Metric conversion advances incrementally

Metric conversion advances incrementally

WILLIAM B. GARDNER

The United Kingdom officially adopted the metric system of measurement on October 1, 1995. This ratification means the United States is the only major industrial nation that has not converted to metric unit standards. However, the commitment to the metric system has been made by several U.S. engineering and technical standards bodies. Encouraging the transition to metric for U.S. fiber-optic cable manufacturers are the Insulated Cable Engineers Association (Icea), the Telecommunications Industry Association (TIA), the American National Standards Institute (Ansi) and the American Society for Testing and Materials (Astm).

Most of the fiber optics industry is already using the metric system. Fiber coatings and connector dimensions are routinely given in micrometers; fiber strength in megapascals; and cable lengths in kilometers. But because many cable routes in the United States are surveyed in kilofeet, customers frequently order fiber cables in customary "inch-pound" units.

In deference to those customers that still prefer U.S. customary unit measurements, cable manufacturers provide a dual-dimensioning policy. For example, in Section 9.5 of the TIA`s TSM-6 and TSM-7 Author`s Guides for preparing technical documents, the association recommends that metric units be listed first and then followed by American units. On the other hand, the Icea stipulates the reverse order.

The transition to metric units by adopters generally takes place in two stages, called "soft" and "hard." In a soft metric transition, for example, a designer would not change an established 2.0-inch-diameter measurement for a part, but would respecify that parameter metrically as 5.08 cm. A hard transition involves changing the 5.08-cm diameter to 5 cm. In practice, hard transitions have proved more difficult to implement.

Nevertheless, Section 9.5 of the TIA`s Author`s Guides states that "...the trend to hard metric is accelerating." The trends of the international marketplace are expected to drive the acceptance of the hard metric process.

First proposed by Gabriel Mouton, Vicar of St. Paul in Lyons, France, in 1670, the metric system continues to evolve. The present version--called the International System of Units, or SI--dates from 1960. It supersedes earlier versions such as the centimeter-gram-second (cgs) system.

Some old measurement units that are considered metric, such as the micron and the angstrom, are in fact not part of the SI system. The correct SI equivalents are the micrometer and tenths of a nanometer. A recommended document covering the SI system is the National Institute of Standards and Technology (Nist) Special Publication 811 (1995), titled "Guide for the Use of the International System of Units (SI)." It can be obtained from Nist`s Metric Program Office (tel: (301) 975-3690).

Other useful measurement guides include:

Astm E 380, "Standard Practice for Use of the International System of Units (SI)"

Icea P-57-653-1995, "Guide for Implementation of Metric Units in Icea Publications"

Ansi/Ieee Standard 268-1982, "American National Standard for Metric Practice"

Some technical organizations, including the International Electrotechnical Commission and the U.S. Department of Defense, require the exclusive use of SI units in all new technical standards.

Opponents of the metric system will be pleased to hear that in the post-metric United Kingdom they can still order a pint of ale. q

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