientific notation off the base units; you eliminate opportunities for confusion. The prefixes don't really add anything. The Sun is 1.5 x 10^11 meters from the Earth. Calling it 1.5 x 10^8 kilometers doesn't help you visualize the distance better, nor calling it 150 gigameters. Now you have three numbers that mean the same thing floating around, and if you accidentally write 1.5 x 10^8 m somewhe…

Unless you need to make a dial on a physical piece of equipment. In that case it's helpful not to have to append x10^N to all of your numbers.

Or if you need to write it down anywhere at all; that's why programmers use eN or e+N. Although we really need to get around to standardizing a hexadecimal equivalent.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#140

Post by SilasX »

max76 wrote: post #133
SilasX wrote: post #131

Earlier quoted context omitted.

Why wouldn't the mass loss scale with the mass of the object? Why would it always be the same mass loss in absolute terms?

It's hard to say because the mechanisms for the mass loss are not fully known. In fact, we are just guessing that mass loss actually occurred, and how much did occur. How would we have measured the mass change of the reference kilogram? By definition the reference kilogram was the most stable quantity of mass we could have generated at the time, and the most well protected quantity of mass for the duration of it's se…

Is the concept of a half-life similarly crazy to you?
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