How the kilogram has put on weight

Using a state-of-the-art Theta-probe XPS machine experts at Newcastle University, UK, have shown the original kilogram is likely to be tens of micrograms heavier than it was when the first standard was set in 1875. And they say a suntan could be the key to helping it lose weight.

Post-Christmas and most of us are feeling the over-. But take heart - experts at Newcastle University, UK, have shown even the kilogram itself has put on weight. Using a state-of-the-art Theta-probe XPS machine – the only one of its kind in the world – the team have shown the original kilogram is likely to be tens of micrograms heavier than it was when the first standard was set in 1875.

And they say a suntan could be the key to helping it lose weight.

The original kilogram – known as the International Prototype Kilogram or the IPK – is the standard against which all other measurements of mass are set. Stored in the in Paris, forty official replicas of the IPK were made in 1884 and distributed around the world in order to standardise mass. The UK holds replica 18 at the National Physical Laboratory (NPL).

But despite efforts to protect the IPK and its duplicates, industrialisation and modern living have taken their toll on the platinum-based weights and contaminants have built up on the surface. Now Professor Peter Cumpson and Dr Naoko Sano have used cutting-edge (XPS) to analyse surfaces similar to the standard kilogram to assess the build-up of hydrocarbons – and how to remove them.

Publishing their findings this month in the journal of Metrologia, they reveal how giving the kilogram a suntan could be the answer to helping it lose weight. "Statute decrees the IPK is the kilogram," explains research lead Peter Cumpson, Professor of () at Newcastle University. "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. Around the world, the IPK and its 40 replicas are all growing at different rates, diverging from the original.

"We're only talking about a very small change – less than 100 micrograms – so, unfortunately, we can't all take a couple of kilograms off our weight and pretend the Christmas over-indulgence never happened.

"But mass is such a fundamental unit that even this very small change is significant and the impact of a slight variation on a global scale is absolutely huge. There are cases of international trade in high-value materials - or waste - where every last microgram must be accounted for.

"What we have done at Newcastle is effectively give these surfaces a suntan. By exposing the surface to a mixture of UV and ozone we can remove the carbonaceous contamination and potentially bring prototype kilograms back to their ideal weight."

The kilogram is one of the seven SI base units from which all other units can be derived and is the only one which is measured against a physical object – the IPK – all others are standardised against known constants.

The Newcastle team are now moving on to study the addition of mercury from the atmosphere, something Professor Cumpson first identified while working at the NPL in the 1990's. But it is the development of techniques such as XPS which has allowed them to accurately measure how the build up of chemicals such as hydrocarbons can be most effectively removed.

Newcastle University hosts the £3million National XPS service funded by the Engineering and Physical Sciences Research Council (EPSRC).

Using a Theta-probe XPS machine – the only one of its kind in the world - Professor Cumpson and Dr Sano showed how the UV/ozone wash could be used to remove contamination without damaging the platinum surface. "The Theta probe allows us to look at the composition of very thin layers by measuring the angle at which the electrons emerge from it," explains Professor Cumpson.

"Rather like an MRI scanner, it takes a cross section of the material but at an atomic level. The second part of the machine is the Argon cluster ion gun – which fires charged 'droplets', each containing about a thousand Argon atoms – and it is this which makes the Newcastle machine unique.

"The Argon cluster ion gun allows us to analyse organic materials without damaging the inorganic surface, in this case the platinum alloy."

Work is underway internationally in several National Measurement Institutes to find an alternative to the IPK – a standardised value for the kilogram that is not based on a matchbox- sized piece of metal. But until then, the prototype kilograms are what the world relies on for its mass scale.

"If the does put on weight then it's imperative that we understand exactly how the IPK is changing," says Professor Cumpson.


Explore further

Redefining the kilogram

More information: "Stability of reference masses V: UV/ozone treatment of gold and platinum surfaces." Peter Cumpson and Naoko Sano. Metrologia. January 2013.
Journal information: Metrologia

Citation: How the kilogram has put on weight (2013, January 6) retrieved 19 July 2019 from https://phys.org/news/2013-01-kilogram-weight.html
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Jan 07, 2013
It doesn't really matter what it weighs as long as we are all working to the same exact standard

True to some extent. But if we change the standard (even globally) then that makes it difficult to build on research that was conducted prior to that point.
You'd have to be very careful whether you're basing conclusions (or meta studies) on pre-change or post-change papers.

And when it comes down to it: All research builds on previous research.

