The Future of the Kilo: a Weighty Matter (theguardian.com) 207
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.
"Chaos" is overstated (Score:5, Insightful)
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.
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Unfortunately, the reference kilograms and the master no longer agree.
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The US reference is now only about 0.45 of the master. The liter is also slightly smaller.
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Yeah, who cares?
I mean, only drug dealers use and deal with kilos....it really isn't used for anything else useful.
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Tiny, yes. Known direction, no.
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If the difference has been measured, then they know the direction of the disagreement. What they don't know is which reference is the one that drifted.
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In other words, they have no idea if the grand kg is too heavy or too light. They also don't know exactly how much too heavy or too light since the other references may also have gained or lost mass.
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It does not really matter the origin of the constant, once defined via energy, it is defined. What is interesting is you could kibble and bits, heh, heh. Create a digital transfer of energy by valuing a resources according the the energy it generates compared to other resources, with a constant for the energy, giving it global transfer value.
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And in an instant, literally every recorded mass in the world is wrong.
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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.
... It would be an annoyance to lose the master, but not a disaster.
Remember, they're French. :-)
Re:"Chaos" is overstated (Score:5, Insightful)
the US National Institute of Standards and Technology has a kilogram and a meter
Well, isn't it about time to start using them then?
Re:"Chaos" is overstated (Score:5, Informative)
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They have a meter? What does it measure? Does it measure the length of a metre?
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Given the melting point of the alloy (90% platinum/10% iridium - 1790C), that would have to be one hot fire.
True, but in all seriousness, a reasonbly hot fire would increase the evaporation rate of metal atoms (a real problem now, albeit over longer time periods), fouling up the official standard anyway.
Interesting (Score:2)
I assumed that it would be an electric scale + gravity measurement and not a balance that would ultimately determine the Kilogram.
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I assumed that it would be an electric scale + gravity measurement and not a balance that would ultimately determine the Kilogram.
The kilogram is a unit of mass, not a unit of weight. Mass is constant regardless of gravity (same number of molecules on earth as on the moon or on Jupiter), so although every day "close enough" measurements do measure against gravity to ensure you're getting a kilo of avocados at the grocery store, doing so as a part of defining a measurement of mass defeats half the purpose.
A balance allows the ability to ensure the amount of molecules on the left has the same mass as the right side; a kilo weight on the
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Further reading leads me to believe a "Kibble Balance" is what I have in the past read as being described as an electric scale.
The Kibble Balance looks to be essentially a very precise scale plus gravimeter, which is what I thought would succeed in being the first practical and precise enough option.
set the standard to a single subatomic particle (Score:2)
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Re: set the standard to a single subatomic particl (Score:2)
A good workable definition of pi based on integers is 355/113.
It's so close that it bugs me it isn't used more.
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A good workable definition of pi based on integers is 355/113.
FWIW, using gmp and Rmpfr with 200 (binary) digit lengths, comparing 100-digit representation of pi to 355/113 gives an error of 2.7E-7 .
Aren't you glad you asked?
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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... [aps.org] ) 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 ki
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What do you mean? Ten iMacs or ten MacBooks?
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True, but we aren't talking about machining ingots, we're talking about moving individual particles around, I'd assume in a vacuum.
Make a great Pink Panther movie... (Score:5, Funny)
"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!"
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And the villains should be some "big is beautiful" anachists who's stolen it to pad the reference and redefine them as normal weight. Because that's totally how it works and it'd have instant effect on scales worldwide. And nurses would be oblivious too so skinny people start getting fattened up for severe undernourishment with intravenous HCFS and a prescribed junk food diet. The plot is actually so silly that if you just went all in it might make a good comedy.
Do nanogram quantities require 1 in 10^9 precision (Score:2)
OK, I get it, that some drug gets administered in microgram or perhaps even nanogram quantities.
But does a "kee" of that drug need to be measured to one part in 10^9? You could take that quantity of a drug, "cut" it in two, and keep repeat that process 29 more times to get, say, diluted drug doses containing a nanogram of the drug to 7 percent precision?
How precise do you need to administer a nanogram of active ingredient? Certainly not to 9 sig figs, so do you really need to measure out a kilogram t
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Officer: Sir, you were going 100 mph in a 35 mph zone.
