Metric Units, System International (SI)
Metric Units, System International (SI)
The most widely used system of units and measures around the world is the Systeme International d’Unites (SI), which is the modern form of the metric system. The other system of measurement is the U.S. Customary System Units, also known as “English Units,” consisting of the mile, foot, inch, gallon, second, and pound. Although the English system is gradually being replaced by the metric system in some sectors of U.S. industry, the full conversion of the U.S. to the metric system is still incomplete.
Using metric units or providing metric equivalents is important because of the following reasons:
1. Use of metric units facilitates understanding and communication in technical areas such as engineering, architecture, building codes, and other scientific areas at a global level.
2. Use of metric units is simpler because variations from smaller to larger units or vice versa are in multiples of 10, but in English units the multipliers could vary with unit and with subject. For example, smaller units of an inch are ½, ¼, ⅛, or 1/16 of an inch, each a multiplier of 2 larger than the other. Conversion of inches to feet is at a multiplier of 12 and from feet to yards at a multiplier of 3. For metric, the smaller unit of a centimeter is a millimeter, which is a centimeter divided by 10. Conversion of centimeters to decimeters is by multiplying a centimeter by 10, and conversion of decimeters to meters is by multiplying by 10, and so on. Accordingly, computations and problem solving are prone to less error.
The conversion to metric units can take two forms—soft metrication and hard metrication.
Soft Metrication
Soft metrication is the use of metric units in specifying measurements, sizes, and other dimensions without changing product sizes and without changing the everyday practice of using English units. For example, a wood-stud member commonly used is a 2 × 4, which is actually 1.5 inches × 3.5 inches. To report or to specify the actual size of this member in metric, a soft conversion of 38 mm × 89 mm is used (rounded from actual 38.1 and 88.9). Another example could be the load-bearing pressure of clay soils of 1,500 pounds per square foot being reported as 72 kPa (kilopascals).
Hard Metrication
Hard metrication goes beyond soft metrication and converts production and manufacturing based on metric sizes. For example, instead of manufacturing 2 × 4 wood studs of 1.5 inches × 3.5 inches (38.1 mm × 88.9 mm, actual dimensions), wood studs of 40 mm by 90 mm might be manufactured. Another example is ½-inch-diameter (12.7-mm) U.S. size automotive bolts versus 13-mm metric bolts, which are manufactured with a diameter of 13 mm. The production of other structural or nonstructural members such as structural steel, plywood, nails, pipes, ducts, insulation panels, and all other such elements would also be done in metric rather than manufacturing in English units and reporting metric equivalents.
More information on the SI system in the United States is available from the National Institute of Standards and Technology (NIST), an agency of the U.S. Department of Commerce: http://physics.nist.gov/cuu/Units/index.html or http://physics.nist.gov/Pubs/SP811/contents.html.
Unit Conversion
Tables
SI Symbols and Prefixes
Conversion Factors
