The kilogram is about to be redefined

The redefinition of the ampere involves fixing the elementary charge. This was done so that with the fixing of Planck's constant, the Josephson-effect constant and the quantum-Hall-effect constant (the von Klitzing constant) both have fixed values. Though this is good for precision measurements of voltage and current, it makes the magnetic permeability of the vacuum a measured quantity.
 
lpetrich said:
pound avoirdupois -- the ordinary English-unit pound, defined in 1963 to be exactly 453.59237 grams. The US pound was earlier defined to be 453.5924277 g, and the Imperial pound was earlier approximately 453.592338 g.

The density of water was a good idea in theory, something universally available, but it proved impractical for getting a lot of precision. So that's why we were stuck with platinum cylinders until recently. The IPK is used as a reference for some secondary mass standards, also platinum-iridium cylinders, and these are in turn used as references for more widely-used standards. I think that with this redefinition, the IPK will join the secondary mass standards.

The most successful measurement of mass with the new definition has been with the Kibble balance, formerly the Watt balance. It measures how much electric current is needed to balance the gravitational force on some massive object. That object's mass is then calculated with the help of the acceleration of gravity at the balance's location. The electric current flows through a coil in a magnetic field, and that field's value is found by wiggling the balance and finding out how much induced voltage it makes.

Voltage is measured using the Josephson effect as a standard, making it proportional to h/e.

Current is measured using voltage and resistance, and the latter is measured using the quantum Hall effect as a standard. Thus giving (h/e) / (h/e^2) = e

The balance effectively measures (voltage) * (current) ~ (electrical power) ~ h. Thus its earlier name.
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Whatever. But now that there is a definitive definition, can any mere mortal actually make a KG standard that is any more accurate than the one in Paris? Or is it all theoretical?
 
casebro said:
Whatever. But now that there is a definitive definition, can any mere mortal actually make a KG standard that is any more accurate than the one in Paris? Or is it all theoretical?
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Maybe not a mere mortal like you or me. But a university and certain other places yes. Then if they weigh something and say it has a mass of x kg and then send it to another country with a similar device they will agree.

The current (old) standard doing this experiment they might disagree on its mass. Now the biggest error may be how accurately they can build these instruments.
 
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rjh01 said:
Maybe not a mere mortal like you or me. But a university and certain other places yes. Then if they weigh something and say it has a mass of x kg and then send it to another country with a similar device they will agree.
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Lots of countries have national standards institutes. They would generally be the ones who would actually be doing this,, and they should indeed be able to calibrate mass measurements independently to high accuracy and in agreement with each other with this new standard.
 
rjh01 said:
Maybe not a mere mortal like you or me. But a university and certain other places yes. Then if they weigh something and say it has a mass of x kg and then send it to another country with a similar device they will agree.

The current (old) standard doing this experiment they might disagree on its mass. Now the biggest error may be how accurately they can build these instruments.
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Seriously, and an un-equivicable answer: Has any uni , or anybody else, actually done it?
 
casebro said:
Seriously, and an un-equivicable answer: Has any uni , or anybody else, actually done it?
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Does not take much work to find the answer is yes. Otherwise they would not have defined the kilogram using it. Here is one result of my quick search

http://www.npl.co.uk/educate-explore/kibble-balance/

His proposal was brought to life in 1978, when the first Kibble balance was made.

Over the next 10 years it was used to make measurements which resulted in the setting of the 1990 conventional values of the von Klitzing and Josephson constants which are used throughout the world for voltage and resistance calibration.

By 1990, Dr Kibble and Dr Ian Robinson built the Mark II Kibble balance which used a circular coil and operated in vacuum conditions.

By 2014, Dr Kibble and Dr Robinson published new methods for building cheaper Kibble balances so that National Measurement Institutes across the world could have access to them. Later the same year, Canada's National Research Council used the Mark II Kibble balance, made at NPL, to measure the Planck constant with an accuracy of nineteen parts per billion: sufficient for the proposed redefinition of the kilogram.
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RecoveringYuppy said:
"Sloshing water" around will cut it, because errors are relative, not absolute.

