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Death of the SI unit

By Oliver Batley - Last updated: Wednesday, September 7, 2016

ElephantFeetOn my first day as a young graduate engineer at Rolls-Royce, I remember reading a recruitment brochure whilst nervously waiting for my line manager to collect me. The brochure stated “Did you know: the F136 turbofan can lift ten elephants to the top of the Eiffel Tower in just six seconds”. My first thought – was I honesty expected to know that? Second, who gets to calculate these numbers? And third, what type of elephant?

A quick Google search reveals another interesting fact – the force at the root of a Trent fan blade is equivalent to 13 African elephants! This time I’m pleased they specified the type of elephant, but do they know the male African bush elephant weighs 6000kg, whilst the African forest elephant only weighs 2700kg? That’s a twofold factor of uncertainty!

Joking aside, units play an important part in marketing. At the pub we were discussing Telsa’s interesting choice of units for charging their electric car: miles per hour. Or in other words, how many miles you can travel on 1 hour of charge. I think you’ll agree that charging at 260mph certainly sounds more impressive than 120kW.

There’s another use for unusual units – helping understand the scale of a number. I’m currently designing a pressure housing for a sensor. It needs to withstand 700bar collapse pressure (sorry – 332 African forest elephants standing on one foot*) whilst only deflecting 50 microns (the thickness of a human hair). This problem requires analysis, exotic materials, precision machining and rigorous testing – all part of the day job in the mechanical engineering team at Cambridge Consultants.

If you have a spare moment, Google “unusual units” – if only to impress colleagues with your new found knowledge of Banana Equivalent Doses. In the meantime, I’ll stick to the SI units – happy in the knowledge that my 1.15 safety factor is not out by an African elephant factor of two.

 

* Explanation of calculation: 700 x 10⁵ Pa [collapse pressure in Pascal] / (2700 kg [weight of elephant] x 9.8m/s² [gravity] / (π x 0.2²) [area under elephant’s foot])


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AuthorOliver Batley


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