Wednesday, 18 September 2013

The Problem that is the Kilogram

A few weeks back I had a chance to see and hold the Untied Kingdom's official kilogram weight at the National Physics Laboratory in Teddington. Well, I say I held it, more accurate would be to say I held the special sealed unit that contained it. It's not a thing that people actually touch. The more I chatted with its keeper, Dr Stuart Davidson the more intrigued I became.


 
The official UK kilogram
Let's just back up a little and give some background. Le Système International d'Unités, or SI Units gives us the seven fundamental physical units of science. It's ultimately what defines all measurement. Four of the units are everyday and familiar to us all, one is disguised and the last two are a tad obscure. We all know about kilograms, metres, seconds and amps, and the Kelvin is familiar once you realise it is just degrees Celsius with the scale shifted. The mole is a measure of number of atoms in a defined weight of an element, and the last unit was always my favourite. The candela is a measure of luminosity and was initially established as being the brightness of a single candle flame. 

All of these units are now defined with reference to a specific, measurable phenomenon. So, the metre is the distance light travels in a vacuum in 1/299,792,458th of a second. The second is 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. Not particularly easy to get your head around, but you get the picture. 

There is one exception. The kilogram is defined as the weight of a lump of metal stored in a vault in Paris. It is the only unit whose definition relies on an actual, physical artefact. Various copies of this weight have been made over the years and these have been sent around the world for other countries to use. The UK has copy number 18. It was made in 1889 and like the original it's made from platinum and iridium, it's very shiny and it was this I got to see. 

Unfortunately, there is a teensy problem, the kilogram and all the copies are putting on weight. It's only in the order of tens of micrograms, but when we're talking about the object that ultimately calibrates every set of scales in the world, we have a problem. Over time carbonaceous crud is building up on the surface of all the standard kilograms. So, all the kilograms are periodically sent back to Paris for a rub down by a bloke with a chammy leather soaked in alcohol followed by a steam bath. Problem solved? Not quite, as the cleaning process needs a super skilled chamois operative. Only one man is currently up to the job and he may not be around for the next official mass kilo buffing. 

On hearing this my immediate thought was "why not keep the weights under vacuum?" No atmosphere, no atmospheric contaminants, no weight gain, but Dr Stuart quickly pointed out the error of my suggestion. Each standard kilogram is a working artefact that can't spend it's life sealed in a bell jar. By placing the weight under vacuum, you boil off a 1 nanometre thick layer of water that protects the weight from hydrocarbons in the atmosphere. When the weight comes out of its vacuum, these contaminants leap on and the kilogram starts to gain weight at an alarming rate. Vacuum is not the answer.

The kilogram is the metal cylinder in the middle
Fortunately, the team at the NPL have an idea that may be the solution. The weight is bathed in ozone and irradiated with UV light. The UV splits the ozone, creating oxygen free radicals that then set to attacking the carbonaceous crud on the metal. The crud is turned into carbon divide gas and floats away. It's the ultimate in non-contact cleaning.

Until the scientific community can come up with a way of defining the kilogram based on universal constants, we are stuck with our official kilogram weights stored under lock and key. And while that situation remains, we now have hope that we will be able to keep them all weighing the same by putting the on the occasional Ozone / UV diet.

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Science TV Presenter, live show performer, writer, strange prop builder and all round Science Bloke. All opinions expressed are mine alone. Not the BBC's, just mine.