Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: meems on 02/12/2017 01:38:15
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So NIST are redefining the kg next year : instead of a lump of plat in Paris, a kg will be defined using a formula based on fundamental constants.... and Earth's surface gravity. And there's the prob I see with this definition. Earth's surface gravity is known to vary slightly over time in every place where its tested. Why base a constant on something that is known to vary? Or has Veri got it wrong when he states g is a part of definition? Will the watt balance give the same definition value of 1kg when performed under different gravitys?
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Will the watt balance give the same definition value of 1kg when performed under different gravitys?
Half of this video was not talking about measuring the kg, but measuring Planck's constant.
- It was necessary to know the value of the local gravity in the sub-basement of the NIST building very accurately in order to calculate Planck's constant very accurately.
- This will be used to define the value of Planck's constant in terms of quantum-defined units like the second, the meter, the Josephson junction voltage and the quantum Hall effect.
However, as I understand the video, the subsequent measurement of mass in other labs will use the defined value of Planck's constant, plus these other quantum-defined values. It will be independent of the value of g in the other lab.
Oops! I left the constant "c" out of Planck's constant :o
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This brings back fond memories of working at the UK National Physical Laboratory. Tom Kibble was the archetypal quiet genius with chalk dust on his tweed jacket, and Ken Hursey, the mechanical engineer who designed the Watt balance*, played piano in my band.
It's true that the Watt balance measures force as a function of h (universal) and g (local), but the argument is that since g can be measured to the required degree of accuracy against the (now fundamental) standards of length and time, instead of defining the kilogram as the mass of an agreed object, we can now measure the mass of any object in terms of two universal constants, h and c.
For convenience I assume that most standards laboratories will retain legal reference kilograms of platinum or silicon, but instead of carting them off to Paris for comparison with what has now become a rather arbitrary lump of metal, they will be able to calibrate them in situ against h and c.
*why does it look like a cathedral clock? Ken also played the organ and his other hobby was horology!
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Will the watt balance give the same definition value of 1kg when performed under different gravitys?
Yes, it does.
Essentially, you get a coil of wire on a balance and a magnet set on the ground
You set the thing swinging- it doesn't matter how, but gravity is the easiest way to do it, and you measure the voltage induced in the coil. That gives you a measure of the strength of the field from the magnet.
That expression includes g in the calculation
Then, you put the "kilo" on the balance and measure the current needed to bring the balance back to "level".
That current depends on the force produced by gravity on the kilo and on the strength of the magnetic field.
The force also depends on stuff like the number of turns and the size of the coils and lots of other stuff that's hard to measure accurately (you may thing it's easy to count the turns in a coil- but how do you account for the wires feeding current to an from the magnet?)
The really clever bit is that all those thing affect the voltage produced when you move the coil through the field of the magnet in exactly the same way that they affect the current needed to give a particular force.
A lot of the "difficult" terms cancel out; unfortunately, g isn't one of them.
So, the value of g needs to be measured in order to calculate the mass, but measuring g is "relatively easy".
https://en.wikipedia.org/wiki/Gravimeter#Absolute_gravimeters
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The vid mentions that local g for the laboratory was measured and modelled before the watt balance was installed. This suggests g is not measured during the measuring of h ( or m ), but instead a value for g is assumed. Local g is known is vary by 14ppb over 24hrs. I've seen graphs from google search that measure local g continuously over years, but can't find them now, lost in the blizzard of google search images. But here's one that shows g varying 14ppb in 1 day. ( light green test in the middle ).
I wonder what their plan is to deal with varying g. If not continuously monitoring g in the lab, they should measure it periodically.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fsgf.rgo.ac.uk%2Fsgf-old%2Fgravimetry%2F2006%2520oct%2520-%25202007%2520feb.jpg&hash=d18c2f190f261eea530a9320ce721a11)
Strikes me they ought to set up another apparatus to be measuring g in the lab at the same time as they are measuring h or m. That should improve precision from 20ppb to around 1ppb. Still, if they only periodically measure lab g, like once a month, a measurement with 20ppb over time accuracy would still be an improvement over the artifact kg wandering by a few ppb per year.
If they are planning on only measuring lab g once and then assuming it won't wander over years, then that's a fail.
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They don't plan to just measure the Kg once and they will have relative gravimeters in place to measure changes in local g.
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They don't plan to just measure the Kg once and they will have relative gravimeters in place to measure changes in local g.
I hope that's true. I wonder if you could release your sources on this instead of choosing to keep them secret. I'm searching but so far I haven't found a word from NIST saying they're planning to measure lab g more than once. Although it does seem unlikely they'll make such a principle error, I've been surprised before about what experts have missed.
