ISO 31-0:1992 pdf download.Quantities and units - Part 0: General principles
This part of ISO 31 gives general information about principles concerning physical quantities, equations, quantity and unit symbols, and coherent unit systems, especially the International System of Units, SI.
The principles laid down in this part of ISO 31 are intended for general use within the various fields of science and technology and as a general introduction to the other parts of ISO 31.
2 Quantities and units
2.1 Physical quantity, unit and numerical value
In ISO 31 only physical quantities used for the quantitative description of physical phenomena are treated. Conventional scales, such as the Beaufort scale, Richter scale and colour intensity scales, and quantitles expressed as the results of conventional tests, e.g. corrosion resistance, are not treated here, neither are currencies nor information contents.
Physical quantities may be grouped together into categories of quantities which are mutually comparable. Lengths, diameters, distances, heights, wavelengths and so on would constitute such a category. Mutually comparable quantities are called “quantities of the same kind”.
if a particular example of a quantity from such a category is chosen as a reference quantity called the unit, then any other quantity from this category can be expressed in terms of this unit, as a product of this unit and a number. This number is called the numerical value of the quantity expressed in this unit.
An empirical relation is often expressed in the form of an equation between the numerical values of cer-tain physical quantities. Such a relation depends on the units in which the various physical quantities are expressed.
An empirical relation between numerical values can be transformed into an equation between physical quantities, containing one or more empirical con-stants. Such an equation between physical quantities has the advantage that the form of the equation is independent of the choice of the units. The numerical values of the empirical constants occurring in such an equation depend, however, on the units in which they are expressed, as is the case with other physical quantities.
Physical quantities are related to one another through equations that express laws of nature or define new quantities.
For the purpose of defining unit systems and intro-ducing the concept of dimensions, it is convenient to consider some quantities as mutually independent,i.e. to regard these as base quantities, in terms of which the other quantities can be defined or ex-pressed by means of equations; the latter quantities are called derived quantities.
It is a matter of choice how many and which quan-tities are considered to be base quantities.
The whole set of physical quantities included in ISO 31 is considered as being founded on seven base quantities: length, mass, time, electric current,thermodynamic temperature, amount of substance and luminous intensity.ISO 31-0 pdf download.