For Relax Max: Metric Arrogance

>> Friday, November 13, 2009


Relax Max didn't ask me anything, per se, but we've been going back and forth about the metric system (or more appropriately, SI) . See, I'm a scientist so I'm for it. And Relax Max doesn't agree with me. There's nothing unusual about the latter, I might add, but this happens to be one of my pet subjects, so I'm doing a double header on this today, here and on Rocket Scientist (which is fielding an oldie but a goodie).

So, I'm going to use this question he posed in a comment: Why is it so wrong to just slip in and out of whatever system a person wants to use, or NEEDS to use for a particular job?

That's a wonderful question, Relax Max. Let's see if I can't give you the answers you were looking for in my tyrannic and smug way. 'Cause, hey, it's the only way I know. :)

Below are the sources I used on my paper two years ago. Hopefully some of the links are still good and can help you with more detail if I don't provide enough, or actual examples. As I mentioned earlier, I'm also going into why I think standard units are needlessly complex and ambiguous on Rocket Scientist, so feel free to check it out, but, here, I'll focus on your question.

The simplest way to answer is to ask what units of measure are for. I would say (and you're welcome to provide your own definition, but I'd be surprised if it didn't boil down to the same thing) that units of measure do two things. They enable one to convey the magnitude of some amount/size/distance, etc. They enable you to use those values to figure out important things about an object without having to test it. What do I mean?

A measuring device can let me know I have six cups of flour (or perhaps a half kilo - I have no idea what flour masses and, as it's compressible, it's variable). Knowing that, I can figure out how many batches of cookies I can make before I must go to the store for more. If I know the fuel economy of my car on the highway, I can figure out how much gasoline (or how often) I will need to stop for gas to fill my tank of 16 gallons (~60 l).

Now, if I never had to worry about anyone else or conveying things to anyone else or do any sorts of calculations, I could have a truly independent unit of measurement system, as many a chef does, with dashes and pinches and smidges and handfuls or whatever. As long as I knew what the amount referred to, I'd be fine. But, if I'm going to pass my famous recipe for molasses cookies to my niece, Charlotte, I'm going to have to units that she can identify so that she can reproduce my quantities or, at least, proportions. I already know what I want, why I want it, and in what proportions. Units allow me to communicate or convey those quantities to someone else. Hopefully reliably or why use them at all?

And that's the answer for the first part of your question. Changing/switching/using whatever unit system you want defeats the purpose of having a unit of measurement: to reliably convey the quantity to someone else (or yourself, say, so you can figure out how much carpet you need, for instance). Now, if all "standard units" were actually standard, there wouldn't be an inherent issue with using units as long as we didn't have to do any calculations with them or convert them (like figuring out how many inches of wire I get from my 75 yards of beading wire, so I can make how many 25 inch necklaces). But, for just conveying an amount, no unit is better than another as long as what is meant by that unit is clear.

And that's where non-metric units already start to lose their lustre. Did you know that there are at least 7 different types of "foot", all close to the length the US calls a foot (although that's actually a slightly different length than a survey foot), but the variance is as much as 27%. For pounds (of which there are at least three) NOT counting pounds-mass, the variance is also 10-20%. There are ten different versions of mile and, yeah, they vary even more. Let's not forget fluid vs. weight vs. mass ounces, all of which are different and, can vary in comparison. Yikes!

Therefore, the value of the unit, because there are multiple definitions out there, is inherently ambiguous--which kind of defeats the purpose. Get into the nightmare of determining whether a pound is a mass or a weight (only 32X difference) and it gets more ambiguous. Even things that are generally well known, like pounds per square inch (psi) can be ambiguous. When people read a pressure gauge, for instance, what they are really reading is "psig" or the amount of pressure above atmospheric pressure (which can vary by quite a bit). But there is also psia - absolute pressure - or psid - differential pressure. These can all be quite close (relatively) or they can be drastically different depending on the circumstances, but, quite often, they are all described with the same term: psi.

Guess how many kg there are? Yep. Just one. If you force, though, it's Newton (1 kg m/s^2), but you can calculate force, kinetic energy, inertia, and handfuls of other things without having to change any units, not a single conversion.

Then, there are the jillions of non-standard units included in the standard system: yards, mils, microinches (don't get me started), rods, leagues, furlongs, acres, bushels, hogsheads, barrels (not all the same size, of course), acres, hectares, well, I think you get the point. For a unit of measure to be of use, it must be clear and consistent.

But, that's only half what you need a unit for. The other half is using that unit to calculate what you need and how you need it, stress margins or sufficient building materials, anything. How do I convert inches to feet? Divide by twelve, you might say, so what? How about inches to yards? (72 inches= 1 yard). So, how many inches in 9 3/4 feet? Or how many feet in 189 inches? Don't worry, I'll wait. How many feet in a quarter mile? Um, which mile did you have in mind? How many linear feet of cable would you mean to get electricity down a 2 mile street?

But, I know how many meters of wire I need to get electricity down a three km street? 3000

I've got quite a bit of examples of calculation differences described in Rocket Scientist so I won't do it here.

But, it's more than just clumsiness and ungainliness on conversion. Physics in "standard" units is problematic, for instance. If I want to calculate the kinetic energy of a moving train, I can write it up in Newtons (or megaNewtons) by multiplying the mass of the train by the velocity in m/s squared. No problem. The units are unambiguous and each unit has one and only one meaning. No "conversion" is required.

