Standard Kilogram Gains Weight 177
mrbluze writes "The standard kilogram weights used by countries around the world for calibration have variably increased in mass by tens of micrograms. This poses a threat to the precision and comparability of measurements in science, engineering and trade. The problem is due to surface contamination, but a safe method of cleaning the weights has only recently been devised by the use of ozone and ultraviolet light (abstract). 'The ultraviolet light-ozone treatment removes hydrocarbon contamination that has built up on the metal surface, gunk that comes from the emissions of an industrial society. Cumpson suspects that because the kilos living in national labs have been retrieved and handled more frequently than the international kilo, more carbon-containing contaminants have built up on them over time. Incubating the kilograms with a set amount of ozone and ultraviolet light "gently breaks up the carbonaceous contamination at the surface."'"
Re:Mititant metric user (Score:5, Informative)
French do not eat bacon and cheese croissant...
This affects all measurement units (Score:3, Informative)
Just to preempt all comments about imperial or home-grown measurement systems: All measurement systems in the world are defined from the metric base units, which are in turn defined from a few physical constants and this kilogram prototype. When the kilogram prototype gains mass, this affects the kilogram, pound, liter and fluid ounce equally.
Re:Why isn't there a precise atomic standard? (Score:5, Informative)
The mass of X number of molecules of element Y = 1 gram.
Like there is for the second [wikipedia.org]:
"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."
Because we don't yet have an accurate-enough measure of the Avogadro or Planck constants.
Revised Standard (Score:5, Informative)
two things... (Score:5, Informative)
Although I'm sure you're kidding, it's probably worth bringing up the following 2 bits of trivia
1. Sadly, the American "pound-weight" has mostly been defined in terms of the kilogram and has its most recent official relationship updated in 1959 (now exactly 0.45359237 kg, down from 0.4535924277 kg back 1901).
2. The kg artifact itself is soon to be rendered obsolete. In 2014, the kg is likely to be redefined in terms of the planck constant (well technically, planck constant will be fixed to a specific number and since it has the units kg*m^2/s, and the second and meter are defined in terms of oscilations of a Ce133 atom and the speed of light, these will now determine the kilogram).
That is until we discover a grand unifying theory where the Planck constant is not actually a constant. Then you can really see the world unravel...
Re:It used to be losing mass (Score:4, Informative)
50g! Forget Avogadro's constant and spheres of silicon, I could do better than that with a brick and a shoebox.
Oh sorry, I typed "50 ug", but I used an ASCII "mu" but it seems to have been eaten by Slashdot and I didn't notice it in the preview. For the record, Slashdot doesn't accept the µ HTML entity either)
Re:Definition (Score:5, Informative)
Re:two things... (Score:3, Informative)
2. The kg artifact itself is soon to be rendered obsolete. In 2014, the kg is likely to be redefined in terms of the planck constant (well technically, planck constant will be fixed to a specific number and since it has the units kg*m^2/s, and the second and meter are defined in terms of oscilations of a Ce133 atom and the speed of light, these will now determine the kilogram).
I suspect that you are a little bit confused. "Planck constant" has no real meaning without agreeing on some units beforehand, and "some specific number" certainly doesnt convey the likely choice.
The Planck Units [wikipedia.org] are based off the 5 known fundamental physical constants of the universe, where each constant is given the non-arbitrary value of exactly 1.0.
Re:It used to be losing mass (Score:5, Informative)
There is no ASCII mu. ASCII is a seven-bit encoding which only covers unadorned latin alphabetic characters, arabic digits, and some random punctuation. Even latin1 (aka ISO8859-1) lacks a mu character. I'm not sure what you think you typed, but it definitely wasn't ASCII.
There's also the problem of potential confusion between U+00B5 MICRO SIGN and U+03BC GREEK SMALL LETTER MU (among others), but neither of those is remotely ASCII.
Anyway, yeah, slashdot sucks when it comes to international character support.
Re:whats the problem (Score:4, Informative)
Re:The meter is based on the speed of light (Score:2, Informative)
The meter is based on the speed of light.
You can't define a distance based on speed.
You need a well defined speed AND TIME unit.
It doesn't get any better from there...
Then zero the scale with that weight, and fill the container with absolutely pure water. The weight of that water is exactly 1 kg. No special reference needed (although you can make a reference from this).
When do you consider it "full"? Water has the nasty habit to form a spherical surface due to surface tension. Do you consider it full when the middle or the border is aligned with the "full" mark on your comtainer? And good luck getting any "absolutely pure" water that does not fall apart into ions and reforms all the time... look up the definition of ph-value. millions of hydrogen ions are making water not acidic.
Re:You are wrong (Score:5, Informative)
Perhaps you should read this document [nist.gov] from NIST about the history of weights and measures in the US.
According to this document...
1827 a troy pound was obtained from London.
1828 a brass artifact (which was compared to this troy pound) declared standard for the US mint, not the avoirdupois pound
1866 the metric system was made lawful for commerce in the US. Legally defines avoirdupois pound as (1/2.2046) kg
1875 17 governments (incl the US) established the international bureau of weights and measures
1890 The US receives standard kilogram artifacts #4 and #20 for use as the national prototype
1894 The US tweaks the definition of the pound relative to this kilogram artifact to make it closer to the UK pound
The US makes various other tweaks over the years in the pound's definition relative to the standard kilogram artifact that the US government maintains.
The "troy" pound artifact is only used for Mint operation in the US and is not related to the avoirdupois pound used in commerce.
Also all NIST calibrations are done in metric units (as of 1959).
Re:begs the question... (Score:5, Informative)
Isn't the definition of a gram the mass of 1 cm3 of water?
No, that was how it was initially defined but in 1799 they moved to using metal prototypes.
Would this not be a standard you could employ in any lab without the need for the international 1Kg prototypes?
It's trickier than it sounds. Between the difficulty of accurately measuring out the liquid, the fact that density changes with temperature and pressure, the fact that isotopic mixes can vary and the fact that our units of pressure are in themselves based on the kilogram making the definition recursive this becomes a difficult definition to apply accurately.
There is talk of moving to a definition based on fundamental physical units but it's difficult to get the experimental results consistent and stable enough (IIRC two different methods for determining avagadros number were giving slighly different results).
Re:begs the question... (Score:5, Informative)
That was the original definition, but it's not precise enough. It's extremely difficult to get water with an exact isotopic composition. VSMOW is used, but even that is not reliably reproducible to the necessary level of precision.
http://en.wikipedia.org/wiki/Vienna_Standard_Mean_Ocean_Water [wikipedia.org]
Also, the density of water is very much related to the temperature and air pressure. Pressure is measured as a unit of force per unit area. Typically, Newtons per squre metre (the Pascal unit). Force, is then in turn defined as a unit of mass times acceleration, with the Newton being 1 kg * 1 ms-2, which obviously results in a cyclically dependent definition, because it would be defined as 1 kg of pure water at a specific temperature and pressure measured in:
Pa = N/m^2 = kg/m*s^2
To get around this problem, you would need to define the Newton in terms of its relationship to other units, ultimately ending up linked to a fundamental constant of nature. The Watt balance approach is trying to do this, by linking the definition with the Ampere. That would reverse the relationship of the Ampere, which is currently defined in relation to the kilogram.
That would then gives a direct way to link those units with the kilogram, and there is no need to precisely measure 1 cubic decimetre of water. You just develop an extremely precise scale that can measure any test mass very precisely and accurately based on the new definition. The difficulty is actually putting that into practice and eliminating as much measurement error as possible. NIST and other laboratories around the world are trying. The problem is, the margin of error in the measurements are still higher than desired.