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

Why the kilogram is getting heavier (and why a “sun tan” is the remedy)
Artist's rendering of the International Prototype Kilogram (Image: Greg L)
Artist's rendering of the International Prototype Kilogram (Image: Greg L)
View 1 Image
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

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.] 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.
Jim Bowman
I have had the same problem and have picked up lots of extra carbon myself, probably need a little extra UV to help out.
Philip N
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
Richard Dinerman
Wasn't the weight of 1 kilogram equal to 1 liter of water (H2O)?
@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.
I've been telling the wife that I'm not really getting fatter. The bathroom scale must be wrong. Now I have scientific proof.
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!
Cheers Iwesson d'-)
Pieter Rossouw
And a half litre of stout is a lot more delightful than a Pint
Kris Lee
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.
Load More