Mass Movement: The kilogram is getting an update [unit of measure]

[Sea Six]

https://www.apnews.com/e0ac7c33674445aca44ca1a3aa9ee38b

SEVRES, France (AP) — The kilogram is getting an update.

No, your bathroom scales won’t suddenly become kinder and a kilo of fruit will still weigh a kilo. But the way scientists define the exact mass of a kilogram is about to change.

Until now, its mass has been defined by the granddaddy of all kilos: a golf ball-sized metal cylinder locked in a vault in France. For more than a century, it has been the one true kilogram upon which all others were based.

No longer.


Gathering in Versailles, west of Paris, governments are expected on Friday to approve plans to instead use a scientific formulation to define the exact mass of a kilo. The change is expected to have practical applications in industries and sciences that require ultra-precise measurements of mass.

And it will mean redundancy for the so-called Grand K, the kilo that has towered above them all since 1889.

Made of a corrosion-resistant alloy of 90 percent platinum and 10 percent iridium , the international prototype kilo has rarely seen the light of day. Yet its role has been crucial, as the foundation for the globally accepted system for measuring mass upon which things like international trade depend.


In an underground laboratory at the International Bureau of Weights and Measures, scientists are working to perfect this cutting-edge balance that will assist with measurements after the International Prototype Kilo, kept in another of the bureau's vaults, goes into retirement. (AP Photo/Christophe Ena).
Three different keys, kept in separate locations, are required to unlock the vault where the Grand K and six official copies — collectively known as “the heir and the spares” — are entombed together under glass bell-jars at the International Bureau of Weights and Measures, in Sevres on the western outskirts of Paris.

Founded by 17 nations in 1875 and known by its French initials, the BIPM is the guardian of the seven main units humanity uses to measure its world : the meter for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for the amount of a substance and the candela for luminous intensity.

Of the seven, the kilo is the last still based on a physical artefact, the Grand K. The meter, for example, used to be a meter-long metal bar but is now defined as the length that light travels in a vacuum in 1/299,792,458th of a second.

“This, if you like, is a moment of celebration because it’s like the last standard remaining from 1875 that will finally be replaced by new innovation,” Martin Milton, the BIPM director, said in an Associated Press interview. “Everything else has been recycled and replaced and improved. This is the last improvement that dates back to the original conception in 1875. So that’s a tribute to what was done in 1875, that it’s lasted this long.”


Although it will no longer be the gold-standard definition of a kilogram, the Grand K and its copies won’t be freed from their safe after the change. Scientists want to keep measuring them occasionally to see whether, over time, their masses change. “It’s a very interesting historical experiment,” Milton said. “We’ve been studying this thing for 140 years and it’s going to be interesting to continue to study it now we have better ways of determining whether its mass has been changing. We just don’t know at the moment." This photo, by AP's Christophe Ena, shows a replica of the precious International Prototype Kilo.
Only exceedingly rarely, and exceedingly carefully, have the BIPM’s master kilos been gingerly taken out so that other kilos sent back to Sevres from around the world could be compared against them, to be sure they were still properly calibrated, give or take the mass of a dust particle or two.

Although many Americans commonly think of weight in pounds and ounces, the United States is officially a kilo country, too: It was one of the original 17 founders of the BIPM in 1875. The United States’ primary kilo is called K20 and was assigned to the country in 1889 by the BIPM, along with another, K4. One kilo is equivalent to 2.2 pounds.

The U.S. also has six other platinum-iridium kilos: K79, 85, 92, 102, 104 and 105. They are all looked after by the National Institute of Standards and Technology, a branch of the U.S. Commerce Department.

To verify their mass, K20 and other kilos from around three dozen other countries were measured in Sevres against the BIPM’s master kilos in a painstaking calibration exercise from 1988 to 1992. K20 was most recently then measured again at the BIPM in 2014.


The artisans who forged, polished and perfected the International Prototype Kilo more than 130 years ago worked with mind-boggling precision and skill to craft the platinum/iridium cylinder. “If we did this work today, we’d probably come up with exactly the same composition. It has turned out to have really stood the test of time very well," Milton said. This AP photo, by Christophe Ena, shows a high-precision weight in use at the International Bureau of Weights and Measures.
Even as humans argued, fought and slaughtered each other by the tens of millions in the 20th century, they shared the kilo. The kilo allocated to China in 1983, as it started to embrace market reforms that subsequently turned it into an economic behemoth, was the first manufactured with ultra-high precision diamond machining. Allocated to Japan in 1894, K39 was later ceded to South Korea in 1958.

The kilo is “a tribute to man’s ability to collaborate,” Milton says. “It’s been called a great work of peace, actually, because it’s one of the areas where all of the states of the world come together with absolutely the same objective.”

The metal kilo is being replaced by a definition based on Planck’s constant, which is part of one of the most celebrated equations in physics but also devilishly difficult to explain . Suffice to say that the update should, in time, spare nations the need to occasionally send their kilos back to Sevres for calibration against the Grand K. Scientists instead should be able to accurately calculate an exact kilo, without having to measure one precious lump of metal against another.

Milton says the change will have applications in computing, manufacturing, pharmaceuticals, the study of climate change and other sciences where precise measurements are required.

“The system will be intrinsically correct by reference to the laws of science, the laws of nature,” he said. “We won’t have to depend on just assuming that one particular object never changes.”

[DAB]

Current definition: The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.


Proposed definition: The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.
A consequence of this change is that the new definition of the kilogram is dependent on the definitions of the second and the metre.

https://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_base_units
yep, that clears things up nicely.

