Kilogram Reference Losing Weight

doubleacr writes "Ran across a story on CNN that says the "118-year-old cylinder that is the international prototype for the metric mass, kept tightly under lock and key outside Paris, is mysteriously losing weight — if ever so slightly. Physicist Richard Davis of the International Bureau of Weights and Measures in Sevres, southwest of Paris, says the reference kilo appears to have lost 50 micrograms compared with the average of dozens of copies.""

Kilogram Reference Losing Weight

[MrYotsuya]

It's not losing weight, it's losing mass!. The kilogram is not a measure of weight, but mass. Silly pound-centric editors :p

Re:The Kilogram is not losing weight

[Meshach]

I thought that originally the kilogram was defined in terms of water, the mass of 10 square cm of water. The meter is defined in terms of the speed of light so that gives an empirical way to define the kg independent of anything else. It would be interesting to see if it has changed relative to that measurement

More fundamental standards

[iamacat]

I am surprised that they are not using more fundamental standards, like the mass of a hydrogen atom. After all, too many things can happen to a chunk of metal - evaporation, oxidation, radioactive decay.

Re:The metre must be shrinking then...

[Anonymous Coward]

Ah, so that explains the obesity epidemic, but my ever increasing middle indicates that the metre must also be shrinking at the same time.

I'm sorry dude, but unlike the kilogram, the metre isn't defined based on an artifact but rather it is defined based on the speed of light, so unless that changed, the metre hasn't either.

Re:Relativity?

[sanyasi]

Thats not relativity. The twin paradox wont degrade the mass over time. It would make it 'younger' according to the situation you described, but not lighter.

Re:More fundamental standards

[AvitarX]

I read an article about this.

It is apparently really hard to get the right amount of atoms reliably and constantly. This is why mass is still using a reference while time and length have ways to reproduce them in a lab (I believe it is measuring the speed of light, and the waves coming ff some substance that is heated up).

There is some work being done making spheres with a silicone chrystal structure, but the margin of error is a few hundred atoms (molecules?), and they wanted it down to around 50. This was a few years ago, things may have changed.

Original article

[Toinou]

The study comes from the BIPM ( international bureau for weights and measures) , and here is the original article : http://www.bipm.org/en/scientific/mass/verifications.html [bipm.org]. In fact it seems to be very old news since the study is carried every 40 years and the last one was in 1992, according to the BIPM :

On three occasions, roughly 40 years apart, the mass of the official copies, the national prototypes and the working standards of the BIPM have been compared with the mass of the international prototype. [...] the last of these occasions (1988-1992) [...]

Re:The Kilogram is not losing weight

[Bender0x7D1]

I thought that originally the kilogram was defined in terms of water, the mass of 10 square cm of water.

We can't use water as a reference since the molecules in the water are constantly splitting into ions and reforming as molecules. So it is essentially impossible to get 1000 cm^3 of "pure" water. It will be some mixture of H2O, H+ and O-- ions. Also, it would be incredibly hard to prevent other molecules from being disolved in the water. A few stray molecules hitting the surface will ruin your reference mass. Not to mention you need a container to keep it in...

The meter is defined in terms of the speed of light so that gives an empirical way to define the kg independent of anything else.

As mentioned above, we could measure a 1000 cm^3 volume, but we couldn't guarantee the purity of the water in that volume.

That's one reason we are trying to make a perfect sphere [slashdot.org] to replace the reference kilogram. Then we will have a definition of the kilogram in terms of number of silicon atoms.

Not any more

[benhocking]

A meter is defined as the distance light travels in a vacuum in 1/299,792,458th of a second.

Re:The Kilogram is not losing weight

[Anonymous Coward]

The meter was originally 1/10,000,000 of the distance between the equator and the North Pole along the meridian running through Paris. (No chauvinism there...) Someone made up a brass reference. Later, the meter spent some time as the distance between a couple of scratches on a platinum-iridium bar. Then we tried a fwe wavelengths of cesium, then krypton-86. Eventually, we adopted a definition for a similar length based on c.

You don't think anyone would really pick a number like 1 / 299,792,458 if they got to start from scratch, do you? Why not 1/300,000,000, just to make the calculations easier? Or, since powers of ten are supposed to be so vital to the system, why not 1/ 10,000,000, or 1/100,000,000?

Ultimately the meter is as long as it is because it's about a yard long, and that's a useful length for measuring on a human scale. It's not "scientific" at all.

Similarly, a kilogram is a useful weight about the same size as a pound. It happens to be about the mass 10 cm cubed of water, much as a pint of water weighs a pound (the world around, and takes 1 BTU to raise temperature by 1 degree F). Later they made a reference standard for this fairly arbitrary amount of mass.

Re:The Kilogram is not losing weight

[TheRaven64]

The density of water changes when you vary the temperature or pressure, so you'd need an accurate measure of distance, temperature, and pressure in order to get your 1Kg of water.

Re:The Kilogram is not losing weight

[snowgirl]

The density of liquid water is essentially invariant relative to temperature. (Given minor fluxuations of entropic internal force.)

Gases are the liquid with varying density.

