Metric System in Engineering
Engineers of my vintage often find we are more comfortable with the old English system of weights and measures than with the metric system. It was a great relief when the US, which seemed about to go metric in the late 1980's, reversed itself and stuck with the English system. Metric system proponents claim this puts us on a different system from the rest of the world, making it difficult to maintain competitiveness in the global market. This might be true, plus there are definite advantages to the metric system.
For example....quick, which is bigger, an 8 mm nut or a 9 mm? Right, the 9 mm! A total no-brainer, right? Now, which is bigger, a 3/8" or a 5/16" socket? Had to think about it, right? You might even have had to find the "Least common denominator," something that could be done very easily and quickly by 6th graders when I was in school, but which most adults can't do any more. This feature of metric linear dimensions is probably the strongest aspect of the metric system or Systeme Internationale (SI), as it's properly called in its native language, French. For the everyday, general user, this may be the motive behind adopting or switching to the metric system.
For engineers, however, things are more complicated. We do calculations that involve interchanges between various forms of energy. The conversion between mechanical and thermal energy is a common example: 778 ft-lb-force is the same amount of energy as in a BTU of heat. In metric, the energy unit of heat is the calorie, or joule. Alas, for reasons I don't understand, these units are not the same numerically. A calorie is about 252 BTU's, while a joule is 1055 BTU's. The joule is the same numerically as a watt-second. Since the watt is defined as a newton-meter per second, just as the horsepower is 550 foot-pounds per second, the rate of doing mechanical work, the watt's connection to engine torque, RPM, and propeller thrust is direct and straightforward.
However, things go less well when converting weight to volume -- something naval architects do every day using Archimedes' Law, the buoyancy being equal to the weight of water displaced. In the English system, a long ton of seawater occupies almost exactly 35 cubic feet of volume. In the metric system, devised by landlubbers, a cubic meter of distilled water has a mass of exactly 1000 kg. A cubic meter of seawater, at 15 degrees C and at the earth's surface, has a mass of about 1026 kg. It gets worse -- European naval architects "round" this cumbersome number to 1025.
Then there is the question of weight versus mass. The two are related very closely. If I understood Physics 101 correctly, mass is a measure of an object's inertia or momentum, while weight is a force -- the force of attraction the object has exerted on it by the Earth. Now it might well be that the displacement of a ship is the same as its mass. However, we do not use mass units for an individual's weight in the English system, and buoyancy as "the mass of water displaced" makes no sense since buoyancy is an inherent product of gravity with no meaning in outer space. (Mass, unlike weight, is independent of gravity and still has meaning in outer space where objects are "weightless".) Therefore, the metric ton (spelled "tonne" by most naval archtitects in the US to differentiate it from the more familiar long and short tons) is actually a unit of mass, not weight. This is not a particularly helpful distinction and may actually be misleading for designers.
Weight is defined in most systems as mass times a gravitational constant "g" with units of acceleration. In this system, the mass in English units would be in "slugs", which are rarely used. Conversely, in metric units, it's common to use "kilogram-forces" in computations, even though there's already a perfectly good unit of force, the newton, approved by Paris for that purpose. This may be related to the fact that the SI units have a supervisory committee in Paris overseeing them, making sure they follow their inherent logic. The English system may once have had such a committee in London, but no longer; therefore, it is somewhat more free to evolve.
The value of "g" is about 9.81 m/sec squared. To me, that value shows a lack of imagination. Had the French scientists thought about it more, they would have realized that a slightly shorter second would have allowed them to have newtons whose value was exactly 10 times the mass of a kilogram. This in turn would have opened the way to a 100-second minute, and maybe even a 100-minute hour. I wish I could convince myself that the math works out so that these units could be combined with a 20-hour day, but I think the problem is overconstrained by fixing so many variables. The French scientists at the time of their Revolution were willing to invent a new calendar and convert Notre Dame Cathedral into a Temple of Reason. (Both these actions were overturned later). They would certainly have been willing to change the length of a second had they realized how important it was to future engineers.
As it is, the metric system comes off, to me, as different without being much better.