"honey, do i look fat?" "no, you must have the wrong Planck constant this week."

[DAB]

guess they'll have to update the Couric as well.

https://www.urbandictionary.com/define.php?term=Couric

[VatorMan]

We have one of those under the same security as France. Truthfully, it’s about time.

[DAB]

so how did they figure out how heavy to make the first kilo. what did they compare it to? or did they just wing it and hope for the best?

[DAB]

barley corn kernels and water:

http://www.npl.co.uk/reference/faqs/...ss-and-density)

[JRD77VET]

Originally Posted by DAB

barley corn kernels and water:

http://www.npl.co.uk/reference/faqs/...ss-and-density)

bad link

[Sea Six]

Personally, I think those frogs should cut up all the remaining kilos and share them with all the other countries that don't have any.

#RedistributionOfWeightsNOW

[Stevedore]

Proper link:

History of Weighing

[Cybercowboy]

Originally Posted by DAB

so how did they figure out how heavy to make the first kilo. what did they compare it to? or did they just wing it and hope for the best?

I saw a video on this a couple of years ago. IIRC they based the size of the sphere on the composition of specific isotopes of platinum and iridium, in a precise ratio of each (roughly 9:1) so that they would have a very precise molecular spacing when mixed and crystallized. Then they could simply make a sphere of that alloy that is as perfectly spherical as possible to a very precise diameter and therefore volume. Then they would have a known amount of platinum and iridium atoms which, when added up, would mass exactly one kilogram. That's more or less it.

Essentially they said "If a kilogram would be created as a physical object, we could do it by having the exact number of atoms of a certain type make up that object. What would be a good candidate for the element(s) to make up that object so that it would be very stable in size and shape over a certain temperature range, wouldn't have any radioactive decay, and would be robust enough to be handled occasionally without losing or gaining atoms - at least not many?" A platinum/iridium alloy was chosen. The choice of a sphere was made so that there would be no part of it more likely than any other to change in dimension.

[DAB]

Originally Posted by Cybercowboy

I saw a video on this a couple of years ago. IIRC they based the size of the sphere on the composition of specific isotopes of platinum and iridium, in a precise ratio of each (roughly 9:1) so that they would have a very precise molecular spacing when mixed and crystallized. Then they could simply make a sphere of that alloy that is as perfectly spherical as possible to a very precise diameter and therefore volume. Then they would have a known amount of platinum and iridium atoms which, when added up, would mass exactly one kilogram. That's more or less it.

Essentially they said "If a kilogram would be created as a physical object, we could do it by having the exact number of atoms of a certain type make up that object. What would be a good candidate for the element(s) to make up that object so that it would be very stable in size and shape over a certain temperature range, wouldn't have any radioactive decay, and would be robust enough to be handled occasionally without losing or gaining atoms - at least not many?" A platinum/iridium alloy was chosen. The choice of a sphere was made so that there would be no part of it more likely than any other to change in dimension.

from the above link:

"After the 1791 report, measurements were made to decide an appropriate volume of water for the standard of mass. In 1799 it was agreed that the unit should be the mass of one cubic decimetre of water at a temperature of 4 °C, which would be called a kilogram (kg). The mass of one cubic centimetre of water would be called a gram (g)."

so 1000 CC of water at 4C is a kilo.

the standard Kilo cylinder was made to match this mass exactly. that's the basis for the kilo.

[DAB]

Originally Posted by Cybercowboy

I saw a video on this a couple of years ago. IIRC they based the size of the sphere on the composition of specific isotopes of platinum and iridium, in a precise ratio of each (roughly 9:1) so that they would have a very precise molecular spacing when mixed and crystallized. Then they could simply make a sphere of that alloy that is as perfectly spherical as possible to a very precise diameter and therefore volume. Then they would have a known amount of platinum and iridium atoms which, when added up, would mass exactly one kilogram. That's more or less it.

Essentially they said "If a kilogram would be created as a physical object, we could do it by having the exact number of atoms of a certain type make up that object. What would be a good candidate for the element(s) to make up that object so that it would be very stable in size and shape over a certain temperature range, wouldn't have any radioactive decay, and would be robust enough to be handled occasionally without losing or gaining atoms - at least not many?" A platinum/iridium alloy was chosen. The choice of a sphere was made so that there would be no part of it more likely than any other to change in dimension.

from OP, it's a cylinder:

' a golf ball-sized metal cylinder"

[Cybercowboy]

Originally Posted by DAB

from OP, it's a cylinder:

' a golf ball-sized metal cylinder"

Ah, well I saw two videos then and got them mixed together. There is indeed a sphere that is the "roundest object in the world" but it's not the kilogram standard. It's made of silicon and is what the new standard for Avogadros's number and is related to the new kilogram standard.

World's Roundest Object! - YouTube

[Cybercowboy]

Essentially what they are doing is using the new Planck's constant method and pairing it with the silicon-28 method (which redefines Avogadro's constant to be silicon-28 based) and together they will be checks and balances that don't require actual physical objects, only concepts. In the silicon-28 case, it will be the concept of "if you grow a perfect silicon crystal of silicon-28 and make it into a perfect sphere of precisely X diameter, you will have Avogadro's constant of silicon-28 atoms times 1000." Something like that anyway. And it will then be a quick calculation to relate that to a standard kilogram.

[Cybercowboy]

And look what just showed up!

The kg is dead, long live the kg - YouTube

[DAB]

As a protest, I’m going to make something that is one cubit long.