But pounds-force (weight of the train) means nothing when it comes kinetic energy unless gravity's a factor (which it isn't here), you can't just multiply weight by miles/hour - there is no energy unit that corresponds to pounds-force-miles^2/s^2. You'll have to change miles/hour to feet per s (Doh, that's why you need to know how many feet in a mile!) and find some unit of mass because, if you use pounds-force, it will be wrong (by a factor of 32) - perhaps slugs (which is an ugly horrible little unit of mass that's as ugly and ungainly as its name). And you'll still have to change the units into something else because, hey, who uses slugs-ft^2/s^2? (Is that even a unit? There are so many "standard" units) You'll either be converting to joules or you'll be using something like horsepower, ironically enough, converting it back to pounds-force-feet (although you can't just use pounds force for the calculation) and neither go smoothly into that good night.

Go into space where inertia and mass matter but "weight" means something different depending on where you are, and English units make even less sense. However, in space, a Newton is always a Newton, a kg is always a kg and the combine nicely into Joules (=1 N-m).

So what? Let's be "individuals" and conform to the way we've always done it since dark ages!

Except, if all this complexity and opportunity for error means that your seat belt can handle 1/32 of the force it was supposed to handle because those distinctions weren't clear, or someone made an error because, hey, a pound's a pound (and if you think that can't happen, you don't know enough engineers), someone's kid can die. It won't happen, of course, because

Meanwhile, our children are taught, year after year, to convert fractional values of ambiguous measurement units into others, to perform twenty complex calculations to figure out what can be easily deduced with one or two in SI, to teach themselves what literally hundreds of units are, sort of. Why? Because, apparently, you must conform if you want to be a rugged individualist, no matter how painful it is.

9 comments:

  • Boris Legradic
     

    Hear hear!

    Living as I am in Europe, I don't run into the problems you mention nearly as often - except I do, in research, because people like to use (sometimes somewhat understandably) the cgs system or other, non-SI units in their papers, because the numbers look better.
    But this leads to me tearing out my hair for a week because I can't get my simulation to run properly, only to discover that I forgot to convert from Townsend (a unit for electric field over density, or 10^17 Vcm^2) to Vm^2, thereby introducing a small error of twenty-three magnitudes! The error was, of course, mine, but I still feel the need to grab my trusty dead fish, seek out Professor Kücükarpaci who wrote the (rather good) paper I got my constants from, and slap him with it.

    Vive le Système Internationale d'Unités!

  • Stephanie B
     

    Boris, you think YOU have it bad. I work in US aerospace where they're actually moving backwards. Admittedly, we had an early advantage because Von Braun and his German team (oddly enough) used metric and let the US folks convert. Now, even the guidance navigation and control has been told to use "standard" units, so they have to use slugs. I kid you not.

    Kinda makes you wonder why so little progress has happened since (though, admittedly, I think that's more politics, to be fair.). Heck, the lunar lander, which was desperately short on processing power and memory, did all the calculations in metric and converted for the pilots. It still overloaded the computer, but can you imagine if they'd tried to do it in "standard" units? *Shudders*

  • Stephanie B
     

    I have some more great links for those interested:

    Comparison of units
    History of US and the metric system - including the key involvement of our (US') founding fathers
    How our inability to use standard energy terms confuses the discussion on global warming

  • The Mother
     

    I would like to point out that the standard SI system happened when the French decapitated their kings, and decided they needed a more egalitarian system. It was largely developed and propagated by a tax collector known as Antoine-Laurent Lavoisier.

    Who lost his own head a few years later, also to the French Revolution.

    Isn't history fun?

  • Stephanie B
     

    Actually, there I have to disagree with you, the Mother (though not that history is fun). SI as a idea for a system was born way earlier (1688). Check this out. And I have a bad link in my previous comment for history of metric and our founding fathers. Sorry!

  • Stephanie B
     

    Wikipedia says that Lavoisier was a scientist and a noble who was also an investor in a tax collection company. No surprise he was "axed."

    One could probably say that the French Revolution became an impetus to put into practice what many had been striving toward for years, but many who fled the "new regime" were ironically the movers and shakers behind the original change including LaPlace.

  • Relax Max
     

    I have to be polite here because you were so nice. But boy are you ever going to get it elsewhere.

    Thank you, ma'am.

  • The Mother
     

    Most historians date the SI units to the French Revolution, although the movement was started earlier, by Louis, as one of his few actual contributions. The revolutionary govt took credit for it, in a "look, we don't have a king's foot!" kind of way. It's not much of a surprise that Jefferson and Franklin, who were huge Francophiles as well as scientists in their own right, immediately adopted the new units.

    Lavoisier was a founding father of chemistry. He is almost single-handedly responsible for the "atoms in = atoms out" concept that we call conservation of matter. When Louis wanted a better measuring system, it was Lavoisier who was tapped to do most of the work, being a chemist and all. But chemistry didn't pay, and he was given the tax job by Louis to keep him in francs.

    Ironically, he was guillotined largely not because he was a tax collector for Louis, although that certainly influenced his "trial," but mostly because he was a chemist. He was denounced to the revolutionaries by a rival chemist whom Lavoisier kept out of the Royal Society (apparently because his methods weren't as meticulous as Lavoisier thought they should be).

  • Stephanie B
     

    Well, I'll defer to you on Lavoisier - I'd heard the name but he was and isn't an area of expertise. Interesting story though. But I've read from multiple sources about SI being older, including contemporary letters dated before the French Revolution was a gleam in someone's eye, so I'll maintain my view.

    However, I will conceded that the French Revolution certainly took credit for it.

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