Let's say a drug company has a standard 1 Kg reference mass that's off by a full gram. That's 1 part in 1,000 or .1%. That difference doesn't lead to a full gram mistake in their definition of the microgram. It leads to a single nanogram discrepancy in their definition. That's no problem and it's 5 or 6 orders of magnitude greater than other errors being discussed in this thread.
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Excellent comment. If their kilogram is off by 0.1%, so too with their microgram. As I see it, that's 1 × 10-⁴ error in 1×10-⁶, for a total of 1×10-¹⁰, or 0.1 nanograms, or 100 picograms. So it's not worth losing sleep over.

Maybe sloshing around a litre of water will cut it after all. :)
 
rjh01 said:
Does not take much work to find the answer is yes. Otherwise they would not have defined the kilogram using it. Here is one result of my quick search

http://www.npl.co.uk/educate-explore/kibble-balance/
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Didn't answer my question.. They made a scale, but did they make a weight standard? That scale weighs to nineteen parts per billion, how can they cut a standard to that magnitude ?

Because without a physical standard one would need to use a Kibble scale for everything.
 
casebro said:
Didn't answer my question.. They made a scale, but did they make a weight standard? That scale weighs to nineteen parts per billion, how can they cut a standard to that magnitude ?
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You misunderstand the process. You don't need to cut a physical standard to that accuracy. You cut a physical standard to something, it doesn't really matter exactly what, and then you measure that physical standard to that accuracy with the Kibble balance. Nothing requires that your calibration weight be 1.0000000000000 kg. If you know your physical standard is 0.313457815784 kg to that level of accuracy, then you can use it to calibrate other scales to that level of accuracy as well.
 
In fact, for the drug companies we're talking about a microgram range standard weight might be most appropriate. And that might mean a cheaper Kibble balance for small weights, not sure how cost scales for this thing.
 
One would not use a Kibble balance for *everything*, just as a primary mass reference. One can use it to calibrate secondary mass references, like the way that the International Prototype Kilogram Pt-Ir cylinder has been used.
 
Brainster said:
I always thought that designing the kilogram as equal to the weight of a liter of water was a particularly brilliant way to ensure a standard value.
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rjh01 said:
It is a good standard. Though trying to get a litre of pure water using certain concentrations of isotopes of hydrogen and oxygen will be very hard. Remember if the measurement is out by one part in 10^7 it is wrong. That is why I suggest it went out in the 19th century.
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You can still use your litre of water as a guide and I have to measure mass on a crude beam balance I have made many times. I then measure the volume of water to get a result. It is accurate enough for many measurements.

People tend to use the term exact too freely. You cannot make anything to an exact measurement. You may be able to measure to a high degree of precision but that is as far as it goes.

The metric system is great to use and it's derivation interesting.

Those French scientists measured the length of 1 degree along a Meridien line, multiplied it by 90, and divided by 10,000,000 to give us the Metre.

A cubic decimetre was called a litre.

A cubic centimetre of water became the unit of mass, the Gram.

The unit of force was defined as the force needed to accelerate a mass of 1 Kilogram at a rate of 1 metre/sec/sec, the Newton.

If a force of 1 Newton pushes for a distance of 1 Metre, a Joule of energy is used.

If a Joule of energy is expended in 1 second the power is a Watt.

What a beautiful and simple system it is. And yet we still have the intransigents clinging to their imperial system with its multitude of different units, bearing little relationship to each other. :(
 
Bump. I missed this by 11 days or so. The definition of a kilogram has been changed as of 20 May 2019.
The proposal was accepted at the 2018 CGPM and, effective from May 20, 2019, the kilogram would be defined by Planck’s constant.
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Ref: https://www.britannica.com/science/kilogram
https://en.wikipedia.org/wiki/Kilogram
https://www.sciencealert.com/it-s-official-the-definition-of-a-kilogram-has-changed

But not all reference have caught up. Examples
https://www.merriam-webster.com/dictionary/kilogram
 
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