Of SI and Standards

Cicely Enright

Including Units in Standards

Whether current versions of time-honored standards such as specifications for steel tee rails and cement or brand new guides for search and rescue certification groups and tissue scaffold permeability, ASTM International standards are subject to the dictates of Form and Style for ASTM Standards. That includes use of the International System of Units, commonly called SI units.

Part H of today's Form and Style, the guidance document for the format of ASTM standards, states, "ASTM technical committees are urged to give diligent consideration to the use of rationalized SI (metric) units in their standards." The manual also indicates that "each technical committee shall have the option of using rationalized SI units, or rationalized inch-pound units, or both, as the standard units of measure."

ASTM staff members are working with committees to bring into compliance any standards that have yet to catch up with Part H requirements. Over the past several years ASTM has tagged all 12,000 standards as either solely SI, combined, inch-pound, no units or not in compliance. Staff is working with committees to address standards that are not in compliance with one of the Form and Style options.

Consequently, a great deal of progress has been made in ensuring that committees use SI units, non-SI units or both, appropriately. This effort has revealed that about 17 percent of standards, such as most terminology standards, do not use units. Of the standards that do use units, a majority, about 51 percent, use SI only; 33 percent use non-SI units; and 16 percent use both units combined.

The main issue with noncompliance occurs when inch-pound units are given in parentheses. When this work began in 2008, 66 percent of standards followed Form and Style, and today, this number has increased to 85 percent.

ASTM International staff continues to lend a hand to technical committees where changes can be managed editorially, with the approval of the responsible technical subcommittee, as with the following situations.

Standards that do not have combined units but already contain rationalized SI and inch-pound units can be brought into compliance with a scope change and putting one set of units in brackets. This change is reflected by an epsilon after the standard's designation number.

Standards listing SI units as primary that actually contain nonrationalized SI and rationalized inch-pound units in parentheses can be made into inch-pound standards with nonrationalized SI in parentheses, with an accompanying editorial scope change and addition of an epsilon.

When scope statements are missing or inaccurately reflect the standard's unit usage, changes can be made without the addition of an epsilon.

Should a standard need conversions from inch-pound units to SI or combined, however, such conversions must be balloted after you and your fellow task group or subcommittee members make such revisions.

Guidance for these situations can be obtained by contacting your committee's standards editor. If you're at a committee meeting and need editorial assistance with units, you can consult with a standards editor on site.

Form and Style's Part H, "Use of SI Units in ASTM Standards," describes how to manage units in standards; the manual is available online. Please contact your committee's staff manager or standards editor if you have questions.

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