02-26-2022 comment

1 Scope
This part of I S0 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 IS0 31 are in- tended for general use within the various fields of science and technology and as a general introduction to the other parts of I S0 31.
2 Quantities and units
In IS0 31 only physical quantities used for the quan- titative description of physical phenomena are treated. Conventional scales, such as the Beaufort scale, Richter scale and colour intensity scales, and quan- tities 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 cat- egories 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 cat- egory 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 numeri- cal value of the quantity expressed in this unit.
It is essential to distinguish between the quantity it- self and the numerical value of the quantity expressed in a particular unit. The numerical value of a quantity expressed in a particular unit could be indicated by placing braces (curly brackets) around the quantity symbol and using the unit as a subscript. It is, how- ever, preferable to indicate the numerical value ex- plicitly as the ratio of the quantity to the unit.
2.2.2 Equations between quantities and equations between numerical values
Two types of equation are used in science and tech- nology: equations between quantities, in which a let- ter symbol denotes the physical quantity (¡.e. numerical value x unit), and equations between numerical values. Equations between numerical val- ues depend on the choice of units, whereas equations between quantities have the advantage of being in- dependent of this choice. Therefore the use of equations between quantities should normally be preferred.
2.2.3 Empirical constants
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.

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