Observations about the universe, life, Lausanne and me

Thursday, February 12, 2009

Obscure Units, how I hate you

Caution: Research Rant Ahead!I wish I had a time-machine. I would go back in time to 1660, travel to London (maybe the other way round, in fact), burst into the founding of the Royal Society and beat the snot out of them, until they agreed to use the SI and nothing but the SI for all existing and future units, forevermore and until the end of time, but at least until 2009.
For weeks I have intermittently tried to calculate a theoretical breakdown curve, without much success. One of the difficulties (after you have written a program to solve the stupid transcendental equation that laughingly calls itself "breakdown condition") is finding values for the diffusion coefficients, Townsend coefficients and electron mobility in the literature, particulary for the high electrical fields I am using. All those values depend on the electric field over the concentration of your neutral gas, E/N, which is Volts times square metres in good, sensible SI units. But noo, nobody likes using those. Instead, you find Vcm^2, V/(cm torr), V/(m torr), V/(cm mbar), V/(m bar) and my personal favourite, Townsend (Td), which is Vcm^2 x 10^(-17).
Converting all those units is a pain in the ass in and of itself, but the problem is excasperated in that I was using the wonderful little Boltzmann equation solver BOLSIG to calculate my coefficients, and only now found out that BOLSIG expects Townsends, not Vcm^2 as an input. Typical example of garbage in, garbage out - I got diffusion coefficients that were for values 14 orders of  magnitude smaller than I was looking for.

Anyway, this is now resolved, so behold the diffusion coefficients (times N) for argon, from 10 to 10000 (grmbl) Townsend.


  1. Oh, sister, I can so sympathize. Here, at NASA, the scientists do everything in SI while the engineers do everything in "standard". They still use "slugs" even but just the confusion over pounds is enough to make one pull one's hair. Pounds-force and pounds-mass are used, often in the same literature, but described as "pounds" with no distinction. They use "mils" which is a thousand of an inch rather than something useful like micrometers.


  2. Wait, do I have the wrong gender? Sorry.

  3. NO excuse, really. For some reason, on your comments to The Secret of Newton, I got it in my head that you were female despite signing Boris. I have no idea why.

    My deepest apologies.

  4. No reason to apologise so deeply - I am not offended. Although now I have to count my chest-hairs or something, to re-assert my masculinity!

    By the way, for all my whining I should be happy that I don't live in the sole (well, together with Lybia I guess) non-SI holdout left in the world - I can't imagine the pain of constantly converting back and forth between imperial and SI! I feel for you... Anybody that snapped and started to use microfortnights and attoparsec yet?

  5. No but in college, my physics professor once provided a conversion problem: SI to minirod/microfortnight. I was completely spoiled in physics the first three years and then, when I took my engineering courses my senior year (My degree is in engineering physics), I was completely unprepared for slugs and foot-pounds of torque and calculating the strength of a cable 5/16" in diameter.

    I'm STILL irked by it. And I've been dealing with non-SI silliness for two decades now.