Me: Officer, I was trying something that one day might help us develop better pharmace
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Tell me about it. This is like the 54983289th time the Universe gets rebooted because of your damn drugs.
The "kilo" remains at exactly 1000 (Score:5, Funny)
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The American pound is quite precisely defined. It's exactly 0.45359237 kg.
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Oddly enough, the kilogram is considered the base unit of mass -- not the gram. It's the only such metric unit. There's a standard kilogram, there is no standard gram.
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This is slashdot, kilo changes to 1024 when you add "byte" behind it.
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This is slashdot, kilo changes to 1024 when you add "byte" behind it.
We should switch to kibigrams when we need t ojust cram in a little more extra such as airline baggage fees.
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Which only means he's even more confused... (Score:3)
And I would not be surprised if the next vote there is on re-introducing imperial units, once the Brexit is done.
What about the guys in Italy and Austria (Score:3)
Would you like to know more? [chemistryworld.com]
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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?
The Kibble balance folks got there first.
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Firstly - is the "Watt balance" approach currently better than the "standard mass"; if so by how much, and how long on current trends is it likely to remain good enough?
Secondly - is the "atom count" approach currently better than the "standard mass"; if so by how much, and how long o
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What about the other units? (Score:5, Insightful)
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.
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* candela depends on the definition of kg
Does it? How?
All of the other units are currently being considered for redefinition.
Re:What about the other units? (Score:4, Informative)
> How?
Too lazy to do dimensional analysis?
*sigh*
candela [wikipedia.org] is a source that emits monochromatic radiation of frequency 540e12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
1 Watt [wikipedia.org] = 1 joule per second,
1 Joule [wikipedia.org] = 1 Newton meter.
1 Newton [wikipedia.org] = 1 kg * m/s^2
QED.
So, yeah, candela is a derived unit, not a fundamental unit.
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Yes. A candela is defined in terms of lumens per Watt.
A Watt is 1 J/s (Joule/second), or 1 (N-m)/s, or 1 (kg-m^2)/(s^2) or 1 Volt-Amp.
It's really the fact that these units can be defined in terms of Watts that causes them all to be dependent on the kilogram.
Incidentally,l the imperial system is locked to the metric system as well. An inch is, by definition, exactly 2.54cm (25.4mm). Similarly, there are exact conversions for mass and time.
Length, Mass and
Re:What about the other units? (Score:4, Insightful)
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* Kelvin depends on the definition of kg
How is the triple point of water affected by mass?
These units should be ORTHOGONAL; not dependent on one another.
But that would mean we'd have 2.54 Amp per kilogram, how is that useful?
You don't notice the pattern in your post? (Score:2)
You might notice they don't all depend on each other, they are all defined in terms of the kilogram. That means you can know exactly how much a liter of water weighs. Further, that density of water doesn't change as the accuracy of our measurements improves.
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Not a "clusterfuck" at all, useful for over a century and designed the way it is for good reason.
What is a clusterfuck is amount of kids like you whining thinking you have some special insight.
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No, they should not be orthogonal, because then we'd have a mess of transformational constants everywhere when doing simple calculations.
SI is set up to so that you can easily make calculations directly on the units. No need for constants everywhere. This is the entire point of SI; that every unit is normalized against the other make calculations of many things trivial.
Besides, it would not make sense at all to decouple units from each other. What would be gained from that?
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That's absurd. Defining fundamental units is difficult, problematic (as shown by all the work needed to replace the kilogram definition). Instead, is much more reasonable to have a system of interconnected units.
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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.
yes, except you can't because, you know, physics?
Why does this make it sound like (Score:2)
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What are you blathering on about?
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What are you blathering on about?
I think he's an imperialist, but doesn't realise that the imperial system is simply the matric system with a bunch of annoying conversion constants and weird conventions.
I call bullshit on the article's tabloid claims. (Score:2, Interesting)
> 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, t
bad timing (Score:2)
Dammit, I just bought a new jar of kilogram polish.
https://www.design-engineering... [design-engineering.com]
Re:How do they make this work? (Score:5, Informative)
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.
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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.
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.
I'm not saying
Re:How do they make this work? (Score:5, Informative)
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 [wikipedia.org] 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.
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You missed "and you can determine local g accurately by ...", but realising that has given me the last part of the puzzle.
Presumably it's possible to measure that at a given site by various means, perhaps timing the drop of an object in a vacuum.
