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The Future of the Kilo: a Weighty Matter (theguardian.com)
A lump of metal in a building near Paris has long served as the global standard for the kilogram. That's about to change. From a report: Later this month, at the international General Conference on Weights and Measures, to be held in France, delegates are expected to vote to get rid of this single physical specimen and instead plump to use a fundamental measurement -- to be defined in terms of an electric current -- in order to define the mass of an object. The king of kilograms is about to be dethroned. And crucially much of the key work that has led to the toppling of the Paris kilogram has been carried out at the National Physical Laboratory where the late Bryan Kibble invented the basic concepts of the device that will replace that ingot in the Pavillon de Breteuil. The Kibble balance works by measuring the electric current that is required to produce an electromagnetic force equal to the gravitational force acting on a mass. A second stage allows the electromagnetic force to be determined in terms of a fundamental constant known as the Planck constant which will, in future, be used to define a kilogram. These machines will provide the standard for weighing objects -- and that means no more dusting of old lumps of alloy to ensure they stay pure and accurate.
[...] "One key reason for doing this work is to provide international security," says Robinson. "If the Pavillon de Breteuil burned down tomorrow and the kilogram in its vaults melted, we would have no reference left for the world's metric weights system. There would be chaos. The current definition of the kilogram is the weight of that cylinder in Paris, after all." [...] Another major motivation for the replacement of le grand K is the need to be able to carry out increasingly more and more precise measurements. "Pharmaceutical companies will soon be wanting to use ingredients that will have to be measured in terms of a few millionths or even billionths of a gram," says Prior. "We need to be prepared to weigh substances with that kind of accuracy." Suggested reading: A thread on Twitter which discusses SI units and the redefinition of the kilogram.
[...] "One key reason for doing this work is to provide international security," says Robinson. "If the Pavillon de Breteuil burned down tomorrow and the kilogram in its vaults melted, we would have no reference left for the world's metric weights system. There would be chaos. The current definition of the kilogram is the weight of that cylinder in Paris, after all." [...] Another major motivation for the replacement of le grand K is the need to be able to carry out increasingly more and more precise measurements. "Pharmaceutical companies will soon be wanting to use ingredients that will have to be measured in terms of a few millionths or even billionths of a gram," says Prior. "We need to be prepared to weigh substances with that kind of accuracy." Suggested reading: A thread on Twitter which discusses SI units and the redefinition of the kilogram.
If the Pavillon de Breteuil burned down tomorrow and the kilogram in its vaults melted, we would have no reference left for the world's metric weights system. There would be chaos.
That would absolutely be inconvenient, because it is the master reference.
However, other reference kilograms exist, for example, the US National Institute of Standards and Technology has a kilogram and a meter. These secondary references are sometimes used to compare against the primary reference kilogram to ascertain drift.
It would be an annoyance to lose the master, but not a disaster.
Anyway it will soon be redefined in terms of nonphysical objects so the window of problem is small.
How do they make this work as the force of gravity is not constant over the surface of the Earth? Does it only work in one place?
You use a balance, which works by comparing weights, not a scale, that works by measuring force.
"Somebody has stolen the kilogram ingot - the world is about to be thrown into chaos!"
"Never fear Prime Minister, I, Inspector Clouseau am on the case and will find this horrid thief who has stolen this kilogram of nougat!"
"Ingot"
"Zat is what I said!"
I understand that some journalists from some backward countries used to "pounds" (that are not even well defined enough for precise mass measurements) are confused with words they don't understand, but "The Future of the Kilo" is unspectacular: It continues to be a prefix meaning a factor of exactly 1000. No changes or re-definition planned.
How does that NOT only tell you that you have the same weight on both sides (and by proxy the same mass, under the assumption 'g' isn't changing across the relatively small dimensions of the device) ? If that's so then you still only have a way to compare a new mass to a reference mass.
Because a balance doesn't compare two masses, it compares two forces.
In this case, on one side of the balance, the force is generated using electricity,
with parameters tracing back to fundamental constants, which have defined values.
Once you get that, the explanation on how this can be used to
define a mass normal is not that complicated.
And if you tell me "well, it's a test mass on one side, and then electromagnetic force on the other", then surely the latter is then balancing against local 'g' and we're back to the variable 'g' problem.
But g in fundamental units is m/(s*s), so does not depend on the mass of your
object used to measure it, and thus is independent of the definition of the kg.
A precise measurement of g at the location of your balance can therefore be
used in defining the kg.
I get the reasoning behind looking for an alternative, but I do not see this as a sensible solution. And anyway, isn't 1 kilogram already defined at 1 L of pure H2O?
Depends what you mean by "defined".
Getting the exact temperature and pressure correct is hard. It come down to what is more reproducible with a certain degree of precision, and what effort that entails.
As measurement has become more precise, it is observable that the "exact copies" of the official kilo are drifting slightly differently from the original.
What about the researchers at the National Institute for Metrology Research, Italy, and the Australian Nuclear Science and Technology Organisation who are working on the silicon-28 sphere to redefine the kilogram in terms of the Planck constant by determining Avogadro’s constant?
