Introduction

0.1 Arrangement of the tables

The tables of quantities and units in this International Standard are arranged so that the quantities are presented on the left-hand pages and the units on the corresponding right-hand pages.

All units between two full lines on the right-hand pages belong to the quantities between the corresponding full lines on the left-hand pages.

Where the numbering of an item has been changed in the revision of a Part of ISO 31, the number in the preceding edition is shown in parenthesis on the left-hand page under the new number for the quantity; a dash is used to indicate that the item in question did not appear in the preceding edition.

0.2 Tables of quantities

The names in English and in French of the most important quantities within the field of this International Standard are given together with their symbols and, in most cases, their definitions. These names and symbols are recommendations. The definitions are given for identification of the quantities in the International System of Quantities (ISQ), listed on the left-hand pages of the table; they are not intended to be complete.

The scalar, vectorial or tensorial character of quantities is pointed out, especially when this is needed for the definitions.

In most cases only one name and only one symbol for the quantity are given; where two or more names or two or more symbols are given for one quantity and no special distinction is made, they are on an equal footing. When two types of italic letters exist (for example as with ϑ and θ; ϕ and φ; a and a; g and g) only one of these is given. This does not mean that the other is not equally acceptable. It is not recommended to give such variants different meanings. A symbol within parenthesis implies that it is a reserve symbol, to be used when, in a particular context, the main symbol is in use with a different meaning.

In this English edition, the quantity names in French are printed in an italic font, and are preceded by fr. The gender of the French name is indicated by (m) for male and (f) for female, immediately after the noun in the French name.

0.3 Tables of units

0.3.1 General

The names of units for the corresponding quantities are given together with the international symbols and the definitions. These unit names are language-dependent, but the symbols are international and the same in all languages. For further information, see the SI Brochure (7^th edition 1998) from BIPM and ISO 80000‑1¹⁾.

The units are arranged in the following way.

a) The coherent SI units are given first. The SI units have been adopted by the General Conference on Weights and Measures (Conférence Générale des Poids et Mesures, CGPM). The use of coherent SI units is recommended; decimal multiples and submultiples formed with the SI prefixes are recommended even though not explicitly mentioned.
b) Some non-SI units are then given, being those accepted by the International Committee for Weights and Measures (Comité International des Poids et Mesures, CIPM), or by the International Organization of Legal Metrology (Organisation Internationale de Métrologie Légale, OIML), or by ISO and IEC, for use with the SI.

Such units are separated from the SI units in the item by use of a broken line between the SI units and the other units.
c) Non-SI units currently accepted by the CIPM for use with the SI are given in small print (smaller than the text size) in the “Conversion factors and remarks” column.
d) Non-SI units that are not recommended are given only in annexes in some parts of this International Standard. These annexes are informative, in the first place for the conversion factors, and are not integral parts of the standard. These deprecated units are arranged in two groups:
1. 1) units in the CGS system with special names;
2. 2) units based on the foot, pound, second, and some other related units.
e) Other non-SI units given for information, especially regarding the conversion factors, are given in another informative annex.

0.3.2 Remark on units for quantities of dimension one, or dimensionless quantities

The coherent unit for any quantity of dimension one, also called a dimensionless quantity, is the number one, symbol 1. When the value of such a quantity is expressed, the unit symbol 1 is generally not written out explicitly.

EXAMPLE

Refractive index n = 1,53 × 1 = 1,53

Prefixes shall not be used to form multiples or submultiples of the unit one. Instead of prefixes, powers of 10 are recommended.

EXAMPLE

Reynolds number Re = 1,32 × 10³

Considering that plane angle is generally expressed as the ratio of two lengths and solid angle as the ratio of two areas, in 1995 the CGPM specified that, in the SI, the radian, symbol rad, and steradian, symbol sr, are dimensionless derived units. This implies that the quantities plane angle and solid angle are considered as derived quantities of dimension one. The units radian and steradian are thus equal to one; they may either be omitted, or they may be used in expressions for derived units to facilitate distinction between quantities of different kind but having the same dimension.

0.4 Numerical statements in this International Standard

The sign = is used to denote “is exactly equal to”, the sign is used to denote “is approximately equal to”, and the sign := is used to denote “is by definition equal to”.

Numerical values of physical quantities that have been experimentally determined always have an associated measurement uncertainty. This uncertainty should always be specified. In this International Standard, the magnitude of the uncertainty is represented as in the following example.

EXAMPLE

l = 2,347 82(32) m

In this example, l = a(b) m, the numerical value of the uncertainty b indicated in parentheses is assumed to apply to the last (and least significant) digits of the numerical value a of the length l. This notation is used when b represents the standard uncertainty (estimated standard deviation) in the last digits of a. The numerical example given above may be interpreted to mean that the best estimate of the numerical value of the length l (when l is expressed in the unit metre) is 2,347 82, and that the unknown value of l is believed to lie between (2,347 82 − 0,000 32) m and (2,347 82 + 0,000 32) m, with a probability determined by the standard uncertainty 0,000 32 m and the probability distribution of the values of l.

