>> 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.
- United States Metric Association Homepage - http://lamar.colostate.edu/~hillger/#metric.
- Metric Conversion Act of 1975. (http://ts.nist.gov/WeightsAndMeasures/Metric/pub814.cfm)
- Omnibus Trade and Competitiveness Act of 1988.
- Executive Order 12270 signed by President Bush in 1991. (http://ts.nist.gov/WeightsAndMeasures/Metric/pub814.cfm#president)
- Savings in Construction Act of 1996.
- ES, G-00-21 NASA Public Affairs Policy on NASA’s Use of the Metric System, 2001. (http://nodis3.gsfc.nasa.gov/displayDir.cfm?Internal_ID=N_PD_8010_002D_&page_name=main)
- Le Système International d'Unités (SI), 2006: The International System of Units (SI), 8th Edition, Bureau International des Poids et Mesures (BIPM), Pavillon de Breteuil, F-92310, Sèvres, France. (http://www.bipm.org/en/si/si_brochure/)
- Eyles, Don. Tales From The Lunar Module Guidance Computer: Apollo 11, AAS 04-064. (http://www.doneyles.com/LM/Tales.html)
- Kverneland, Knut. ANSI Metric Standards for Worldwide Manufacturing 2005, 5th Edition.
- Lobrano, Barbara and James B. McCracken. U.S. Manufacturers with Products Conforming to Metric Standards: An Analysis, NIST GCR 99-783. (http://ts.nist.gov/WeightsAndMeasures/Metric/upload/thomas2.pdf)
- 13. NASA Problem Reporting and Corrective Action database, JSC, March 2006.
- 14. Restriction of the use of certain Hazardous Substances in electrical and electronic equipment (RoHS), 2002 and Waste Electrical and Electronic Equipment (WEEE) directives. (http://www.rohs.gov.uk/content.aspx?id=9)
- 15. Webb, Ted, “General Motors Goes Metric,” Elements of Technology, Winter 1976.
- 16. Naughtin, Pat, “What is the Cost of Not Going Metric?” Metrication Matters website (www.metricationmatters.com), 2005. (http://www.metricationmatters.com/docs/CostOfNonMetrication.pdf )
- 17. Reif, Rita, “The Quiet Revolution: The United States Goes Metric,” New York Times, December 7, 1975.