Would you like to know more?
Sorry, teleporters just kill you and then make a copy. A perfect, soul-less copy.
You keep using this fundamental units, it doesn't mean what you think it means.
The SI system is a complete clusterfuck of "fundamental units":
* Amp depends on the definition of kg
* candela depends on the definition of kg
* Kelvin depends on the definition of kg
* Mole depends on the definition of kg
These units should be ORTHOGONAL; not dependent on one another.
They do need to know the local gravity to measure mass. There are absolute gravimeters that measure the local value of g by dropping an object in a vacuum chamber and measuring its acceleration to very high accuracy using a laser interferometer and an atomic clock. This does not depend on the mass of the test object by General Relatively. See the Wikipedia article on Gravimeter.
And anyway, isn't 1 kilogram already defined at 1 L of pure H2O?
No, it is not. RTFA. It is defined as the mass of a slug of platinum-iridium alloy in Paris.
Where are you going to get a liter of pure H2O? Water contains about 0.1% deuterium and three different stable isotopes of oxygen, all in varying concentrations depending on the source of the water. You could distill it, but never get it completely pure. And how are you going to determine the purity? By weighing it?
Using water as the basis is way worse than using metal, because water evaporates, absorbs gases from the air, absorbs ions from the container, etc.
Uh, you DO realize that gravity is non-uniform and depends on the distance away from the core, right?
i.e.
The acceleration has its maximum at 3480 km and a value of 10.68 m/s^2
because where he lives dozens of inconsistent "pounds" have been used as units, and not only for mass, but still today for a currency of decreasing value and relevance.
And I would not be surprised if the next vote there is on re-introducing imperial units, once the Brexit is done.
Yep.. You can measure the acceleration of any sized mass to sufficient accuracy by dropping it and measuring it's speed. The easy way to do this is to have a spark generator that creates a regularly timed arc from a point of a falling pointed mass to a plate. You put a sheet of paper between the falling weight and the plate and the arc creates little burn marks on the paper. You measure the distances between the marks to calculate the speed of the falling mass at various points along the paper. If you have enough distance and a fast enough period between arcs you can calculate the acceleration of gravity with pretty good precision, even without using a vacuum chamber.
"File to fit, pound to insert, paint to match" - Aircraft Maintenance 101
One of the goals is to eventually tie as many standards as possible to fundamental constants of nature. The meter, for example, is now defined as the distance light travels in a certain amount of time. This means that rather than having a standard for time and for length, you have a single standard, for time, and then the other is related to that by a fundamental constant of nature. One doesn't measure the speed of light anymore, one measures distances in terms light propagation time. Your water-based definition doesn't work for that because the properties of water can't be computed to high precision in terms of fundamental constants of nature. In this case the goal is to move to a mass standard that is expressed in terms of the Planck mass, which is a fundamental constant of nature. So now instead of having separate scales for mass and acceleration, and then needing to measure the G in Newton's law to relate them, there is just one scale used and the other is related by a fundamental constant since the relation of G to the Plank mass is a fundamental one.
Did you hear about the homeopath that didnt take his pill and overdosed?
> If the Pavillon de Breteuil burned down tomorrow and the kilogram in its vaults melted, we would have no reference left for the world's metric weights system. There would be chaos.
Bullshit. There are 6 master copies and over 200 certified copies of the kilogram etalon, each country in the UN received at least one, some more (e.g. Hungary has the #16 copy). Their minute deviations from "Le Grande Kilo" are well known and marked down. (Being physical copies they cannot be perfect). In case of LGK loss, their consensus would re-establish the etalon.
> we would have no reference left for the world's metric weights system
Note that the imperial / customary systems of measurement have no reference whatsoever, even without a hypothetical blaze. UK / USA just says the pound is 0.453 kg, the foot 0.3048 is meters and let the frenchies (the SI) do the heavy lifting. So damn convenient...
These machines will provide the standard for weighing objects -- and that means no more dusting of [sic] old lumps of alloy to ensure they stay pure and accurate.
Providing that "these machines" are constructed and calibrated accurately, and the devices they use in performing their function (ampere, gram-force, et.al.) are themselves positively known. Especially in a world where you can't even depend upon the gravitic 'G' to be constant anywhere on the surface.
That's not how it works. In the new regime, the kilogram will be calculated from the second, defined in terms of the frequency of radiation produced by a particular atomic transition, using fixed values of the speed of light and Planck's constant. It doesn't depend on any kind of machine.
The machine is needed to work out the definition in these terms sufficiently accurately that the actual mass of the kilogram will not change enough to cause any problems when we shift to the new standard. Once the shift is done, the machine is no longer needed (except as one available type of very accurate mass measuring device).
Incidentally G is constant everywhere. g varies
With a laser functioning as "optical tweezers" one can isolate single subatomic particles (electron, proton, well-characterized ions ref: https://journals.aps.org/rmp/a... ) set the standard kilogram to the appropriate number of one of those and bid all your metal alloys under bell-jars bye-bye. That is, define the kilogram to be something like 1e30 electron masses or 6e26 proton masses. whichever is more convenient.
