A system of measurement which has been in existence for approximately two hundred years. It is widely used in Europe; however variations in terms and units existed between countries, and in 1960 this variety of metric units was replaced by the International System of Units (SZ).For more complete information on the SI system, including units, symbols and conversion practices, refer to ANSVAWS Al.1, Metric Practice Guide for the Welding Industry.
SI is a modernized metric system of measurement that has been officially recognized by all industrial nations. It has features that make it superior to the U.S. Customary and to other metric systems. These features are the following:
(1) An Absolute Base. A base that is not defined by the action of gravity.
(2) Coherence. Coherence is the characteristic which relates any derived unit to any other, or to base units from which it is formed, without conversion factors.
(3) Unique Units. The use of only one unit for each physical quantity; for example, SI units for force, energy, and power are the same regardless of whether the process is mechanical, electrical, or thermal.
(4) Decimal System. SI is a decimal system; it is easier to use because it is easier to work in multiples of ten and in decimal notations than in fractions and decimalized fraction equivalents common to the U.S. Customary system.
This combination of features makes SI a reliable system suitable for all kinds of measurements. Although areas remain that can and no doubt will be improved, the SI system is practical for universal application and is rapidly becoming the commonly
used world measurement system.
SI Units Pertaining to Welding. The recommended SI units used in welding nomenclature are shown in 14. The selection of these terms was based on the use of (1) SI base units where practicable; (2) numbers of reasonable size, and (3) accepted units currently in use or anticipated to be used.
Special Conversions for Welding. Terms that are commonly used in the welding industry and conversions between U.S.Customary and SI units are shown in 14.