NOM-008-SCFI General System of Units of Measurement - General System of Units

The standard “NOM-008-SCFI General system of units of measurement - General system of units”, published under the Official Mexican Standard (NOM), describes the definitions, symbols and notation rules for the units of the International System of Units (SI) and for other units outside this system adopted by the General Conference on Weights and Measures (CGPM). Together they form the General System of Units of Measurement used in various fields of science, technology, industry, education and commerce.

For the purposes of this standard, the following terms and definitions apply:

• The International System of Units (SI) refers to the coherent system of units adopted by the General Conference on Weights and Measures (CGPM). This system consists of SI base units and SI derived units.
• SI base units are units of measurement for the basic quantities of the International System of Units.
• Magnitude is a quality of a phenomenon, object or substance that can be distinguished qualitatively and determined quantitatively.
• A coherent system of units (measurement) is a system of units consisting of a set of base units and compatible derived units.
• Fundamental quantities are quantities that are accepted as independent of each other by convention within a system of quantities.
• SI derived units are units formed by combining base units with each other or with derived units according to algebraic expressions that relate the corresponding quantities according to simple physical laws.

There are 7 SI base units and they correspond to quantities such as length, mass, time, electric current intensity, thermodynamic temperature, luminous intensity and amount of matter. The names of the units are meter, kilogram, second, ampere, kelvin, candela and mole, respectively. The quantities, units, symbols and definitions are given in the table titled "Names, symbols and definitions of SI base units" given in the standard.

SI derived units are derived from base units and are expressed using mathematical symbols for multiplication and division. There are three classes of units:

• The first are SI units derived from base units. Some examples of these are given in the tables “Names of quantities, symbols, and definitions of derived SI units” and “Examples of derived SI units without special names”.
• The second one is formed by the SI derived units having a special name and symbol, the full list is given in the table titled “SI derived units having a special name and symbol”.
• The third is formed by expressing derived SI units with special names. Some examples of these are given in the table titled “Examples of derived SI units expressed with special names”.

There are many derived units used in scientific fields. For ease of reference, they are grouped into 10 tables corresponding to the equivalent numbers of the most important fields of physics, according to the following relationship:

• Fundamental quantities and units of space and time
• Principal magnitudes and units of periodic and related events
• Fundamental quantities and units of mechanics
• The main quantities and units of heat
• Principal quantities and units of electricity and magnetism
• Fundamental quantities and units of light and electromagnetic radiation
• Fundamental quantities and units of acoustics
• Fundamental quantities and units of physical chemistry and molecular physics
• Fundamental quantities and units of atomic and nuclear physics
• Principal quantities and units of nuclear reactions and ionizing radiation

Some non-SI units are also widely used and have been divided into three categories by the General Conference on Weights and Measures (CGPM):

• Units retained for use with the SI: These are widely used units, so it is considered appropriate to retain them. However, it is recommended that they not be combined with SI units in order not to lose their consistency advantages. The relationship between these units is given in the table titled “Non-SI units retained for use with the SI”.
• Units that can be used temporarily with the SI: These are units that should be avoided. They are temporarily retained because of their widespread use at present, but it is recommended that they not be used with SI units. The relationships between these units are given in the table titled “Non-SI units that can be used temporarily with the SI”.
• Units that should not be used with SI: There are other units that are not SI units. These are currently in some use. These units do not correspond to any of the categories specified in this standard. Therefore, they should not be used because they cause the SI to lose its consistency. It is recommended to use the relevant SI units instead. Some examples of these units are given in the table titled "Examples of units that should not be used".

A list containing the names and symbols of the prefixes forming decimal multiples and submultiples of the units in the tables is included in the table titled “Prefixes forming multiples and submultiples”.

The rules for the correct writing of SI unit symbols and prefixes are given in the table titled “General rules for writing SI unit symbols”.

The rules for writing numbers and decimal marks, in accordance with the recommendations of the International Organization for Standardization (ISO), are given in the table titled "Rules for writing numbers and decimal marks".

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