Friday 26 September 2014

Talkin' deep about sulphur hexafluoride

We're at the end of week 2 of the teaching term at The Institute and remarkably, I've got 100% attendance record at my lecture courses.  Too few points on that plot to be considered data but it's a change from last year where I gave my first lecture in The Goldfish Bowl on the 1st floor in full view of three members of my final year chemistry class who were chatting in a corner of the floor below. But apart from turning up and sitting, down this year's cohorts are also contributing to the discourse. On Tuesday I was doing my schtick about James Lovelock and CFCs, which I use as a softening up exercise at the beginning of the course to show some examples where embedded certainties and unquestioned assumptions were exposed as wrong.  It takes people with moral courage and creativity to buck the trend and point out that there is another way of looking at the world. I am trying to inspire my students with those qualities because I so conspicuously lack them myself. My front-row prop-forward student piped up saying
 "don't CFCs make your voice go all deep if you inhale?"
"No that's helium" I said, before I'd started thinking.
"That makes your voice pitch higher" and he forbore to add "duh" because these students are scrupulously polite. And of course he was correct. Helium is much less dense that the components of air and CFCs like Freon are much more dense and the sounds we generate are altered by the physics of the local environment.
I tasked him to find a youtube video of the phenomenon in the 3 hour gap between the two lectures. But I couldn't contain my curiosity and went hunting on my own account to find this demonstration that sulphur hexafluoride is so much denser than air that you can float a tin-foil boat on a tank of the stuff.  It then shows a lot of grown-ups in suits falling about giggling as they talk funny having inhaled SF6. from the vat.  As I said in class, the clip is about twice as long as it needs to be, and you had to be there to find it hilarious.

Turns out the SF6 is really interesting. Like CFCs it is remarkably inert, very stable - which means it doesn't react with anything under normal terrestrial conditions. The breathe-in-and-talk-sexy exercise shows, for example, that it doesn't react with the wet epithelium of your lungs. As an inert fluid, it is used as a highly effective electrical insulator - it packs in round the smallest components on circuit boards and stops any wayward sparking. In a similar way halons (BrFCs) are potent fire-suppressants because they are heavier than air and so displace oxygen by surrounding the seat of flames and filling the area from the bottom with inert gas.

But SF6 acts quite differently to CFCs in the upper atmosphere.  CFCs react with UV light in the stratosphere to shed the fluorine and chlorine atoms which catalytically and catastrophically react with ozone. That depletes the ozone layer which protects us all from the DNA-damaging UV radiation that causes skin cancer. SF6 on the other hand is still stable up there and acts as a greenhouse gas nearly 25,000x more potent than CO2. We don't need to worry too much about it, though, because compared to carbon dioxide it is present in vanishingly small quantities so that its overall effect contributes only an estimated 0.2% to global warming.

In March 2007, nearly 2 years after the 7/7 bombing spree in London, SF6 was used to flood a London underground station to track the progress of a volatile gas through the system in case the next terrorist outrage copied the Tokyo Sarin attack carried out by Aum Shinrikyo in 1995. The British government was a lot more pro-actively transparent in their handling of the emergency test than the US Navy was when it sowed the San Francisco sky with a "harmless" microbe in 1950.

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