Metre Convention and Evolution of Base Units

Abstract SI surpasses previous metric systems in coherence, simplicity, distinction between force and mass, choice of units, and uniformity of usage. Coherence eliminates conversion factors, other than powers of ten, within the system. Through simplification, only seven base units, two supplementary units, fifteen named derived units, and fourteen prefixes are needed to describe the entire system. Introduction of the newton as a unit of force and retention of the kilogram for mass clarifies the difference between the two quantities. Other new units are: pascal for pressure, Siemens for electric conductance and mole for amount of a substance. International agreement minimizes proliferation of specialized units or usage. Adopted conventions include spelling, abbreviations, style, and usage, as well as definitions for the units. Tolerances, rounding of numbers, and elimination of the comma in multidigit numbers assume new significance. Conversion to SI offers a unique opportunity to reduce the number of sizes of many products. Several schemes for metric modularism have been developed for this purpose. In changing from conventional to SI units the key idea is “think metric” rather than use dual dimensioning.

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SI Units

PEDIATRICS ◽

10.1542/peds.83.1.129 ◽

1989 ◽

Vol 83 (1) ◽

pp. 129-131

Author(s):

Keyword(s):

United States ◽

American Medical Association ◽

Scientific Communication ◽

The United States ◽

Coherent System ◽

Common Language ◽

Si Units ◽

Drug Concentrations ◽

Significant Figures ◽

Base Units

Le Systeme International d'Unites or SI units is a system of measure that is an outgrowth of the metric system, widely used in countries other than the United States. This system comprises seven base units from which all other measurements are made (Table). The aim of SI units is to (1) provide a coherent system of measure, (2) ensure that quantities and units are uniform in concept and style, and (3) minimize the number of multiples and submultiples in common use. A more complete review of metrification and SI units has been published in this journal. The conversion to SI units was mandated in December 1984 by the American Medical Association House of Delegates to "provide physicians and other scientists in the United States with an improved common language for fluent scientific communication between nations as well as between sciences." In several published articles there have been suggestions that the United States should, during the next year, begin uniformly reporting laboratory test data in SI units, including drug concentrations in molar units. Although drugs would continue to be prescribed in mass units, the desirability of prescribing and dispensing drugs in molar units has been suggested. The focus of this commentary is to summarize the problems and inconsistencies of expressing drug concentrations in molar units, assuming that prescribing and dispensing would continue in mass units. PHARMACOKINETIC CALCULATIONS In a recent study tenfold dosing errors were demonstrated, resulting from mathematical miscalculations while using traditional units. The introduction of additional calculations for conversion to SI units, and the fact that SI units involve decimal places with several significant figures, will likely magnify this type of error.

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