That being said, it's STP (standard temperature and pressure, or "Schiffkuhlschrank" ("refrigerator on a ship")).

Re:The Kilogram is not losing weight

[Ferzerp]

I think you mean that the density of water is essentially invariant to pressure.

It very much fluctuates with temperature.

Re:Possible reason?

[shawnce]

The simplest way is to use a balance and since the two sides of the balance are in such close proximity to each other any variation of gravity would affect both masses being compared. More complex ways involve measurements of inertia of the masses when a known forces act on the mass.

Bogus story, I think

[fm6]

This entire story (which has appeared on a lot of general news sites, but no science news sites) is probably just a case of a reporter misunderstanding something a scientist said. According to the UK NPL site [npl.co.uk], fluctuations in the physical objects used to define fundamental metric units has always been a problem. Back when they were created, the ideal material for them seemed to be a hard, dense iridium-platinum alloy. This turned out to be a nasty mistake: the alloy is slightly radioactive, which means that some of its mass flies off into space all the time. No mystery there.

This is why most fundamental units are now based on natural constants. For example, the meter used to be the distance between two notches on a platinum-iridium stick. (Before that, it was defined as 1 ten-millionth of a line that goes from the equator to the north pole; except they miscalculated the length of the line!) Now it's based on how far light travels in some tiny amount of time. But there's no consensus as to the best way to get rid of the physical kilogram.

In other words, all we have here is a clueless reporter trying to fill up a slow news day.

Re:The Kilogram is not losing weight

[CrimsonAvenger]

The meter was original defined such that the diameter or radius of the earth at GMT would be a simple power of ten of the meter.

No. The meter was originally defined as one ten-millionth of the distance from the North Pole to the Equator. Which isn't really too far off - less than one fifth of one percent.

Re:tag this "dirtycopies"

[John Hasler]

Yes, it probably is. The copies get handled much more, after all. They are much more likely to have picked up contaminants.

Not a problem

[Anonymous Coward]

This is not a problem. The unit of measure in the metric system, the metre, is defined in terms of speed of light (so it can even measure relativistic distances). Why bother mentioning it? Well in the metric system, a decimeter is 1/10 of a meter. 1 decimeter cubed is called a liter. Now when you fill a 1 liter container (1 dm^3) with pure *pure* water, it weighs exactly (actually by definition) 1 kilogram. Now on this planet we call earth, all you need to do is find *somewhere* 1/1000 of a cubic meter of water (or 1 liter) and somehow make sure that this mysterious 'water' is pure, and it will (by definition) weigh 1 kilogram. Good luck!

Re:The Kilogram is not losing weight

[jmv]

It will be some mixture of H2O, H+ and O-- ions.

I really doubt you'll see O-- ions in water. H2O actually splits into H+ and OH- and the H+ often ends up (IIRC) forming an H3O+ ion [wikipedia.org].

Re:Bogus story, I think

[fm6]

That's the original definition of a kilogram. It's been abandoned. Apparently it's hard to guarantee that water has consistent density.

Re:The Kilogram is not losing weight

[The One and Only]

There's currently a project (which was reported on Slashdot months ago) to make a new reference kilogram of a specific element and geometry. From that, they can define the kilogram as a certain number of atoms of a certain element.

Re:The Kilogram is not losing weight

[Guppy06]

"You could specify the density of water at $PRESSURE and at its maximum density (somewhere around 4 C)."

Self-referential. Pressure is (force)/(length^2), or breaking it down further, (mass)/[(time^2)(length)]. This is why BIPM abandoned the "cubic deciliter of water" definition in favor of the current platinum-iridium artifact (less compressible, less affected by temperature, etc).

"The only problem with doing this for high-precision measurements is: what is water? Some fraction of the hydrogen will be deuterium, and that'll throw off the density. What fraction of the hydrogen should be deuterium for "standard water"?"

Not an issue, as the average rates of naturally occurring isotopes in the universe is already known (hence the non-integer masses in periodic tables). You'd have a greater problem establishing the purity of the water sample in question, at least if you insist on using it in its liquid state; they don't call it the "universal solvent" for nothing.

Re:The Kilogram is not losing weight

[Guppy06]

"We can't use water as a reference since the molecules in the water are constantly splitting into ions and reforming as molecules."

OK, exactly how far up your ass did you have to reach to pull that one out?

See, we have this thing called "The First Law of Thermodynamics." At the molecular scale, water molecules don't just decide to break up and go their own way willy-nilly, not the least because both elements involved (hydrogen and oxygen) really don't like being alone (the two hydrogen atoms can go off on their own merry way as a diatomic molecule, but the oxygen will be lonely). Breaking molecular bonds in water takes energy, otherwise cracking water to produce hydrogen would be more cost-effective than cracking methanol (the carbon atoms have a more independent personality and are better able to get over any rejection issues it might have).

Beyond that, even if the energy to crack an individual water molecule were as trivially small as you believe, the energy would have to come from somewhere. Cracking water is endothermic, but so is making it (oxygen atoms, at least, need to be pried apart against their will first, assuming they're not in some kinky threeway), but even if one of those two reactions was exothermic, the energy required to do one act must necessarily equal the energy released by the other, meaning a net change in energy, and a net change in the number of water molecules, of zero.