0.5 Remark on logarithmic quantities and their units

The expression for the time dependence of a damped harmonic oscillation can be written either in real notation or as the real Part of a complex notation

F ( t ) = A e − δ t cos ω t = Re ( A e ( − δ + i ω ) t ) , A = F ( 0 )

This simple relation involving δ and ω can be obtained only when e (base of natural logarithms) is used as the base of the exponential function. The coherent SI unit for the damping coefficient δ and the angular frequency ω is second to the power minus one, symbol s^–1. Using the special names neper, symbol Np, and radian, symbol rad, for the units of δt and ωt, respectively, the units for δ and ω become neper per second, symbol Np/s, and radian per second, symbol rad/s, respectively.

The corresponding variation in space is treated in the same manner

F ( x ) = A e − α x cos β x = Re ( A e − γ x ) , A = F ( 0 ) γ = α + i β

where the unit for α is neper per metre, symbol Np/m, and the unit for β is radian per metre, symbol rad/m.

The taking of logarithms of complex quantities is usefully carried out only with the natural logarithm. In this International Standard, the level L_F of a field quantity F is therefore defined by convention as the natural logarithm of a ratio of the field quantity and a reference value F₀, L_F = In(F /F₀), in accordance with decisions by CIPM and OIML. Since a field quantity is defined as a quantity whose square is proportional to power when it acts on a linear system, a square root is introduced in the expression of the level of a power quantity

L P = ln P / P 0 = ( 1 / 2 ) ln ( P / P 0 )

when defined by convention using the natural logarithm, in order to make the level of the power quantity equal to the level of the corresponding field quantity when the proportionality factors are the same for the considered quantities and the reference quantities, respectively. See IEC 60027‑3:2002, subclause 4.2.²⁾

The neper, symbol Np, and the bel, symbol B, are units for such logarithmic quantities.

The neper is the coherent unit when the logarithmic quantities are defined by convention using the natural logarithm, 1 Np = 1. The bel is the unit when the numerical value of the logarithmic quantity is expressed in terms of decimal logarithms, 1B = (1/2) ln 10 Np≈ 1,151 293 Np. The use of the neper is mostly restricted to theoretical calculations on field quantities, when this unit is most convenient, whereas in other cases, especially for power quantities, the bel, or in practice its submultiple decibel, symbol dB, is widely used. It should be emphasized that the fact that the neper is chosen as the coherent unit does not imply that the use of the bel should be avoided. The bel is accepted by the CIPM and the OIML for use with the SI. This situation is in some respect similar to the fact that the unit degree (^◦) is commonly used in practice instead of the coherent SI unit radian (rad) for plane angle.

Generally it is not the logarithmic quantity itself, such as L_F or L_P, which is of interest; it is only the argument of the logarithm that is of interest, i.e. F/F₀ and P/P₀, respectively.

To avoid ambiguities in practical applications of logarithmic quantities, the unit should always be written out explicitly after the numerical value, even if the unit is neper, 1 Np = 1. Thus, for power quantities, the level is generally given by L_P = 10 lg(P/P₀) dB, and it is the numerical value 10 lg(P/P₀) and the argument P/P₀ that are of interest. This numerical value is, however, not the same as the quantity L_P, because the unit decibel (or the unit bel) is not equal to one, 1. This applies to field quantities where the level is generally given by L_F = 10 lg(F/F₀)² dB.

EXAMPLE 1

The implication of the statement that L_F = 3 dB (= 0,3 B) for the level of a field quantity is that it is to be read as meaning: lg(F/F₀)² = 0,3, or (F/F₀)² = 10^0,3. (It also implies that L_F ≈ 0,3 × 1,151 293 Np = 0,345 387 9 Np, but this is not often used in practice.)

EXAMPLE 2

Similarly, the implication of the statement that L_P = 3 dB (= 0,3 B) for the level of a power quantity is that it is to be read as meaning: lg(P/P₀) = 0,3, or (P/P₀)= 10^0,3. (It also implies that L_P ≈ 0,3 × 1,151 293 Np = 0,345 387 9 Np, but this is not often used in practice.)

Meaningful measures of power quantities generally require time averaging to form a mean-square value that is proportional to power. Corresponding field quantities may then be obtained as the root-mean-square value. For such applications, the decimal (base 10) logarithm is generally used to form the level of field or power quantities. However, the natural logarithm could also be used for these applications, especially when the quantities are complex.

BS EN ISO 80000-3:2013

Quantities and units - Space and time

Published:

31 Oct 2007

Withdrawn:

5 Nov 2020