Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: theThinker on 12/01/2023 10:09:18
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We now (May 2019) have a new SI definition of the base unit of the kilogram. How is one kilogram standard mass being calibrated? Who now issues such mass standards?
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The international prototype (material) mass standard has been abolished and national laboratories are generally encouraged to adopt the new definition for their own primary standard use, but there is no limit on the production of practical secondary and commercial standards that can be calibrated against the new primary, so whatever appears in your factory test lab or local government trading office will be pretty much the same piece of metal with a slightly different number attached to it as time passes.
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Who now issues such mass standards?
The standard is no longer a piece of metal, but a definition.
https://en.wikipedia.org/wiki/Kilogram#Definition
It was issued by
https://en.wikipedia.org/wiki/General_Conference_on_Weights_and_Measures
And, in principle, you can use that definition at home to check if your bag of sugar is the correct mass or not.
In practice, you will probably compare the sugar against a certified mass issued by your country's local legal authority.
For the UK it would be NPL
https://www.npl.co.uk/
For the US I think it's
https://www.nist.gov/pml
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Previously, the secondary mass standard was produced by certain institution based on "that precious platinum-iridium prototype". If I am the chief of my national lab, please tell me how I could produce one sample of a standard 1 kg.
Please give the exact method as the best physicists in my group belong to the fourth world and their Ph.D is from Somalia National University.
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I think they were actually produced by Johnson Matthey.
https://matthey.com/about-us
I presume that, if you paid them, they could do it again.
Please give the exact method as the best physicists in my group belong to the fourth world and their Ph.D is from Somalia National University.
The problem isn't physics.
The problems are engineering, metalurgy etc.
There is, in principle, no metal part of a car engine which couldn't be made by a man with a piece of metal and hand tools- given enough time. But it would be insane to try to do so.
Making a mass comparator would be a comparable problem.
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I think they were actually produced by Johnson Matthey.
https://matthey.com/about-us
I presume that, if you paid them, they could do it again.
Please give the exact method as the best physicists in my group belong to the fourth world and their Ph.D is from Somalia National University.
The problem isn't physics.
The problems are engineering, metalurgy etc.
There is, in principle, no metal part of a car engine which couldn't be made by a man with a piece of metal and hand tools- given enough time. But it would be insane to try to do so.
Making a mass comparator would be a comparable problem.
It seems the "exact" method is a commercial secret.
What if the Vatican claims it is going to produce standard one kilogram mass to sell. Which authority is going to certify its accuracy?
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If I am the chief of my national lab, please tell me how I could produce one sample of a standard 1 kg.
Take a chunk of reasonably non-corroding material and machine it to your best approximation of a kilogram, then weigh it with a Kibble balance and record its exact mass. It doesn't really matter whether your national primary standard is 1.000001 kg or 0.99999 kg, as long as you know its mass to at least an order of magnitude better than your clients need to make their measurements.
But don't assume it's easy! You can buy a Kibble balance and save yourself the 5 years it took my friend Ken to build the first prototype but the general consensus among those who devote their lives to primary standards is "20 man-years per decimal point".
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What if the Vatican claims it is going to produce standard one kilogram mass to sell. Which authority is going to certify its accuracy?
All they have to do is demonstrate how they derived its mass from Planck's Constant, then offer it for sale at a competitive price.Other manufacturers already offer kilograms traceable to primary standards at various levels of accuracy from "trade" via "test" to "research and international comparison", with prices as appropriate.
There is no commercial secrecy about this stuff, just friendly international rivalry to see who can reduce the systematic and random errors to the lowest level. If anyone makes a better balance than Brian Kibble's design, the championship may change hands, but it's a multi-marathon, not a sprint, and the prize is merely your reputation among professional hairsplitters - after 13 years' work in a related field I got half a chapter in a textbook! It is however enormous fun if you enjoy pushing physics and engineering to their limits.
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If I am the chief of my national lab, please tell me how I could produce one sample of a standard 1 kg.
