## Wednesday 24 July 2013

### Troy

A couple of days ago, I was on about Troy and Avoirdupois and Metric units in the context of gold prices.  It's hard to drag people into the uniform and mathematically simple world of the Metric system.  When we lived in the Netherlands it was normal to hear people in the market asking for een pond van kaas when they wanted 500g.  It was six or eight generations since NL had officially adopted the Système International (SI) created by French revolutionaries nearly 200 years before.  It's nearly twenty years since they changed all the roadsigns in Ireland to km, and more than that since they started selling petrol in litres, but you still hear car-buffs talking of how many miles per gallon their wheels do for them.  All my students at The Institute are happy to give their weight in stones and their height in feet and inches.  They've never learned anything but SI in school, but 80% of them couldn't tell their weight in kilos without a calculator.  And, of course, the births of babies are reported in pounds and ounces.

Troy measures may be derived from the city of Troyes in France. A Troy ounce is 480 grains 31.1g, whereas an Avoirdupois ounce is 437.5 grains 28.3g.  The grain, nominally the weight of one wheat-seed, in both cases is the same at 15.43 to the gram. To (over)compensate, there are 16 AvdP oz in a pound (lb) - 7000 grains, but only 12 Troy ounces in that pound 5760 grains.  As I said in the last post, Troy measures are only used for gold, silver and gems.

Matter a damn, you say?  It can matter to the tune of \$125million if you mix your measures and don't pay attention and don't communicate properly with your partners and traders.  The most famous case was NASA's cunning plan to measure the weather on Mars.  It seemed a bit premature to add a bunch of Martian data to the stream when we were/are so long from knowing how Earth weather works.  But they made their case to Congress and launched the Mars Climate Orbiter MCO in late 1998.  The spacecraft never achieved stable orbit round the red planet when it arrived there 10 months later but rather took a figgairy to plunge straight at the surface and burn up in the atmosphere.  Whooomph!

The physics and engineering required to drop a man to a precise spot on the moon is so complex you need rocket-scientists to make it happen. NASA missions are extra complex because they have to share out the contents of the pork barrel among the clients of their paymasters in the US Congress.  As I blogged about before, they also have to be relentlessly optimistic and play down the risks or they'd never get any support.  In the case of the MCO, one set of directional control software was programmed to deal with measures in the US-friendly pound-force while an interacting piece of code written by another company assumed (we're all scientists and engineers here) that the data would be in newtons - the SI units.  Even before the destination was reached, the MCO appeared to be behaving a bit lively:  weaving left and right like a drunk trying to walk a straight line for the cops.  Even with that information nobody back in Houston realised what the problem was.  If they had they might have been able to modify the code to read:
THRUST = THRUST/4.448
even
THRUST = THRUST/4.5
would probably have saved the day.  They didn't make that mistake again.  But several other missions went wrong before NASA finally (Jan 2004) landed a vehicle on the Martian surface that worked.  Indeed the Spirit Rover was so well engineered that it lasted 6 years longer than its expected 90 day shelf life.  It wasn't NASA's fault that Spirit drove into a patch of soft sand and couldn't get out again. Three weeks later, a sister vehicle Opportunity landed on the other side of Mars and she's still going; as is Curiosty which started work in August 2012.  For most of us, including pretty much everyone in RTE except their canny and redoubtable new science correspondent Will Goodbody, its as if these marvels of engineering chutzpah didn't exist.  But they show us what science can achieve - we're not just primates any more.