Argon is heavy!

Argon

Argon

Yesterday, I had a curious afternoon weighing a sphere. The sphere in question is a rather perfectly made object manufactured out of two hemispheres so that when they are bolted together the internal surface encloses a highly perfect spherical shape. We are hoping to determine the average internal radius of the sphere by filling it with water and weighing it. However we had severe problems with the water leaking out of gaps through which it should not have leaked. We think we have it sorted now and earlier in the week we tested it with a helium leak tester.

To do this we filled the sphere with helium gas. Helium gas  has light and mobile molecules and they can be guaranteed to find the tiniest of leaks. We then use a ‘sniffer’ outside connected to a mass spectrometer which can very sensitively detect helium exiting the sphere. Actually its all a bit trickier than this – but that is basically what happens. We left the helium in the sphere until just before the weighing when it occurred to me that the helium might affect the weighing, and so I quickly decided to flush it out. However for some reason I chose to flush it out with argon gas rather than compressed air.

After flushing it out for 10 minutes we took the sphere to the rather tasty weighing machine at NWML. As my colleagues set up the balance, they noticed the sphere – which weighs 9 000 g was 300 milligrams (0.3 grams) heavier than the last time weighed it. That may not sound like much – but we need a measurement uncertainty below 0.000 1 grams. After a moment the penny dropped – it was still full of argon. Argon molecules have a relative atomic mass of 40 compared with the average mass of an air molecule of about 28.8. So the 1 litre of argon in the sphere weighed about 1.67 grams rather than 1.2 grams that 1 litre of air would weigh. The difference (0.47 grams or 470 mg) was plenty large enough to explain the observed discrepency if we imagined it also contained a bit of air.

So first I tried using a ‘puffer’ to put air into the sphere, and after re-weighing the mass had fallen by 30 mg. Eventually I had to hold the sphere upside down and shake it in order to empty out the argon while my colleague used the puffer to flush air into the sphere. Eventually we seemed to get rid of most of the argon and the weight of the sphere returned to within a few milligrams of its previous value.

Anyway. I just thought I would mention this because (a) it was a curious moment and (b) it really brought to light very graphically the heaviness of gases in a way in which we are not usually aware – by simply weighing them!

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2 Responses to “Argon is heavy!”

  1. melindwr williams Says:

    This is very entertaining and just the kind of anecdote that should be used in schools to trigger children’s natural curiosity and channel it down the route of science enquiry. The mental picture of someone holding a sphere upside down (does a sphere have an upside?) and shaking it to flush out its contents is not only comical but an intriguing starting point for powerful questions to children.
    My wife used to work in food analysis, and used helium gas often in her work. One day she came home from work and mentioned that bttles of the gas had become more expensive recently because of difficulties in Algeria, drilling and extracting the reserves. Our own converstion about this eventually tended towards the ridiculous (well, it was a Friday evening) as we speculated on the growth of an international smuggling trade in helium … and the images that conjured up.

  2. Emma Says:

    Wonderful Michael! This is the sort of story that reminds me why I love metrology!

    I’m hoping to test my hypothesis that the gas inside the blackbody doesn’t change the radiance measured outside next week – Graham says he’ll be doing the experiment filling the furnace with argon then helium and melting Pt-C. Would be nice to discuss the results with you.

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