The SI System

Compressed Air Wiki Basic Theory

Any physical quantity is the product of a numerical value and a unit. Since 1964, the International System of Units (SI system) has gradually been adopted worldwide, with the exception of Liberia, Myanmar and the United States.

Basic information can be found in standard ISO 31, which is under revision and will be superseded by ISO/IEC 80000: Quantities and Units. Units are divided into four different classes :



Base units, supplementary units and derived units are called SI units. The additional units are not SI units, although they are accepted for use with SI units. Base units are any of the established, independent units in which all other units can be expressed.

There are 7 base units in the SI system.

Derived units are formed as a power or product of powers of one or more base units and/or supplementary units according to the physical laws for the relationship between these different units.

Additional units: A limited number of units outside the SI system cannot be eliminated for different reasons, and continue to be used along with the SI as additional units.

The 15 most important derived units above have been given generic names.
The units above are common additional units for technical use.

Prefixes may be added to a unit to produce a multiple of the original unit. All of these multiples are integer powers of ten, for example:
kilo-denotes a multiple of thousand (10³)
milli-denotes a multiple of one thousandth (10-3)

Fourteen prefixes are listed in international recommendations (standards) as set out in the table.

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