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A Guide to SI Usage for the Reluctant

by Stan Jakuba

A great number of Americans are reluctant to deal with the metric system. Unfortunately for them, it’s a language spoken by nearly everyone else on the planet. Stan Jakuba offers some plain and simple information on the ins and outs of metric.

(This manuscript is published with the permission of the author and ASME Mechanical Engineering magazine where most of the text was published in the April 2001 issue (www.asme.org).)

Engineers in the United States rarely need one of the requirements for their counterparts in most countries. They typically don’t have to learn a second language. In most of the world, however, engineers are at least bilingual. Most of them speak English in addition to their native tongue.

The arrangement may make it convenient for Americans to venture abroad, but it also contributes to cultural isolation. And the isolation extends to the system of measurement.

There has been an undue amount of criticism over the years condemning the metric system for its inconsistency (as compared to the English—or inch-pound—system, presumably). We little realize how inconsistent, illogical, and unsystematic the English “system” is because we are not readily confronted with alternatives.

The inconsistencies the critics of the metric system focus on are miniscule and most are due to the system’s evolution. The evolution is inevitable. Any system that wants to keep up with the evolving society must change.

Among metric users, as in any population, there are people who do not want to accept change, or are not aware of revisions or the need for them, who prefer to cling to established ways. Inconsistencies arise, then, in the way that people use the system, not in the system itself.

The Metric System Is a Standard

There indeed is one, and only one, metric system—always the latest revision of the international standard that describes it. For engineers worldwide, the standard is ISO 1000. This standard is subject to change, as any standard must be, and a user is expected to follow the latest version, which is the practice with all standards. The standard describes a universal, international language of measurement.

Essentially, all units created in a coordinated manner in modern times are metric in every country of the world, including the United States. The evolution is guarded by an international committee in which the United States has participated since 1875.

It may be comforting to many to learn that the standard, in the section most people use, is not expected to change for a long time. And the last major revision took place in 1960—two generations ago.

Note that there is no officially recognized forum for the development of any other system of units, of which there used to be dozens, including the English system and any version of it.

The modern system of measurement is properly called SI, not metric. SI, for the French Systeme International, is based on seven units, called base units, and a set of names, called prefixes, that stand for certain multiples.

Base units measure such basic physical quantities as length, mass, or time. Alone or in combination, they let mankind measure everything.

Derived Units

There are, of course, hundreds of units needed for measuring “everything,” but they are all derived from those seven. The derivation is done in a way that provides the marvelous and unique feature of SI: there are no conversion factors.

Some of the derivations were given a special name. This was applied in cases where the combination would be too long and cumbersome for frequent use, or where confusion could result. Most have been used in the English system also because no official non-metric equivalent ever existed.

All derived units can be expressed in terms of the base units. The derivations are as straightforward as the relationship of length to breadth to compute area, m2. The unit newton, for example, is derived from mass times acceleration, the kilogram accelerating a metre per second per second or, in graphic symbols, kg·m/s2.

As convenient, units may be expressed in a combination of both base and derived units. Knowing that torque, for example, means force times distance, leads to the newton metre, N·m. Or, for energy density (energy per mass or volume), the same logic leads to the units J/kg or J/m3. Pressure is force per area, hence the unit pascal is N/m2.

Many derived units can be expressed in more than one form, but professional use usually settles on a single convention. For example, the unit of dynamic viscosity could be expressed as kg/(m·s) or N·s/m2 or Pa·s. Only the last form is prevalent.

Holdovers From the Past

As pointed out by SI’s critics, there are inconsistencies in the sense that non-SI units and terms remain in local (and, in some cases, general and approved) use. They mostly reflect a tradition that is slow to die.

Here are several examples of terms carried over from the past that are still in common and approved use.

• The degree Celsius (symbol °C) designates a temperature on the Celsius scale. Note that as an increment, the degree Celsius is identical to the kelvin.
• The degree in plane angle (symbol °) is an alternative to the SI radian.
• The litre and millilitre are the everyday usage alternates for dm3 and cm3, respectively.

Handling Long Numbers

Prefixes (kilo-, centi-, milli-, etc.) often precede the name of an SI unit. They were devised to shorten long numbers; for example, 20 000 kg shortens to 20 Mg, and 0.0008 A to 0.8 mA.

