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International System of Units overhauled in historic vote

npl.co.uk

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Re: International System of Units overhauled in historic vote

#191

Post by azernik »

pfbtgom wrote: post #83

How would you use the new definition to calibrate instruments? My current understanding is that there is a lineage of artifacts calibrated against another artifact until one of those was calibrated against the one true artifact. So how would NIST or another certifying source say that my 1kg standard is 1kg +/- tolerance? Is it anyone with a kibble balance can now certify calibrations? How do you know your kibble bala…

To abbreviate - these definitions change the means by which those calibration artifacts ("transfer standards") are created.

Re: International System of Units overhauled in historic vote

#192

Post by Waterluvian »

kibwen wrote: post #188

See, the US wasn't being stubborn in not adopting metric, it was just waiting patiently for it to progress from beta to 1.0. :) (Fun fact: all the US customary units have been officially defined in terms of their metric counterparts since 1893: https://www.nist.gov/sites/default/files/documents/pml/wmd/m... )

So it's fair to rib my American friends with the fact that Imperial is implemented using metric?

Re: International System of Units overhauled in historic vote

#193

Post by mbreese »

maccam94 wrote: post #173
jackpirate wrote: post #153

Earlier quoted context omitted.

I think the GP is making the claim that Avogadro's constant should be 1 (effectively eliminating it), not that there's no need to count molecules.

An Avogadro's constant number of molecules is one mole. One mole of hydrogen has much less mass than one mole of iron. You have to choose an arbitrary mass of a single element as the base quantity. Hydrogen might be the best theoretically, it's just a proton and an electron, but it is tricky to work with because it's a gas at room temperature. So instead the mole has been defined as the number of molecules in a speci…

I think this is exactly the rationale. But, from a more practical perspective...

If instead of g/mol, we referred to molecules/g -- we would end up populating tables and charts with really big numbers. This would make lookup tables hard to read, difficult to publish, and hard to work with. Imagine if you had to do math with a bunch of 10^23 exponents all of the time.

Instead, it was agreed to effectively pull out a constant value from each of those to make the math significantly easier. Now, instead of dealing with a lot of big numbers, all of the lookup tables could now list smaller g/mol values. And we would be left with just the one single large (Avogadro's) number in the equations.

Honestly, we don't need a set mole constant, but it makes chemistry significantly easier to do so. Unlike the other constants mentioned in the OP, Avogadro's number is completely arbitrary. It could be '1' as the parent suggested, except then it makes the rest of the math more difficult.

Even for this SI overhaul, we didn't really even need to redefine the mole, except for the fact that it was previously defined in terms of the old kg. This was just "fixing a glitch".

Re: International System of Units overhauled in historic vote

#195

Post by amptorn »

3pt14159 wrote: post #12

This is probably the dumbest thing I'll type on HN. In university I just gave up trying to understand why we even needed the Avogadro constant / mole as a fundamental constant. It still confuses me. Why have a difference between molar mass and mass? Why couldn't it just be "1" and everything else change around it?

I don't believe the Avogadro constant needs to be an SI unit. It's like having 12 as an SI unit, or a million, or 1. Sure, it's a useful constant scaling factor, but it doesn't need to be canonized at the heart of the SI system. It doesn't actually express anything fundamental about how we measure our universe.

And if nothing else, it can be derived: a mole is the number of atoms in a kilogram of carbon-12. Done.

Re: International System of Units overhauled in historic vote

#196

Post by gus_massa »

zamadatix wrote: post #102
jbay808 wrote: post #87

Earlier quoted context omitted.

Is there any scale in the world that can measure a kilogram to within the accuracy of a microgram? And if you buy a kilogram of something and they accidentally short you by a few micrograms, haven't you only overpaid by a few parts per billion? Even on a billion dollar order you've only overpaid by a few dollars...

For your first question some Kibble balances have accuracy parts per billion in measurement of the kilogram. For your second question, agreed - no trade deals care about the accuracy of the new kilogram vs old.

