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Why the kilogram is getting heavier (and why a “sun tan” is the remedy)

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January 7, 2013

Artist's rendering of the International Prototype Kilogram (Image: Greg L)

Artist's rendering of the International Prototype Kilogram (Image: Greg L)

According to researchers at Newcastle University in the United Kingdom, the kilogram is very likely getting heavier. How can this be? Mainly because we’re talking about the definitive kilogram, the International Prototype Kilogram (IPK) kept at the International Bureau of Weights and Measures in Paris. But because this is the kilo against which all kilos are defined, in a theoretical sense at least, all kilograms will technically be heavier too.

The IPK is a small cylinder of platinum-iridium alloy, about 39 mm (or an inch and a half) in both height and diameter. Using a unique Theta-probe XPS machine, Professor Peter Cumpson and Doctor Naoko Sano of Newcastle University have analyzed surfaces similar to those of the IPK to quantify the build-up of hydrocarbon contaminants. Their research indicates that the IPK is likely to have gained tens of micrograms in mass since the standard was introduced in 1875.

XPS stands for X-ray Photoelectron Spectroscopy, a process which involves irradiating a material with X-rays, and analyzing the quantity and energy of the electrons that are emitted, granting insights into the surface chemistry of the material, and the differences in it before and after some process (such as cleaning). What makes the researcher’s XPS machine unique is its argon cluster ion gun which emits charged clusters of argon, each containing around a thousand atoms. It’s this component that allows analysis of the organic layer – the gunk – without damaging the underlying inorganic material.

“It doesn’t really matter what it weighs as long as we are all working to the same exact standard – the problem is there are slight differences,” said Cumpson, in a press release put out by the University. “Around the world, the IPK and its 40 replicas are all growing at different rates, diverging from the original.” The 40 replicas Cumpson refers to were made in 1884, and distributed across the globe to help countries conform to the standard.

Though the tens of micrograms may sound insignificant, Cumpson claims any discrepancy between the IPK and its replicas could be problematic. “There are cases of international trade in high-value materials – or waste – where every last microgram must be accounted for,” Cumpson added.

However, the build-up of hydrocarbons can be removed with exposure to a mixture of ultraviolet light and ozone. “What we have done at Newcastle is effectively give these surfaces a suntan,” said Cumpson. “We can remove the carbonaceous contamination and potentially bring prototype kilograms back to their ideal weight, he added.” However, the real breakthrough is the quantifying of the build-up of contaminants through XPS analysis.

The kilogram is the only one of the seven base units of the the International System of Units (SI) to be derived from a physical artifact. The meter, for instance, is defined as distance traveled by light in a vacuum over the course of 1/299,792,458 of a second. Professor Cumpson and Doctor Sano's research may add further weight (if you'll pardon the phrase) to National Institute of Standards and Technology's 2010 proposal that the kilogram should be redefined with reference to the Planck constant.

Source: Newcastle University

About the Author
James Holloway James lives in East London where he punctuates endless tea drinking with freelance writing and meteorological angst. Unlocking Every Extend Extra Extreme’s “Master of Extreme” achievement was the fourth proudest moment of his life.   All articles by James Holloway
12 Comments

How do they actually compare the masses of the 40 other weights to the Standard without transporting them to the same location and "weighing" them at the same time.... (It is hard to weigh something very accurately in different locations, as it is difficult to callibrate a scale and transport it without losing accuracy.. (easier to transport a lump of metal....)

At present we are in Limbo, According to Wikipedia, the kilogram will be standardised to the Planck constant (some how) probably in 2014....

I thought it funny, when looking at a few sites noting that the USA has several different silogram standards (things weighing a kg) made of various materials, ...

[it may be an experiment so that they can choose which one to use in any given circumstance, as the different materials will corrode/exaporate (due to vaporisation of the metal) (or whatever else stuff does..) at different rates.]

...as the USA is the Only country to maintain officially its Imperial (avoirdupois) heritage, (sounds French). (The UK has been metric for decades, though you wouldn't know it to visit... People just love their Miles and Pounds.) lol.

MD
7th January, 2013 @ 09:45 am PST

I have had the same problem and have picked up lots of extra carbon myself, probably need a little extra UV to help out.

Jim Bowman
7th January, 2013 @ 09:52 am PST

I believe the new method of defining the kg is based on a project NMI have been working on using silicone balls. When it works they can sell the old units for scrap. I wonder what the going rate is for Platinum Iridium alloy?

See

http://www.measurement.gov.au/SCIENCETECHNOLOGY/Pages/MassandRelatedQuantities.aspx

Philip N
7th January, 2013 @ 01:30 pm PST

Wasn't the weight of 1 kilogram equal to 1 liter of water (H2O)?

Richard Dinerman
8th January, 2013 @ 01:16 am PST

@Richard Dinerman

And what is the unit of length and how is it defined? The best bet is to find the most stable thing in the universe and base all other units on it.

kwarks
8th January, 2013 @ 06:46 am PST

I've been telling the wife that I'm not really getting fatter. The bathroom scale must be wrong. Now I have scientific proof.