Take a chunk of reasonably non-corroding material and machine it to your best approximation of a kilogram, then weigh it with a Kibble balance and record its exact mass. It doesn't really matter whether your national primary standard is 1.000001 kg or 0.99999 kg, as long as you know its mass to at least an order of magnitude better than your clients need to make their measurements.
But don't assume it's easy! You can buy a Kibble balance and save yourself the 5 years it took my friend Ken to build the first prototype but the general consensus among those who devote their lives to primary standards is "20 man-years per decimal point".
But we already have the Kibble balance before this new standard. You are making the assumption that the Kibble balance can measure mass without recourse to the previously kept "platinum-iridium piece". It may be true, but can you explain the physics behind it.
The only said change that can do away with the prototype is just taking Planck constant as an exact value. It is assumed this changed alone can allow us to measure mass free of any prototype. But how?
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It may be true, but can you explain the physics behind it.
Yes, but Wikipedia saves me the bother.
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It may be true, but can you explain the physics behind it.
Yes, but Wikipedia saves me the bother.
OK. I accept that - I am not going to read that article yet.
Somehow I assume the Kibble balance can measure mass "free of any mass prototype" as long as Planck constant is given an exact value.
Thanks.
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It seems the "exact" method is a commercial secret.
It seems that way to you.
But I already posted the link to the WIKI page which tells you the method, so I don't know why you think it's secret.
Any method you choose will work as long as it refers back to the data there on the values of the fundamental constants.
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What if the Vatican claims it is going to produce standard one kilogram mass to sell. Which authority is going to certify its accuracy?
Any or none. Who cares.
Companies make standard masses.
There's plenty for sale on ebay or whatever.
A competent authority certifies how good they are.
In the lab where I work, we used to get the local trading standards authority to certify our test masses.
They compared our weights to their reference weights and they, in turn, were compared to the NPL's standard kilo.
As long as you have a traceable certified path to the reference it doesn't matter who does the work.
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Somehow I assume the Kibble balance can measure mass "free of any mass prototype" as long as Planck constant is given an exact value.
Yes. That's the point.
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The interesting engineering point is that the Kibble is a force balance, not a mass comparator: it measures the gravitational force on a single object. So you need to know the local value of g to determine its mass. Weirdly, it seems that we can measure the acceleration of a falling object to a greater degree of precision than the observed variance between prototype standard masses.
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The interesting engineering point is that the Kibble is a force balance, not a mass comparator: it measures the gravitational force on a single object. So you need to know the local value of g to determine its mass. Weirdly, it seems that we can measure the acceleration of a falling object to a greater degree of precision than the observed variance between prototype standard masses.
Your recommendation that the details of the method could be in wiki is wrong - they don't give any detail method. I found the detail in a 2015 paper describing the physics underpinning the watt balance (pdf available):
"A LEGO watt balance"
https://www.nist.gov/publications/lego-watt-balance-apparatus-demonstrate-definition-mass-based-new-si
The watt balance will measure a electromagnetic force to counterbalance the mass(weight mg). It happens they have ways to measure the force with a high degree of precision that relies on the quantum Josephson effect and the quantum hall effect. Somehow, the mass could then be obtained through measuring voltages, current, g and having a fixed Planck constant.
To be exact, there is no way that a mass could be determined through the Planck constant. The Planck constant is defined strictly through the relation E = hν. So, there is no relation between Planck constant and any mass. They needed new phenomena and it the Josephson and quantum hall effect. They finally obtain a relation between h, V, I, g and mass m. They could measure V,I and g and by fixing h, they could weigh a mass without any calibrated mass, doing away with the traditional way that cannot escape mass comparison or the simple scale balance.
Defining a standard in a system of units is always about reproducibility and precision. I think 200 years from now, people following the same technique could reproduce this new kilogram standard without error.
