Why I love weighing

The UK's kilogram shown in its storage case. It matters more than you think.

The UK’s kilogram shown in its storage case. It matters more than you think. Image Courtesy of NPL

Weighing is probably not the most glamorous field of metrology, but nonetheless it forms the bedrock of many measurements: more than you might think.

I was reminded of this recently while visiting the Scottish Enterprise Technology Park in East Kilbride near Glasgow

I had two visits to make:

  • The first was to the Scottish Universities Environmental Research Centre (SUERC) with whom we are hoping to carry out some measurements of the isotopic composition of argon gas.
  • And the second was to the National Engineering Laboratory (TUV-NEL) who are the UK’s national centre of excellence in flow measurement.

As I walked across the park from one site to the other I realised that SUERC needed to weigh samples of just a few milligrams in order to count atoms, and TUV-NEL needed to weigh – literally – tonnes of fluid in order to measure flow.

Both these institutions relied ultimately on weighing procedures to tell them ‘the truth’ – but they weighed amounts that differed by a factor one billion!

SUERC

As I mentioned in the previous posting, I am working with SUERC to measure the relative  amounts different argon isotopes in some samples of argon gas.

SUERC have a mass spectrometer, called ARGUS devoted entirely to measuring argon isotopes, but there is no simple way to prove that it is equally sensitive to each type of argon isotope.

A photograph of the ARGUS mass spectrometer. Argon molecules enter on the left and are ionised and accelerated towards the magnet. The trajectories of the lighter argon-36 molecules are more strongly affected by the magnetic field than the heavier argon-40 moleculs - and so end up in a different detector at the end of their flight.

A photograph of the ARGUS mass spectrometer. Argon molecules enter on the left and are ionised and accelerated towards the magnet. The trajectories of the lighter argon-36 molecules (shown as a red line) are more strongly affected by the magnetic field than the heavier argon-40 molecules (shown as a blue line) – and so end up in a different detector at the end of their flight. The process is analogous to the way white light is split into different colours – that’s why this is called a ‘spectrometer’. Courtesy SUERC

In order to evaluate the sensitivity of the spectrometer to different types of argon isotope it is necessary to first create a sample in which the relative amounts of the different types of argon isotope is already known.

And to achieve that it is necessary to very carefully weigh samples of gas containing just a single argon isotope – typically weighing just a few milligrams – and mix them together. So the ultimate accuracy of this super sophisticated instrument is assessed, in the end, by weighing.

TUV NEL

Measuring the rate of uniformly flowing liquid or gas down a pipe is hard. But not too hard.

Measuring the rate of flow of (say) oil when it is mixed with water and air is very, very hard.

At TUV NEL they calibrate flow meters, and their ultimate measure of the accuracy of a flow meter is to place the meter in a pipe and flow fluid past: and then have the pipe empty into a giant tank.

They then weigh the tank as the water pours in – tonnes of it! – and measure how much fluid arrives as a function of time.

It’s a pretty basic measurement – but how else can you ultimately know that your flow meter is reading correctly?

The kilogram

And if SUERC and TUV NEL want to make sure that their measurements will be comparable internationally, then they need to make sure their measurements are traceable to the SI definition of the kilogram.

Seeing how these diverse measurements in different realms  related back to the mass of a lump of metal we keep in a safe at NPL, made me reflect that the work we do at NPL really does matter. And often in ways that perhaps even we don’t realise.

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2 Responses to “Why I love weighing”

  1. edhui Says:

    It’s not really ‘the weight of stuff matters so much’ that is surprising. It’s that the miniscule differences in relative amounts of two types of atoms with different numbers of neutrons of a very pure and utterly inert substance can make such a difference to YOU personally. That’s an astounding thought that I’m guessing you’d think was wildly unlikely even 10 years ago!

  2. The Zen of Weighing and Running | Protons for Breakfast Blog Says:

    […] I love weighing. […]

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