Jan 07, 2013
Distributed around the world? The standard kilogram will weigh 1% less at the equator, which is far more deviation than a few micrograms. Presumably the inertial mass will remain undisturbed, although it's debatable if that's the same type of mass

Jan 07, 2013
Don't confuse weight with mass.

"The standard kilogram will weigh 1% less at the equator" - kochevnik

Jan 07, 2013
Work is underway internationally in several National Measurement Institutes to find an alternative to the IPK

It would be interesting to follow these efforts. It seems likely we will continue to need ever greater degrees of precision and these physical objects will prove to be ever less useful standards.

Jan 07, 2013
Distributed around the world? The standard kilogram will weigh 1% less at the equator, which is far more deviation than a few micrograms.

This is accounted for. Also note that we're talking about the standard kilogram (mass) - not the standard Newton (weight, force) in this context. This is about he scientific meaning of the kilogram - not the colloquial one.
The mass is the same at the equator as anywhere else. The reference to the stanrdad kilogram having become slightly 'heavier' is misleading. (Yes, it has become heavier - but the real problem is that it has become more massive)

Jan 07, 2013
I don't think it is all that well understood how the prototypes, especially the original, collects mass. But ironically this will make a good study of degradation by the environment.

@AP: The standard is already "reversibly" cleaned and measured 3 times with 2 different methods. [ 3 times, 2 methods: http://en.wikiped...of_units ]

The exact method won't change much, but it definitely merits to maintain control. However, since the -14 meeting (see below) is expected to retire the prototype mass, I wouldn't worry about this. It is a backup work, at best, but interesting on its own I think (see above).

@natello: This is about a cleaning procedure and how it could affect the mass unit up till -14, when the mass definition most likely will change.

[ http://en.wikiped...ew_SI.22 ]

It is not the cause to natter about irrelevant insane crackpot physics.

Jan 07, 2013
A kilogram's mass is decidedly not the same everywhere. At the speed of light, it would have infinite mass. At the equator, the kilogram has a mass-added value because of relativity, because its velocity on the x-axis around the Earth's y-axis at that latitude is over a thousand miles per hour. As the distance to the y-axis decreases at higher latitudes so does the velocity of rotation decrease.

Jan 07, 2013
What manifests this reality on this, the macro level, are particles like leptons, whose boundaries' rotational characteristics are focused around the x-y-z trisection singularity and which help to define the matter that acknowledges this peculiar generational relationship. One wonders what complex of simplicity lies on the Galactic scale.

Jan 07, 2013
At the equator, the kilogram has a mass-added value because of relativity, because its velocity on the x-axis around the Earth's y-axis at that latitude is over a thousand miles per hour. As the distance to the y-axis decreases at higher latitudes so does the velocity of rotation decrease.


The tangential speed maybe 1000 mph, but that is somewhat less than the speed required to relatively affect the mass,,, the difference between the mass at the equator and the mass at one of the poles defies any meaningful difference....

The difference in apparent gravitational acceleration and true gravitational acceleration is many orders of magnitude greater. I think is in the range of 0.01 meters/sec^2. That's many orders of magnitude greater the relativity mass increase.

I think that's right, but may be wrong, so I'd like to hear what someone else has to say on it.

Jan 07, 2013
It would be great to have a unit for mass that is decided by nature, a fundamental constant like that of time and length.

Jan 08, 2013
"Don't confuse weight with mass"

Indeed, but the measurement of this artefact is derived from the weight exerted by the local gravity field. The earths gravity is in a continual state of flux as the core sloshes around. This is a another powerful argument to design an electromagnetic method of measuring mass.

Jan 08, 2013
Indeed, but the measurement of this artefact is derived from the weight exerted by the local gravity field.

Probably the best way would be to measure the spacetime warping of a mass. That way the mass could be connected to the speed of light.
(e.g. by measuring the time dilation that a standardized mass causes when a standardized photon passes by)

However, that would be very crude as the effect is so small (or require a pretty big mass, which may not be very convenient)

Jan 08, 2013
dense vacuum


Dense vacuum? What does that mean?

Jan 08, 2013
It wouldn't work too, if the space-time is already warped with some axions and neutrinos - the lensing effect of the same amount of matter will be just weaker after then.. The principle of my theory is, the space-time lensing is not additive product of mass, but it converges to some saturation value - which is indeed very high, but it prohibits the further space-time curving due the quantum gravity effects and the collapse of matter into singularity.


Are you by any chance related to Zephyr, aka Valeria, aka Yash? Or just a collaborator on the flat-wave-on-the-longitudinal surface theory?

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