I agree that nobody cares for this level of precision in comercial trade, but as a thought experiment ...

According to the first result in Google https://brightside.me/wonder-curiosities/the-16-most-expensi...

> 1. Antimatter — $62.5 trillion per gram

> 2. Californium — $25-27 million per gram

So the difference is a few dollars per milligram of antimatter, and less than a cent per kilogram of Californium.

From https://en.wikipedia.org/wiki/United_States_Bullion_Deposito...

> As of November 2017, Fort Knox holdings are 4,582 metric tons (147.3 million oz. troy), with a market value of over $100 billion.

So the difference is a few hundred dollars.

Re: International System of Units overhauled in historic vote

#197

Post by geekone »

FiveSquared wrote: post #194
yholio wrote: post #23

The actual text of the resolution, for those with low tolerance for dumbed-down PR releases: https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN...

I have low tolerance for long ass papers ;)

But it's only three pages long :(

Re: International System of Units overhauled in historic vote

#198

Post by zamadatix »

24gttghh wrote: post #170
conbandit wrote: post #131

Earlier quoted context omitted.

I believe it means something like: Given the definition of metre and the definition of second, the kilogram is whatever value that makes the Planck constant h precisely 6.62607015×10−34 kg m^2 s^(-1).

But isn't that a bit of a recursive definition? What might one use as the value of kg in that example?

There is only one value a kg could be in that example, everything else is constant. The question I think you're trying to ask is "why 6.62607015×10−34 instead of some other simpler value" and the answer is a bit longer.

Original 1 kg was the mass of a cubic decimeter of water at 4 C at 1 ATM. Why a cubic decimeter at 4 C? Water is densest at 4 C and a cubic decimeter of it is a weight that people can work with on a day-to-day scale. Unfortunately this was a bit hard to measure so they made the IPK (international prototype kilogram) which was a lump of metal. Fast forward 100 years and the lump of metal proved to be too unreliable for modern standards as it kept losing very small amounts of mass, also it Earth's gravity isn't even so it requires you average it out and then calculate the local offset and a whole bunch of other weird things that can mean a microgram or two. This is inconvenient but we still needed a way to say "1 usefull measurement of mass" but unfortunately in the universe 1 Planck's constant is far too small to ever use daily. Thankfully it has become easier to accurately measure Planck's constant against the IPK so now we have solidified 1 kg to be exactly what we measured.

The nice thing is 1 kg will forever be the same thing now and is easy to measure to extraordinary accuracy. The downside I think you're asking about is 1 kg by itself isn't some significant relation of the physical world, it's just a useful-in-daily-life multiple of mass as defined by Planck's constant.

Re: International System of Units overhauled in historic vote

#199

Post by empath75 »

24gttghh wrote: post #170
conbandit wrote: post #131

Earlier quoted context omitted.

I believe it means something like: Given the definition of metre and the definition of second, the kilogram is whatever value that makes the Planck constant h precisely 6.62607015×10−34 kg m^2 s^(-1).

But isn't that a bit of a recursive definition? What might one use as the value of kg in that example?

1

Re: International System of Units overhauled in historic vote

#200

Post by effie »

da_chicken wrote: post #116
3pt14159 wrote: post #12

This is probably the dumbest thing I'll type on HN. In university I just gave up trying to understand why we even needed the Avogadro constant / mole as a fundamental constant. It still confuses me. Why have a difference between molar mass and mass? Why couldn't it just be "1" and everything else change around it?

The one that made it click for me as incredibly useful was Avogrado's Law (now part of the Ideal Gas Law): > Equal volumes of all gases, at the same temperature and pressure, have the same number of molecules. This law, for example, explains why hot air rises. Take two equal quanties of the same gas at the same pressure. They will have the same volume. Now, heat one quantity of gas. By this law, that gas will have a…

> By this law, that gas will have a larger volume at the same pressure.

It will have larger volume, but that is not implied by Avogadro's law. That law is about the surprising property of all gases: no matter the chemical nature of the gases, if they all have same T and P, they all will have same number of molecules per unit volume.

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