I am not too surprise we could now measure g to a high degree of accuracy as our technology is very advanced. Also the earth below should not be moving. I think all these Kibble balances are housed underground, maybe deep to avoid disturbances.
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The Planck constant is defined strictly through the relation E = hν. So, there is no relation between Planck constant and any mass.
There clearly is.
E= MC^2
Therefore M= E/ C^2
Given E= hv
M=hv/C^2
The funny thing is that the scientists think there is, and you think there isn't.
Guess which side is correct.
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I think 200 years from now, people following the same technique could reproduce this new kilogram standard without error.
They don't need 200 years.
Any suitable laboratory could do it now.*
That's the whole point.
You no longer need to worry about the prototype kilogram.
As someone pointed out, if there's a fingerprint on Le Grand K then it still weighs 1 kilo, but the rest of the universe weighs less.
That's absurd.
That's why they redefined the kilo in terms of fundamental constants such as Planck's.
* and that includes a school lab with some lego- as you pointed out. (and thank you for citing that paper- it's fascinating)
It may not be a very precise measurement, but it should be easy to get within 1% or so.
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The Planck constant is defined strictly through the relation E = hν.
Not any more.
"The SI units are defined in such a way that, when the Planck constant is expressed in SI units, it has the exact value
h = 6.62607015×10−34 J⋅Hz−1"
From
https://en.wikipedia.org/wiki/Planck_constant
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The Planck constant is defined strictly through the relation E = hν. So, there is no relation between Planck constant and any mass.
There clearly is.
E= MC^2
Therefore M= E/ C^2
Given E= hv
M=hv/C^2
The funny thing is that the scientists think there is, and you think there isn't.
Guess which side is correct.
E is energy with unit joules. Mass is in kilogram. The two are two different physical dimensions in physics.
E=mc² cannnot be use pedagogically. A photon is energy and never mass. So using the watt balance requires two more physical phenomena, Josephson effect and quantum hall effect .
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The Planck constant is defined strictly through the relation E = hν.
Not any more.
"The SI units are defined in such a way that, when the Planck constant is expressed in SI units, it has the exact value
h = 6.62607015×10−34 J⋅Hz−1"
From
https://en.wikipedia.org/wiki/Planck_constant
You confuse the physical concept of the Planck constant with the operational definition of the constant in a system of units - here the SI unit. We may set the Planck constant to any value we choose.
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I have one conjecture. The Kibble balance or the Watt balance may only be used to calibrate a standard one kilogram in a new SI unit definition. But such balance may not be acceptable as a weighing scale to measure mass in general. It cannot qualify as an acceptable weighing scale in the manner a lab analytical balance or a scale balance may. So you cannot use it to measure the mass of a sample of material in your experiment, say of 312.23 g.
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So you cannot use it to measure the mass of a sample of material in your experiment, say of 312.23 g.
You could, but it wouldn't be sensible.
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The two are two different physical dimensions in physics.
Nobody said that they were not.
Energy and frequency also don't have dimensions of mass, but you are happy enough to use them to define Planck's constant
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So you cannot use it to measure the mass of a sample of material in your experiment, say of 312.23 g.
You could, but it wouldn't be sensible.
I only say the watt balance cannot be acceptable as a general balance to measure mass. On the other hand, the spring balance may be acceptable.
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The two are two different physical dimensions in physics.
Nobody said that they were not.
Energy and frequency also don't have dimensions of mass, but you are happy enough to use them to define Planck's constant
E has the dimension of energy; hν too has the dimension of energy.
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The two are two different physical dimensions in physics.
Nobody said that they were not.
Energy and frequency also don't have dimensions of mass, but you are happy enough to use them to define Planck's constant
E has the dimension of energy; hν too has the dimension of energy.
And mc^2 has units of energy too.
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The two are two different physical dimensions in physics.
Nobody said that they were not.
Energy and frequency also don't have dimensions of mass, but you are happy enough to use them to define Planck's constant
E has the dimension of energy; hν too has the dimension of energy.