The real reasons we don't use water are:

1. Corrosiveness (which you already covered)
2. Compressibility (there is no such thing as an incompressible substance, but liquids are more susceptible than solids)
3. Thermal expansion (something else solids are less susceptible to)
4. Last, but not least: evaporation

"So it is essentially impossible to get 1000 cm^3 of "pure" water."

Very easy, actually; the problem is maintaining its purity after it cools down from superheated steam.

"That's one reason we are trying to make a perfect sphere to replace the reference kilogram. "

Actually, there are a number of different proposals. One involves fixing the Avogadro constant as you say, but the other involves basing mass in terms of an electrical current through a device called a watt balance [wikipedia.org], which would reverse the current relationship between mass and electric current.

Re:Why don't we just do again what we did for the

[Entropius]

If we did physics because it was easy we'd be art history majors.

Re:The Kilogram is not losing weight

[camperdave]

Silly Americans... Your pounds are defined in terms of the kilogram.

The international avoirdupois pound is equal to exactly 453.59237 grams. The definition of the international pound was agreed by the United States and countries of the Commonwealth of Nations in 1958.

Wikipedia [wikipedia.org]

Re:The Kilogram is not losing weight

[gomiam]

Remember this useless thing called pH, used for measuring acidity and basicity? You may remember that the neutral point is 7, the pH of pure water at 25 degrees Celsius, when the amounts of H3O+ and OH- are almost equal (yes, those ions exist even in pure liquid water). See this [wikipedia.org] for more information. Thermodynamics is all right but some of its laws get quirky at sufficiently small distances.

Re:The metre must be shrinking then...

[canadian_right]

The meter has a long history and was in fact once defined as "one ten-millionth of the length of the Earth's meridian along a quadrant, that is the distance from the equator to the north pole". Then it was a number of standard wave lengths and not until 1983 that the meter was defined as how far light travels in a very short time. Wiki has a good article on the meter. [wikipedia.org]

In a vacuum the speed of light is constant - even in a gravitational field as long as your are freely falling.

Re:The Kilogram is not losing weight

[or-switch]

Except that it doesn't matter if it varies or not. Standard Temperature and Pressure (STP) is well defined as 25 degrees C and 1 atmosphere of pressure. There can be precisely controlled. But people who work in the standards groups will always want a solid physical reference that doesn't (or at least shouldn't) vary. The guy overeacted when he said, "Oh no, this could have ramifications for everything including such things as power generation). I doubt a powerstation has any mass measuring system that is precise to 50 ug out of and object weighing 1000000000 ug. Except for the instruments at places like the national institute of standards, very little else has this kind of precision so until it loses something like a whole gram it isn't going ot have a practical consequence.

Re:Not any more

[SorcererX]

Not quite, according to wikipedia "the second is currently defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom."

Re:The Kilogram is not losing weight

[makapuf]

In fact IIRC the definition didn't refer to a specific meridian and neither equator because any country has its OWN meridian, addind to the universality of the definition (which has a main motive then).

Re:Imperial reference units?

[cnettel]

The only reference is conversion into S.I. by well-defined constants, and then trusting the S.I. references.

Re:Not any more

[xYoni69x]

No.

http://en.wikipedia.org/wiki/Beg_the_question [wikipedia.org]

Re:The Kilogram is not losing weight

[turbidostato]

While you are somehow right with the metric system not admiting number 3 so easily, you forget you don't need to be so precise when not needed.

I live on a "metric society" and I can assure you we don't have problems. On the butcher you will ask for a kilogram, half a kilogram, 100 grams, 150 grams, 200 grams, a quarter (250g) or even a "quarter and a half" (which being 250+125=375 grams comes "near enough" to be one third of a kilogram for this kind of practical purpouses).

But regarding woodworking or any other trades, I really doubt you really do in practice what you say. I don't know of anybody on the trade that would really divide any piece of raw material by adding measures: you will always end up adding measure errors on the farest end. You always do it by proportions, so errors get evenly distributed (of course, your average carpenter, or butcher or whatever won't know that, but still will apply some practical recipy that "just works" that probably will be based upon the "even error distribution" principle).

So, going to your example, even if you wanted to hang three hooks on a four meter wall (that makes for a very easy measure of hooking at 1, 2 and 3 exact meters from the beginning) you won't go to one end of the wall, take one meter, hook, take another, hook, take another hook, and then discover your last hook it's clearly out of place, but you'd go to the middle point, hook, then the middle to the right, then the middle to the left and you'll end up better even just by eye-metering.

When dividing by three (or any other odd number) you will see most of the times that disregarding if measuring on decimal or imperial units, people will go using the Thales Principle (projecting a known lenght segment over the one to divide) or the fact than an hexagon's side is exactly the lenght of the radius of the circunscribing circle, or the fact that a twelve units long rope (whatever the units are) will make for you a perfect square angle and will give you 3, 4 and 5 units long segments for free.

All in all, I'm used to the metrical system and I can tell its advantages outweight by faaaaaaaaaaar any minimal problems .