Prefixes are the names of “power” multiples (hundred, thousand, million, etc.). Their use provides an alternative to the scientific notation (50 000 expressed as 5x104), eliminates the need for the creation of unnecessary new units (5,280 feet grouped into one mile), and helps retain only significant digits.

The 10n notation is impractical for the non-scientific person, and the creation of the new units is impractical for everybody, because in the modern world it would necessitate coining thousands of names and subjecting each of us to memorizing hundreds of them.

Instead, the prefixes shorten numbers to make them convenient for everybody to use.

While the SI committee has so far established 20 prefixes, far fewer—perhaps eight—are needed in daily life and in common technical work.

Ten are more than most people need.

*The use of these four prefixes is declining all over the world with hekto (hecto) and deka (deca) not in engineering use anymore, and deci and centi surviving in engineering only with m3 and m4 (and, to be picky, deci- alone with the non-SI unit “bel” as in decibel (dB)).

With rare exception, Americans consider km and mm or cm separate units because schools teach converting among them. That misguided practice leads to the persistent argument against metric for having too many and long-named units.

But km or mm is just a way of saying in shorthand “a thousand metres” or “a thousandth of a metre.” One cannot convert among them as one does not convert between “a thousand inches” and “a thousandth of an inch.” Prefixes are a language—the words can be translated, not converted.

There is hope. We nowadays seem to treat “kilobyte,” “megabyte,” and “gigabyte” quite comfortably as one unit. This author has not heard as yet anyone claiming them to be three different units nor any teacher suggesting converting among them.

Getting To Like SI

Most people, once they understand it, like SI for its logic, for featuring only one unit per physical quantity, and for its lack of conversion factors. On the other hand, some older engineers don’t like to use it. This is understandable. One tends to dislike anything that one does not understand and has little feel for.

The author hopes that this article provides an understanding of the system. To get the feel for the units takes longer, and personal initiative is required—such as reading about SI and trying to remember the commonly used values in units of either system.

Table 1 presents a sample of units and reference values. With them, each unit is shown accompanied by the most suitable prefix. All engineers new to performing their jobs in SI should take the time to write down a similar table for the ball-park figures in their fields, and pin it onto the office wall at eye-level.

Writing the reference numbers and returning to them repeatedly helps in getting the feel for values in SI—a feel of utmost importance for overcoming the resistance to using SI in one’s work.

Seek Brevity and Universality

SI units, prefixes, and rules were established to facilitate data communication worldwide. They represent a compromise intended to suit all languages, to ease arithmetic manipulations, to prevent ambiguity, and to retain some of the traditions of the metric system.

For technical documentation, the preferred way of writing SI prefixes and units is by their symbols; for example, 5 kg, not 5 kilogram, or five kilograms.

If each symbol is written according to the SI rules—distinguishing between uppercase and lowercase letters, and between the Latin and Greek letters—it will be intelligible everywhere, regardless of the script and language a nation uses.

People facing the need to acquire a new skill often delight in debating the pros and cons of the new venture. Nowhere is this more prevalent than with a new system of units. But it should be easy to realize that there cannot be one that satisfies all, just as there cannot be one language that would combine only the “best” features of other languages. What is the best unit in one profession may be the worst in another. SI is the best compromise; its unceasing advancement into the standards writing communities and daily usage in all countries of the world attests to its being “the best.” //

Copyright 2001, ASTM

Stan Jakuba is president of S I Jakub Associates in West Hartford, Conn., a firm specializing in training and consulting in international engineering standards and practices. (See his Web site.)

IEEE/ASTM
SI 10
The American National Standard

The American National Standard on SI usage is the joint effort published for the first time four years ago, IEEE/ASTM SI 10, Standard for Use of the International System of Units (SI): The Modern Metric System. In order to create SI 10, the Institute of Electrical and Electronics Engineers and ASTM merged their two pre-existing standards, ASTM E 380 and ANSI/IEEE 268. The two organizations cooperated to eliminate differences in style and substance between the two documents.

SI 10 provides material that defines and comments on quantities and SI units used to describe such properties as mass, force, temperature, pressure, angle, moment of inertia, kinetic energy, and work. It also includes descriptions of the classes of measurement units and their magnitude, describing prefixes, a section on style and usage, and clear guidance in writing SI symbols correctly. The publication includes conversion tables and information for determining appropriate precision in conversions.

Click here for table describing origins of some SI units.