And mc^2 has units of energy too.
In E=hν, the LHS E is that of the photon; ν is the frequency of the same photon.
In E= mc², you may combine it with the above provided m is the mass of the photon and LHS E is also the energy of the photon; but a photon only has energy, never mass.
Your algebra is correct, but your physics is wrong.
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but a photon only has energy, never mass.
Due to mass-energy equivalence, photons do have a mass that is dependent on their energy. What they lack is an invariant (rest) mass.
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Your algebra is correct, but your physics is wrong.
The algebra and physics I'm using are the ones which earned Louis de Broglie his Nobel prize.
https://en.wikipedia.org/wiki/Louis_de_Broglie#Duality_of_the_laws_of_nature
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I have one conjecture. The Kibble balance or the Watt balance may only be used to calibrate a standard one kilogram in a new SI unit definition. But such balance may not be acceptable as a weighing scale to measure mass in general. It cannot qualify as an acceptable weighing scale in the manner a lab analytical balance or a scale balance may. So you cannot use it to measure the mass of a sample of material in your experiment, say of 312.23 g.
Which is exactly wrong. It measures any mass in terms of the SI unit of mass. A laboratory balance merely compares two masses and tells you which is the heavier.
Apart, that is, from a Cahn Electrobalance which is a sort of proto-mini-Kibble, using electromagnetic force to offset the gravitational force on the unknown mass. The difference is that it doesn't self-calibrate the current and voltage.
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but a photon only has energy, never mass.
Due to mass-energy equivalence, photons do have a mass that is dependent on their energy. What they lack is an invariant (rest) mass.
@BoredChemist is talking about mass used for the watt balance. Do you use your watt balance to weigh rest mass or mass at speed 299792458 m/s?
I vote against your argument.
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A laboratory balance merely compares two masses and tells you which is the heavier.
For about the last 50 years balances in labs haven't looked like this
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They now typically look like this
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Arguably, it's a pity; but they are much easier to use.
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Do you use your watt balance to weigh rest mass
A fundamental aspect of the Kibble balance is that the mass moves.
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I have one conjecture. The Kibble balance or the Watt balance may only be used to calibrate a standard one kilogram in a new SI unit definition. But such balance may not be acceptable as a weighing scale to measure mass in general. It cannot qualify as an acceptable weighing scale in the manner a lab analytical balance or a scale balance may. So you cannot use it to measure the mass of a sample of material in your experiment, say of 312.23 g.
Which is exactly wrong. It measures any mass in terms of the SI unit of mass. A laboratory balance merely compares two masses and tells you which is the heavier.
It's why I use the word "conjecture". It's for those in this forum to figure out why if they show any interest.
The puzzle is: The spring balance is acceptable, but not the watt balance. But please do not ask for the Nobel prize for this.
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@BoredChemist is talking about mass used for the watt balance. Do you use your watt balance to weigh rest mass or mass at speed 299792458 m/s?
I vote against your argument.
It measures mass at whatever speed you happen to be travelling,as long as there is a nearby large mass that doesn't change much. At the poles, you would be travelling around the sun at about 30 km/second. You may as well vote Liberal for all the good it will do.
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The spring balance is acceptable, but not the watt balance.
To whom? A non-metrologist may prefer the simplicity of a spring balance, but a metrologist would recognise its absurdity.
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It's why I use the word "conjecture". It's for those in this forum to figure out why if they show any interest.
I think I have figured out why you use the word "conjecture".
It's because you know that your idea is unsupported by evidence and thus can not be described as a theory.
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The spring balance is acceptable, but not the watt balance.
They are both acceptable.
They have different fields of application.
A Kibble balance has to be maintained in a vacuum chamber to address the issues of air density and currents.
If you want to weigh a fish before you toss it back in the river you would use a spring balance, rather than a Kibble balance.
Most cooks are quite content to use spring balances- the modern digital ones are good to better than 0.1% so they are also useful as a rough balance in a laboratory.