For example....quick, which is bigger, an 8 mm nut or a 9 mm? Right, the 9 mm! A total no-brainer, right? Now, which is bigger, a 3/8" or a 5/16" socket? Had to think about it, right? You might even have had to find the "Least common denominator," something that could be done very easily and quickly by 6th graders when I was in school, but which most adults can't do any more. This feature of metric linear dimensions is probably the strongest aspect of the metric system or Systeme Internationale (SI), as it's properly called in its native language, French. For the everyday, general user, this may be the motive behind adopting or switching to the metric system.
For engineers, however, things are more complicated. We do calculations that involve interchanges between various forms of energy. The conversion between mechanical and thermal energy is a common example: 778 ft-lb-force is the same amount of energy as in a BTU of heat. In metric, the energy unit of heat is the calorie, or joule. Alas, for reasons I don't understand, these units are not the same numerically. A calorie is about 252 BTU's, while a joule is 1055 BTU's. The joule is the same numerically as a watt-second. Since the watt is defined as a newton-meter per second, just as the horsepower is 550 foot-pounds per second, the rate of doing mechanical work, the watt's connection to engine torque, RPM, and propeller thrust is direct and straightforward.
However, things go less well when converting weight to volume -- something naval architects do every day using Archimedes' Law, the buoyancy being equal to the weight of water displaced. In the English system, a long ton of seawater occupies almost exactly 35 cubic feet of volume. In the metric system, devised by landlubbers, a cubic meter of distilled water has a mass of exactly 1000 kg. A cubic meter of seawater, at 15 degrees C and at the earth's surface, has a mass of about 1026 kg. It gets worse -- European naval architects "round" this cumbersome number to 1025.
Then there is the question of weight versus mass. The two are related very closely. If I understood Physics 101 correctly, mass is a measure of an object's inertia or momentum, while weight is a force -- the force of attraction the object has exerted on it by the Earth. Now it might well be that the displacement of a ship is the same as its mass. However, we do not use mass units for an individual's weight in the English system, and buoyancy as "the mass of water displaced" makes no sense since buoyancy is an inherent product of gravity with no meaning in outer space. (Mass, unlike weight, is independent of gravity and still has meaning in outer space where objects are "weightless".) Therefore, the metric ton (spelled "tonne" by most naval archtitects in the US to differentiate it from the more familiar long and short tons) is actually a unit of mass, not weight. This is not a particularly helpful distinction and may actually be misleading for designers.
Weight is defined in most systems as mass times a gravitational constant "g" with units of acceleration. In this system, the mass in English units would be in "slugs", which are rarely used. Conversely, in metric units, it's common to use "kilogram-forces" in computations, even though there's already a perfectly good unit of force, the newton, approved by Paris for that purpose. This may be related to the fact that the SI units have a supervisory committee in Paris overseeing them, making sure they follow their inherent logic. The English system may once have had such a committee in London, but no longer; therefore, it is somewhat more free to evolve.
The value of "g" is about 9.81 m/sec squared. To me, that value shows a lack of imagination. Had the French scientists thought about it more, they would have realized that a slightly shorter second would have allowed them to have newtons whose value was exactly 10 times the mass of a kilogram. This in turn would have opened the way to a 100-second minute, and maybe even a 100-minute hour. I wish I could convince myself that the math works out so that these units could be combined with a 20-hour day, but I think the problem is overconstrained by fixing so many variables. The French scientists at the time of their Revolution were willing to invent a new calendar and convert Notre Dame Cathedral into a Temple of Reason. (Both these actions were overturned later). They would certainly have been willing to change the length of a second had they realized how important it was to future engineers.
As it is, the metric system comes off, to me, as different without being much better.
posted by SToby at 14:43
1 Comments:
The Metric System was created by scientists who desperately needed a better system of measurement than was available. Today, the metric system has many applications in nearly every field. Metric works especially well with engineering, physics, international commerce, and other applications that are mathmatically intensive. If such workers used U.S. customary units, they will need to make more calculations than necessary, increasing the chance of errors, some of them serious. If an employee is accustomed to Imperial, some training would be needed. I myself learned metric in a few minutes-at age 8-on my own! Of course children learn more quickly than older people, and I was eager to learn the new system. This was in 1975, the year of the Metric Conversion Act, when I heard that the USa will be converting.
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