The kilogram is about to be redefined

caveman1917 said:
Can't you use trig? For example a triangle with sides with lengths 3u, 4u and 5u necessarily has a 90 degree angle. If the lengths are accurate then so is the angle.
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You can do reasonably well with determining angles afterwards. But how do you MAKE the angle precise? Especially if it's an interior angle, as it has to be for making a container for water. It's very, very hard to do at the level of precision we're talking about here.

This new move to define the Kg in terms of Planck's constant isn't the first effort to make it artifact-independent. Previous efforts have tried to tie it to Avogadro's number. That effort basically used a sphere of isotopically pure silicon of precisely known volume so that you could in effect count the number of atoms in the sphere. The choice of a sphere wasn't arbitrary: it's actually easier to make a sphere of precise volume than a cube. The reason is that you don't need to worry about angles: just make sure its the same diameter no matter which way you spin it. You can't make a cube to the same level of precision.
 
Ziggurat said:
You can do reasonably well with determining angles afterwards. But how do you MAKE the angle precise? Especially if it's an interior angle, as it has to be for making a container for water. It's very, very hard to do at the level of precision we're talking about here.

This new move to define the Kg in terms of Planck's constant isn't the first effort to make it artifact-independent. Previous efforts have tried to tie it to Avogadro's number. That effort basically used a sphere of isotopically pure silicon of precisely known volume so that you could in effect count the number of atoms in the sphere. The choice of a sphere wasn't arbitrary: it's actually easier to make a sphere of precise volume than a cube. The reason is that you don't need to worry about angles: just make sure its the same diameter no matter which way you spin it. You can't make a cube to the same level of precision.
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Maybe you don't really need a container nor even a cube. You could put water in a larger container that's exactly filled up, submerge your 1 liter object (cube/sphere/...) and measure the water that got displaced instead.
 
caveman1917 said:
Maybe you don't really need a container nor even a cube. You could put water in a larger container that's exactly filled up, submerge your 1 liter object (cube/sphere/...) and measure the water that got displaced instead.
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That's tricky too, because how do you separate that precise volume without spilling any, letting it get exposed to air, or leaving drops behind inside your valves? You can't expose it to air because then you need to worry about dissolved gasses.

The level of precision involved here is mind boggling, even the slightest thing can screw it all up.
 
Ziggurat said:
That's tricky too, because how do you separate that precise volume without spilling any, letting it get exposed to air, or leaving drops behind inside your valves? You can't expose it to air because then you need to worry about dissolved gasses.

The level of precision involved here is mind boggling, even the slightest thing can screw it all up.
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Some of those problems are also going to exist with the container cube. Assume you have such a cube, how are you going to fill it up? Specifically, what is the water going to displace inside the cube? If it's a vacuum then the water will boil, if it's air then the water gets exposed to air. ETA: yeah nevermind about this paragraph, solution is trivial :)

A possibility might be to have a deformable container, think something like a large container with a hole in the top with a balloon attached. As the 1 liter object gets submerged, the water goes into the balloon (expanding the balloon) and you detach the balloon which now has 1 liter of water in it.
 
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caveman1917 said:
A possibility might be to have a deformable container, think something like a large container with a hole in the top with a balloon attached. As the 1 liter object gets submerged, the water goes into the balloon (expanding the balloon) and you detach the balloon which now has 1 liter of water in it.
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None of this is worth doing. Instead of making an incredibly precise volume of something else and then using that to measure the volume of water, just use the thing you made an incredibly precise volume of directly.
 
Ziggurat said:
None of this is worth doing.
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Few things are.

Instead of making an incredibly precise volume of something else and then using that to measure the volume of water, just use the thing you made an incredibly precise volume of directly.
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The point was to accurately get 1 liter of water though:

Ziggurat said:
Plus, getting the volume exact actually isn't trivial either. You might think, just make a cube 10cm on each side. But it's very hard to make a cube where all the sides are exactly 90 degrees apart. Any slight skew, and the volume changes, even if the sides are the perfect length. So even measuring volume precisely is very, very hard to do. Length is much easier to measure than volume.
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The point of a standard is to ensure everyone agrees on the definition. Now if I'm buying a sack of potatoes, I might not worry about a almost non-detectable difference between how the grower and the seller define a kilogram. But if I'm purchasing a potent drug where the difference between non-effective, effective, overdose and lethal may be on the order of micrograms, I want to be damned sure the lab making the drug knows precisely the value of that microgram. In that context, sloshing around a litre of water isn't going to cut it.
 
Blue Mountain said:
The point of a standard is to ensure everyone agrees on the definition. Now if I'm buying a sack of potatoes, I might not worry about a almost non-detectable difference between how the grower and the seller define a kilogram. But if I'm purchasing a potent drug where the difference between non-effective, effective, overdose and lethal may be on the order of micrograms, I want to be damned sure the lab making the drug knows precisely the value of that microgram. In that context, sloshing around a litre of water isn't going to cut it.
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You realize that drug companies have been doing just fine with the IPK standard up until today, right? I mean, I'm sure they're not all weighing their doses by going to Paris to take turns with the IPK artefact.
 