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You can find stuff about this sort of work in a journal called metrologica
http://iopscience.iop.org/journal/0026-1394
It used to be free to access it; sadly no longer, but they still let you read bits of it
http://iopscience.iop.org/article/10.1088/1681-7575/aa7820
If you look at the sort of attention to detail these guys have you will see that you are pretty unlikely to spot something they have missed.
These experiments are run by international consortia of experts in their fields.
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For convenience I assume that most standards laboratories will retain legal reference kilograms of platinum or silicon
I expect so, too. They compare them using a mechanical balance (like the traditional statue of Justice (https://en.wikipedia.org/wiki/Lady_Justice)).
And this is another case where the local value of g doesn't matter, because it cancels out.
However, when calibrating scales at the supermarket checkout, they place a known mass on the scales. The local value of g does matter (to a small extent), so they are effectively calibrating the checkout scales for the value of g at that checkout.
Variations of g in parts per billion don't matter for checkout scales!
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No! The local value of g is irrelevant!
When I buy a kilogram of dingo meat, I want a kilogram mass of protein and gristle to schlep back to Blighty. So the nice guy from the National Measurement Institute puts the Oz Standard Kilogram (it's a stuffed wallaby, of course, like everything else in Oz, but one that has been calibrated on a Watt balance) on Fred the Butcher's scale, and twiddles the knob until it reads 1 kg despite the presence of a bloody great lump of rock sticking out of the desert on the neighboring property (less than a thousand miles away). Then I know that my kilogram will equal the Oz kilogram even if g is upside bloody down, mate, and I won't be charged for excess baggage.
The joy of the Watt balance is that it ensures that the Oz kilo is the same is the Pom kilo without having to refer to the Frog kilo. This is really, really important because World Peace and the future of civilisation depends on the mass of a cricket ball being 0.1630 kg whether in Maida Vale or Woolloongabba and the French just don't understand.
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Don't go all Jeremy Clarkson on us big Al.
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It depends on what you consider "calibration" to mean.
I I buy some bathroom scales they are probably accurate enough to do their job without needing "recalibration" because they were calibrated in the factory.
Strictly, if I calibrate them at sea level and then ship them up a mountain, they won't give (quite) the right answer.
If I get a good electronic analytical balance and set it up on the bottom floor of a skyscraper then move it to the top floor, it will no longer be properly calibrated so I would need to calibrate it again.
The practical way of doing that is with a test mass.
The impractical way is with a watt balance (or a Kibble balance to give credit where it's due for a jolly clever invention).
Only national measurement labs will ever bother (well, actually I wouldn't rule out some amateur enthusiast managing it too).
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When great minds come together to deliberate, sometimes cans of laughter are popping loudly just before New Years day.
Joke aside, space ship Earth is getting overcrowded. Perhaps now is the right time, for all fine labs globally, to think in terms of planetary gravity.
But please, keep the cricket ball weight at 0.1630 kg to keep the old game played.
Wishing everybody a happy 2018.
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I always thought that the weight of a cricket ball was 5.5 ounces but now I learn that it may be as much 5.75 such is the wonder of the internet as a source of knowledge
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When great minds come together to deliberate, sometimes cans of laughter are popping loudly just before New Years day.
Joke aside, space ship Earth is getting overcrowded. Perhaps now is the right time, for all fine labs globally, to think in terms of planetary gravity.
But please, keep the cricket ball weight at 0.1630 kg to keep the old game played.
Wishing everybody a happy 2018.
Kg is not a unit of weight.
If they play cricket on the Moon will they use a ball of the same mass, or the same weigh as they use on Earth?
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Future space travellers, will truly have to concern themselves about the surface gravity of their planet of choice, if their cricket game shall be played successfully.
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There are already golf balls on the moon. Nothing to do with overcrowding, but to quote an old song
Flying too high / up there in the sky / is my / idea of nothing to do
and for deep philosophy
Golf is a good walk ruined.
All of which shows something about the American psyche. Never having had an empire, they have no friends to visit and no international games worth playing, so out of sheer boredom they rush off into space and play golf. Snob value? Wentworth is more expensive,and the selection process takes twice as long.
You will need a very special planet to play cricket. If g is too low, every long-hop will turn into a bouncer, but if you increase the mass of the ball, lbw becomes a death sentence. Within the range 30 - 34 ft/sec2, you just move the boundaries until it's difficult to hit a six - like Trent Bridge rather than Lord's.
Coconut matting will serve for a wicket, provided the outfield grass is not carnivorous. But the atmosphere is crucial: water for tea must boil at 99 - 101 °C and the air must be moist enough to prevent the cucumber sandwiches from curling. Not that I'm a Little Englander, by any means: post-match curry or barbecue is just as welcome as fish'n'chips, as long as there's plenty of beer, so the ambient temperature can be anything from lager to stout.