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Here's roughly how I would try to explain it.
Take a speaker coil, push the cone up and down and measure both the velocity of the movement and the voltage produced.
Because of the way speaker coils work this allows you to derive the relationship between the current through this coil, and the force that is produced;
Force = (Voltage / Velocity) * Current.
Now drop something and measure the local acceleration due to gravity. Force equals mass * acceleration, so now you can derive the relationship between curr
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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
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Wouldn't you need a vacuum for it to be accurate?
The spark isn't going to go in a straight line if it's travelling through ionised air.
Paper wouldn't be good either, as the conductivity/breakdown of it is going to change when it gets burnt. You may end up with an arc going through the same point on the paper when the object has moved.
Why not just drop an object past 3 sensors. You can calculate the average speeds between each gap based on time and measured distance and derive the acceleration from that. No
Re:Been following this stuff (Score:5, Informative)
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.
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I propose that we set the kilogram equal to exactly 1024 grams to end the confusion once and for all.
Re:Been following this stuff (Score:5, Insightful)
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.
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The purity of water is defined by electrical resistance.
That doesn't work for isotopes.
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anyway, isn't 1 kilogram already defined at 1 L of pure H2O?
At what temperature? Room? Ambient? 0C? At what pressure? Sea-level? Sea-level where? At what ratio of tritiated water to heavy water to protium water? In what electric field? Zero? Ambient? Ambient where? etc, etc.
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Not to mention, what exactly is a "liter"? A kilogram of water?
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A litre of water is exactly 1000 cubic centimetres.
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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?
H2O is no longer the definition of the kilogram (Score:4, Informative)
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).
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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
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I usually laugh when Americans use grams to tell me how much fat is in (eg.) a 12oz steak.
https://www.google.com/search?... [google.com]
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How would you use a device like this to determine the mass if you don't know the exact gravitational constant at the specific location you are at? Or is there a way to determine it that does not rely on mass?
As a matter of fact there is. Drop something in a vacuum and measure the acceleration by measuring it's speed at a known distance.
Re: How does this work? (Score:5, Informative)
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Thanks, I had been wondering the same thing, and the solution is obvious in retrospect.
Presumably the new definition will include the precise reference acceleration to be used. You'll never find such a reference acceleration to use of course, but it's easy enough to adjust the rest of the parameters to compensate for the actual acceleration available.
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Uh, you DO realize that gravity is non-uniform and depends on the distance away from the core, [wikipedia.org] right?
i.e.
The acceleration has its maximum at 3480 km and a value of 10.68 m/s^2
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A kilogram is not a measure of weight. It is a measure of mass. So it's not the weight of 1 liter of water, it is the mass of 1l of water.
Most folks don't make the distinction between mass and weight because in their experience on the earth's surface, they are always the same, or at least close enough that the weighting equipment they have cannot tell the difference. A Kilogram mass weighs a Kilogram under normal conditions experienced by people.
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A kilogram weighs about 9.81 Newtons under normal conditions experienced by people, plus or minus about half a percent.
Since this story is about defining a kilogram in a way that makes it fairly easy to measure billionths of a gram, half a percent is *enormous*.
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I'm not sure quite what you're trying to say, but it sounds like you contradicted yourself in the first sentence. Weight is a force (due to gravity) yes. So weight is measured in units of force. Yes, we often use measures of mass and call them weight colloquially. Since we're talking about high precision measurements, that colloquialism isn't just technically wrong, it's completely inadequate since acceleration due to gravity varies by over half a percent, just on the surface.
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Well, it all depends on just how accurate you need to be. If you are selling fruit, then calibrating your scale with a liter of tap water at room temperature is likely good enough. If you are weighing gold dust, you may need something a bit more accurate.
When we are measuring things, being exact is not possible regardless of how you measure. One needs to know what the acceptable accuracy is for the problem at hand and not waste time and effort on unnecessary precision.
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Certainly. However, such inaccuracies are completely intolerable for many modern scientific purposes, and THOSE are the reason heavily protected standardized references are created. Nobody cares if the grocery store scale is off by a percent or two from the the international standard kilogram, but when you're trying to determine the mass of an electron to six decimal places, it becomes extremely important.
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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 app
Re:Quackery is science now? (Score:5, Funny)
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Take 50 mcg of sufentanil and get back to us on that.
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