Blue Mountain said:
The point of a standard is to ensure everyone agrees on the definition. Now if I'm buying a sack of potatoes, I might not worry about a almost non-detectable difference between how the grower and the seller define a kilogram. But if I'm purchasing a potent drug where the difference between non-effective, effective, overdose and lethal may be on the order of micrograms, I want to be damned sure the lab making the drug knows precisely the value of that microgram. In that context, sloshing around a litre of water isn't going to cut it.
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caveman1917 said:
You realize that drug companies have been doing just fine with the IPK standard up until today, right? I mean, I'm sure they're not all weighing their doses by going to Paris to take turns with the IPK artefact.
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Here is another video on the subject. Would have agreed with caveman1917 except they do specifically mention drugs in the video. I think the disagreement over the mass of a kilogram was what its 7th digit is. So a person might export a kilogram of a substance and the person who imports it might claim that it has a mass of only 0.999999 kilograms just because of the ambiguity of what a kilogram is.


In future every country can build a machine that can weigh a kilogram and every such machine in the world will agree on the answer. Though I am sure they will test this just to be sure. Then every non standard method of finding mass will have to be calibrated with the new standard.
 
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rjh01 said:
Here is another video on the subject. Would have agreed with caveman1917 except they do specifically mention drugs in the video. I think the disagreement over the mass of a kilogram was what its 7th digit is. So a person might export a kilogram of a substance and the person who imports it might claim that it has a mass of only 0.999999 kilograms just because of the ambiguity of what a kilogram is.


....
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+/- ONE microgram? or is that a nanogram? By an actual scale? How many of those in the commercial world?
 
rjh01 said:
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In future every country can build a machine that can weigh a kilogram and every such machine in the world will agree on the answer. Though I am sure they will test this just to be sure. Then every non standard method of finding mass will have to be calibrated with the new standard.
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Which would require a thermometer to measure the heat in the object to a micro-degree? No, wait. Since you need to check the temp of the object first, THEN add an exact amount of heat to it (measured in nano-joules? ) THEN check the temp to see how much the temp rose, and calculate the mass from there? So each of those instruments ought to be at least one magnitude more accurate than the desired result.

Do those instruments exist in the kilogram +/- .1 microgram range today?
 
caveman1917 said:
You realize that drug companies have been doing just fine with the IPK standard up until today, right? I mean, I'm sure they're not all weighing their doses by going to Paris to take turns with the IPK artefact.
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Using the IPK standard doesn't require taking out the IPK artifact every time, it means using instruments whose calibration can be traced back to the IPK artifact. And the IPK artifact, while not perfect, was already a lot better than measuring a liter of water. It's the liter of water, not the IPK artifact, which doesn't suffice for drugs.
 
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Ziggurat said:
Using the IPK standard doesn't require taking out the IPK artifact every time, it means using instruments whose calibration can be traced back to the IPK artifact.
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Exactly, so why would using the water standard require one to slosh around a liter of water? What's so special about the water standard that one can't use calibrated instruments as well?
 
Blue Mountain said:
The point of a standard is to ensure everyone agrees on the definition. Now if I'm buying a sack of potatoes, I might not worry about a almost non-detectable difference between how the grower and the seller define a kilogram. But if I'm purchasing a potent drug where the difference between non-effective, effective, overdose and lethal may be on the order of micrograms, I want to be damned sure the lab making the drug knows precisely the value of that microgram. In that context, sloshing around a litre of water isn't going to cut it.
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"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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caveman1917 said:
Exactly, so why would using the water standard require one to slosh around a liter of water? What's so special about the water standard that one can't use calibrated instruments as well?
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You could. But in order to do that calibration, you're still going to need a liter of water sloshing around, because that's what the water standard is. And calibrating against a liter of water is a lot harder to do than calibrating against the IPK standard artifact, and a lot less accurate as well. Even if you don't have to do it very often, you will still have lost accuracy.
 
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.
 
The seven SI base units are the meter, kilogram, second, ampere, mole, kelvin, and candela.

They started out with separate physical realizations, but they now have only one. The second is now the base unit for all the others.

The meter was originally 10-7 of the Earth's pole-to-equator distance, then it became the length of a platinum bar in Paris, then a multiple of the wavelength of light from a certain electronic transition in krypton-86, and then in terms of the second by fixing the speed of light in a vacuum.

The second was originally 1/86,400 of a solar day, later clarified to the mean solar day. It then became some small fraction of some specified year, and finally a multiple of the period of radio waves from the ground-state hyperfine transition of cesium-133.

The mole's name is short for "gram molecular weight". It is defined with Avogadro's number, the number of atomic mass units in a gram. The amu was originally the mass of a hydrogen atom, then 1/16 the mass of an oxygen atom, then 1/12 the mass of a carbon-12 atom. Avogadro's number is now fixed, making the amu a fixed number of kilograms.

The kelvin was originally a degree Celsius, defined as 0 = melting point of water and 100 = boiling point of water at sea-level pressure. The Kelvin scale's zero point is absolute zero, and the kelvin's size became 1/273.16 the temperature of the triple point of water. The kelvin is now defined in terms of energy by fixing Boltzmann's constant.

Visible-light luminosity standards were originally lamps with specified construction, and the candela was originally the blackbody luminosity of melting-point platinum, and then some energy flux.

It's something like what happened with energy: when different kinds of energy were shown to be interconvertible, they were given the same units.
 
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