Tuesday, January 22, 2013



ruler4Reader Jim Tahler, a Senior Engineer with Rolls-Royce Instrumentation & Controls in Chattanooga—and more importantly, a fellow Ramblin’ Wreck from Georgia Tech—was intrigued by my recent opinion piece on the Imperial vs. Metric System. He wasn’t alone, either. At last glance, there were more than 50 comments on the story, making it one of the most-talked-about stories of the year on this site.
But Jim went farther than the other readers, compiling a list of his reasons to hate SI units.
“I think units should work for us, not the other way around,” he told me. “The fact that ‘the rest of the world does it’ is a fairly weak argument to me. There are LOTS of things that are prevalent in the rest of the world that I would not want the U.S. to do.”
So, without further ado, here is Jim’s Top 10 list (or a ‘Jim rant,’ as he describes it) of reasons to hate SI units:

1. I don’t like being told by the government what to do, especially in an area where the free market can make a perfectly good decision. You may not LIKE the decision, but it is almost always the most efficient one. Countries that have switched to SI have done so only because their government made it the law of the land.
2. Many of the SI units are not “metric” in actual use (defined as “a unit that uses the metric prefixes for powers of ten.”) For example, degrees C is never KiloC or milliC or PetaC, it is always just C. What is the temperature of the center of the sun? The Wikipedia article reports the temp as 1.57 x 10^7 K. Celsius is not inherently better than F, just different. In fact, Fahrenheit has more precision since it is about half the size of a degree C and thus is “handier” for everyday use.
3. Most people, and publications, are very spotty about following the SI rules. For example, hardly anyone uses a length unit larger than km: The distance to the moon is almost always quoted as 400,000 km (not the more SI-correct 0.4 Gm or 400 Mm. Note how funny Gm and Mm even look and sound!) The same Wikipedia article on the sun reports the distance to the sun in km, and light minutes. It reports the diameter in km and earth radii. It also reports luminosity as 3.8 x 10^26 watts, and the galactic period in years (not the SI unit of seconds). Seconds are NEVER quoted with metric prefixes when larger than 1 (megaseconds, anyone?).
4. Most SI countries don’t consistently use the SI units. For example, in Europe, tire pressure is almost always quoted in Bars (atmospheres). I did some work on a nuclear station in Spain and was surprised to learn that they do NOT use the Pascal as the pressure unit, rather they use Kg/cm^2 (mass per unit area? Not even a pressure unit in any classical sense) and mmHg, and they also do not use m^3/sec as volumetric flowrate, rather they use a variety of units including metric tons/hr, liters/sec, kg/sec, kg/hr, m^3/hr, liters/min, and kilo-metric tons/hr. They tend to “make up” a unit that is convenient for the application (sound familiar?).
5. Many SI units are just dumb, they are made just to be infuriatingly consistent. For example, the unit of radioactivity that I used in college is the Curie, which is approximately the radioactivity of a gram of Radium (3.7×10^10 decays per second) which is a useful unit size for the real world. The Si unit is the Becquerel which is 1decay/sec. Thus any actual amount of radioactivity will be billions of Bq.
6. Many SI units, in my opinion, were made just so another favored person could have a unit named after them. For Example, the REM is the customary unit of absorbed radiation dose. The recently (late 70’s) named SI unit for absorbed dose is the SEIVERT, which is equal to 100 REM. Not better, just different. When I was a kid, the SI temp was Centigrade, then it was changed to Celsius. That is better?
7. SI units are not handy. The proper unit for volume, m^3, is just too huge to be useful (except for concrete I suppose). The area unit is not handy for land measurement. The meter can’t be divided in thirds exactly (as a yard and a foot can, since they are based on 12ths). The “handy” unit for small volumes, the liter, is not even a proper SI unit. Quick, how many liters in a cubic meter?
8. It is irritating that many groups that purport to be on board with SI don’t use it. Astronomers use light-years or parsecs, for example instead of 9.46 petameters (a light year), and they use AUs and earth radii frequently (very handy units but definitely not SI). Nuclear physics uses barns for absorption cross-section (equal to 10^-24 cm2). Examples abound.
9. There is no standard prefix use for square or cubic units, for example … m^2. What is 1000 of those called? A kilometer squared? Isn’t that a square one km on a side, or a million m^2? I hear the term “thousands of square km” but what exactly do they mean?
10. The vaunted ability of changing the units by merely adding a prefix is highly overrated, in my opinion. For example, I have NEVER wanted to calculate how many teaspoons are in a cubic yard, as I have never wanted to know how many millimeters it is to the moon (start with 400,000 km, it is no easy task for a non-engineer type to convert that to mm, or even harder to dm or cm).

Check out some of the 50 comments on this----

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