This approach was considered -- there were a lot of attempts to make a reasonably large lump of silicon pure enough and with a perfect enough crystal lattice that the number of atoms in it could be counted to sufficient accuracy, whereupon the mass of one atom of (a specific isotope of) silicon would become the reference. The Kibble balance (which ties the kilogram to Planck's constant and so to the energy of photons of specific wavelengths) got to the required accuracy (required so that the mass of the kilogram didn't change too much when the standard changed) first.
Originally perhaps weight was referenced, but by your own quote that was removed only a few years later, and over 200 years ago.
As for trying to use the old mass-of-water definition to recreate the reference: what kind of water? After all, we now know that there are three stable, naturally occurring isotopes of hydrogen, and three of oxygen, all of which will be present in varying amounts in a sample of distilled water that size, meaning that individual water molecules can potentially vary in mass from approximately 18 amu (1H, 16O) to 24amu (3H, 18O). That's an awful wide range of density to span. Even presuming 99.8(ish)% of the water is composed of H1, O16, the variance in the rest would be unacceptably large for a modern reference mass. No two randomly selected samples will weigh exactly the same amount to the limits of modern measurement tools.
That's the main reason all the reference units have been redefined in terms of absolute constants - we've realized that the world is far less homogeneous and far more volatile than we once imagined, and it's basically impossible for any physical object to be recreated, or even maintained in stasis.
--- Most topics have many sides worth arguing, allow me to take one opposite you.
True, but we aren't talking about machining ingots, we're talking about moving individual particles around, I'd assume in a vacuum.
My Other Computer Is A Data General Nova III.
The kilogram was originally created by weighing 1 L = 1 dm3 of pure water at the temperature of maximum density (about 4degC), but it turns out that this is a fiendishly difficult measurement. Water is liquid, so you need a container, and it evaporates, its density is affected strongly by temperature and weakly by atmospheric pressure, surface tension does odd things, there's such a thing as "heavy water", and so on.
It's difficult to make this measurement to better than 1 part per million. So if two laboratories (which we for simplicity assume can measure lengths and volumes perfectly) both try to derive mass from volume using water, they will only agree to 6 decimal places.
But comparing standard kilogram metal weights can be done to micrograms, which is a few parts per billion uncertainty.
So I can weigh the metal weights relative to each other to 9 decimal places, but relative to water to only 6 decimal places. It's better for everyone if we use one of the metal weights as the definition, because that will let us weigh other metal weights to 9 digits, without affecting weighings of water (which will still be accurate to 6 places).
Metrology standards are routinely redefined in this way when new technology comes along which permits measurements relative to a new standard more precisely than was possible using the old standard. Some scientists work very very hard to measure the new and old standards relative to each other to a precision greater than any previous measurement relative to the old standard, so that no previous measurement is invalidated by the change.
This has already happened to the kilogram. The water-based definition was decided on in 1795. In 1799, after having spent a few frustrating years weighing water, a platinum kilogram weight was created as the standard to be used from then on. (The "Kilogramme des Archives". Platinum was chosen because it's very dense, minimizing "air bouyancy" corrections, and because it's extremely chemically inert, so doesn't rust or corrode.) But pure platinum is a bit soft, and the "Kilogramme des Archives" was getting dinged during weighings.
So in 1875 a new kilogram (the "international prototype kilogram") was made out of a platinum-iridium alloy, which has all of platinum's advantages and is much harder to damage.
Anyway, although we can measure metal weights relative to each other to 1 part per billion, it turns out that if you take two identical such weights, store them very very carefully under identical conditions for 50 years, and then re-weigh them, the relative weights have changed by up to 50 parts per billion!
This is a big problem. We don't know what is causing that change (one plausible suggestion is carbon soot and mercury pollution in the air has been sticking to the surface of the weights) or how much any single weight has changed (we can only measure they relativechanges), but clearly at least some of the weights have changed by at least 50 ppb over the last half-century.
So that is a fundamental limit on how accurately any past measurement in kilograms has been.
The new definition is actually not as good as 1 ppb in a single day, and we'll continue to use metal weights for day-to-day operations, but has the big advantage that it doesn't change over time, so in 20 years' time we'll still be able to reproduce it to 10 ppb accuracy.
These days, we know the maximum density of water isn't quite 1 kg/L (it's 999.97495 g/L at 3.983035degC when using VSMOW). But it's equal within the accuracy of any measurement made prior to the redefinition of the kilogram in 1799, so the redefinitions hurt nothing (and helped a lot).
I propose that we set the kilogram equal to exactly 1024 grams to end the confusion once and for all.
It is the volume of a perfect cube where light can travel one of its edges in exactly one 2997924580th of a second. A second being 9192631770 periods of the exact frequency that most efficiently causes caesium atoms to transition between certain energy levels. Who needs water when you have practical definitions like that? People often ask me why I keep ceasium in my kitchen. Well, how else are you going to accurately measure your ingredients?