AJ1
8th January, 2013 @ 06:50 am PST

There is amusement that we have kept our language of measure. Look at it this way, you can say that measurements are tools, and you might assume that you pick the right tool for the circumstance at hand. The English did quite well with their measures while they spread around the World. Even we Yanks did pretty good in WW2, and most right sending Apollo to the Moon with our measures. First use of measures on the Moon, English!

For example, IMDB, the International Movie Data Base, for the longest time measured actors in meters. 1.45 and the like. What? Silly. They have now added Feet & Inches, which is far more readily comprehensible. We NEVER measured people in Yards.

When I built a house in Colorado, I had access to use the tools of Feet & Inches. Only a lunatic would choose Yards. Not only did they work great, a beautiful house was the result. Oh, and cold it was up there. Humm? Fahrenheit is based around the Human experience, like most English measures, and most of the time that is what it is all about, the Human experience in the World. 100ºF is hot, 32º cold. I find it most amusing to hear the Canadians go on about a lovely 30ºC day... Centigrade is dandy for the lab. Kelvin is also a useful tool... Fine, use the correct tool for the occasion.

And it is far from being a "Metric" World. In navigation, Nautical Miles are applied. Airliners use Feet & Nautical Miles. Meters, Yards are simply too large a unit when a mere Foot or so can be most important. We never used Yards in this area. In fluid measure, I find the notion of buying little Liters of fuel, just Hobbit sized crazy. A gallon of gas gets you someplace. Never picked Quarts for gasoline.

And what a jumble, speed of light for a meter? Not very 10 like. And seconds? You mean the seconds used for a minute, 60, and 60 minutes to an hour, and 24 hours in a day, and 365 days in a year, that second? Not at all very Metric. The ancients built the Pyramids just fine without seeing a need to go a kind of Metric route. Humm?

And finally, Pints are delightful if they are full of Stout. Cheers!

lwesson
8th January, 2013 @ 09:37 am PST

Cheers Iwesson d'-)

Jetwax
8th January, 2013 @ 06:00 pm PST

lwesson

And a half litre of stout is a lot more delightful than a Pint

Pieter Rossouw
8th January, 2013 @ 09:29 pm PST

@lwesson

You use what you are used to.

Measure 6'1" does not make any sense to me but 185 cm does. But I do agree that using feet may be a good tool for rough estimation.

Then again Fahrenheit scale is total nonsense for me. With Celsius scale it is pretty clear when it starts to get really cold - it is from 0 degree when water freezes.

PS. I'm not British.

Kris Lee
9th January, 2013 @ 06:57 pm PST

@kwarks: "And what is the unit of length and how is it defined? The best bet is to find the most stable thing in the universe and base all other units on it."

Er, that's exactly what they do, as mentioned in the article. Distance is based on the speed of light in a vacuum (about the most stable thing we know), as timed using a caesium clock, which is accurate to about 1 part in 10^14.

@iwesson - what units you like is just what you're used to: the UK didn't really have any trouble converting from gallons to litres. France converted from many weird units to SI. Taken in isolation, some imperial units have practical appeal, however since they don't fit together in any meaningful way, they immediately lose their utility in anything remotely technical as they so create so many opportunities for error. Put it like this: In a car capable of 40mpg(US), how many inches will a quarter of a pound of petrol take you? The number of conversions and arbitrary constants required to work that out is just crazy.

@Pieter Rossouw

It depends where you are! 500ml is smaller than an Imperial pint (573ml), but bigger than a US pint (473ml). A US 'liquid', as opposed a to 'dry' pint is 16 US fluid ounces, a British pint is 20 Imperial fluid ounces, and of course the fluid ounces are different sizes too (1/160th of an imperial gallon vs 1/128th of a US gallon). Yup, really practical and 'human' these imperial units. Now where's my Sérieux?

Random fact: the first major project done by the British Aircraft Corporation using the metric system was Concorde.

Synchro
15th January, 2013 @ 05:18 am PST

@Synchro you're quite right - except a British pint is 568ml not 573!

Oh, and I think the British engineers used feet and inches when designing Concorde while the French engineers used metres and millimetres.

Other than that, no unit of measurement intrinsically makes more sense than any other - it's purely down to what you're used to and to think anything else is to mistake your own opinion for fact.

On a different point I don't really understand why Americans think that the British use mainly metric units.

Our roadsigns are still in miles (and our speed limits in miles per hour).

Our milk and beer as still sold in pints (although the milk cartons hilariously list 0.57l or 1.14l as their capacity - that's one pint or two pints!).

More people know their height in feet and inches (and their weight in stones and pounds), than do in metres / kilograms.

Some things are metric (fuel in litres, for instance) but I'd say we're largely still Imperial-based.

We actually do temperature using both; some people even use metric when it's cold and Imperial when it's hot, as in the sentence "I can't believe it's eighty degrees today - only two months ago it was minus ten"!

Steve Jones
22nd January, 2013 @ 09:46 am PST
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