




NonSI units accepted for use with the SI, and units based on fundamental constants (contd.)


Table 8 also gives the units of logarithmic ratio quantities, the neper, bel, and decibel. These are dimensionless units that are somewhat different in their nature from other dimensionless units, and some scientists consider that they should not even be called units. They are used to convey information on the nature of the logarithmic ratio quantity concerned. The neper, Np, is used to express the values of quantities whose numerical values are based on the use of the neperian (or natural) logarithm, ln = log_{e}. The bel and the decibel, B and dB, where 1 dB = (1/10) B, are used to express the values of logarithmic ratio quantities whose numerical values are based on the decadic logarithm, lg = log_{10}. The way in which these units are interpreted is described in footnotes (g) and (h) of Table 8. The numerical values of these units are rarely required. The units neper, bel, and decibel have been accepted by the CIPM for use with the International System, but are not considered as SI units.
The SI prefixes are used with two of the units in Table 8, namely, with the bar (e.g. millibar, mbar), and with the bel, specifically for the decibel, dB. The decibel is listed explicitly in the table because the bel is rarely used without the prefix.
Table 8. Other nonSI units

Quantity 
Name of unit 
Symbol for unit 
Value in SI units 

pressure 
bar ^{(a)} 
bar 
1 bar = 0.1 MPa = 100 kPa = 10^{5} Pa 
millimetre of mercury ^{(b)} 
mmHg 
1 mmHg 133.322 Pa 
length 
ångström ^{(c)} 
Å 
1 Å = 0.1 nm = 100 pm = 10^{–10} m 
distance 
nautical mile ^{(d)} 
M 
1 M = 1852 m 
area 
barn ^{(e)} 
b 
1 b = 100 fm^{2} = (10^{–12} cm)^{2} = 10^{–28} m^{2} 
speed 
knot ^{(f)} 
kn 
1 kn = (1852/3600) m/s 
logarithmic ratio quantities 
neper ^{(g,i)}
 Np 
[see footnote (j) regarding the numerical value of the neper, the bel and the decibel] 
bel ^{(h,i)}
 B 
decibel ^{(h,i)}
 dB 
(a) 
The bar and its symbol are included in Resolution 7 of the 9th CGPM (1948). Since 1982 one bar has been used as the standard pressure for tabulating all thermodynamic data. Prior to 1982 the standard pressure used to be the standard atmosphere, equal to 1.013 25 bar, or 101 325 Pa. 
(b) 
The millimetre of mercury is a legal unit for the measurement of blood pressure in some countries. 
(c) 
The ångström is widely used by xray crystallographers and structural chemists because all chemical bonds lie in the range 1 to 3 ångströms. However it has no official sanction from the CIPM or the CGPM. 
(d) 
The nautical mile is a special unit employed for marine and aerial navigation to express distance. The conventional value given here was adopted by the First International Extraordinary Hydrographic Conference, Monaco 1929, under the name "International nautical mile". As yet there is no internationally agreed symbol, but the symbols M, NM, Nm, and nmi are all used; in the table the symbol M is used. The unit was originally chosen, and continues to be used, because one nautical mile on the surface of the Earth subtends approximately one minute of angle at the centre of the Earth, which is convenient when latitude and longitude are measured in degrees and minutes of angle. 
(e) 
The barn is a unit of area employed to express cross sections in nuclear physics. 
(f) 
The knot is defined as one nautical mile per hour. There is no internationally agreed symbol, but the symbol kn is commonly used. 
(g) 
The statement L_{A} = n Np (where n is a number) is interpreted to mean that ln(A_{2}/A_{1}) = n. Thus when L_{A} = 1 Np, A_{2}/A_{1} = e. The symbol A is used here to denote the amplitude of a sinusoidal signal, and L_{A} is then called the neperian logarithmic amplitude ratio, or the neperian amplitude level difference. 
(h) 
The statement L_{X} = m dB = (m/10) B (where m is a number) is interpreted to mean that lg(X/X_{0}) = m/10. Thus when L_{X} = 1 B, X/X_{0} = 10, and when L_{X} = 1 dB, X/X_{0} = 10^{1/10}. If X denotes a mean square signal or powerlike quantity, L_{X} is called a power level referred to X_{0}. 
(i) 
In using these units it is important that the nature of the quantity be specified, and that any reference value used be specified. These units are not SI units, but they have been accepted by the CIPM for use with the SI. 
(j) 
The numerical values of the neper, bel, and decibel (and hence the relation of the bel and the decibel to the neper) are rarely required. They depend on the way in which the logarithmic quantities are defined. 





We are pleased to present the updated (2014) 8th edition of the SI Brochure, which defines and presents the Système International d'Unités, the SI (known in English as the International System of Units).
 SI prefixes

Factor 
Name 
Symbol 

Factor 
Name 
Symbol 

10^{1} 
deca 
da 

10^{–1} 
deci 
d 
10^{2} 
hecto 
h 
10^{–2} 
centi 
c 
10^{3} 
kilo 
k 
10^{–3} 
milli 
m 
10^{6} 
mega 
M 
10^{–6} 
micro 
µ 
10^{9} 
giga 
G 
10^{–9} 
nano 
n 
10^{12} 
tera 
T 
10^{–12} 
pico 
p 
10^{15} 
peta 
P 
10^{–15} 
femto 
f 
10^{18} 
exa 
E 
10^{–18} 
atto 
a 
10^{21} 
zetta 
Z 
10^{–21} 
zepto 
z 
10^{24} 
yotta 
Y 
10^{–24} 
yocto 
y 

 The kilogram
General principles for the writing of unit symbols and numbers were first given by the 9th CGPM (1948, Resolution 7). These were subsequently elaborated by ISO, IEC, and other international bodies. As a consequence, there now exists a general consensus on how unit symbols and names, including prefix symbols and names, as well as quantity symbols should be written and used, and how the values of quantities should be expressed. Compliance with these rules and style conventions, the most important of which are presented in this chapter, supports the readability of scientific and technical papers.
This appendix lists those decisions of the CGPM and the CIPM that bear directly upon definitions of the units of the SI, prefixes defined for use as part of the SI, and conventions for the writing of unit symbols and numbers. It is not a complete list of CGPM and CIPM decisions. For a complete list, reference must be made to the BIPM website, successive volumes of the Comptes Rendus des Séances de la Conférence Générale des Poids et Mesures (CR) and ProcèsVerbaux des Séances du Comité International des Poids et Mesures (PV) or, for recent decisions, to Metrologia.
Since the SI is not a static convention, but evolves following developments in the science of measurement, some decisions have been abrogated or modified; others have been clarified by additions. In the SI Brochure, a number of notes have been added by the BIPM to make the text more understandable; they do not form part of the original text.
In the printed brochure, the decisions of the CGPM and CIPM are listed in strict chronological order in order to preserve the continuity with which they were taken. However in order to make it easy to locate decisions related to particular topics a table of contents is also provided, ordered by subject:
Optical radiation is able to cause chemical changes in certain living or nonliving materials: this property is called actinism, and radiation capable of causing such changes is referred to as actinic radiation. Actinic radiation has the fundamental characteristic that, at the molecular level, one photon interacts with one molecule to alter or break the molecule into new molecular species. It is therefore possible to define specific photochemical or photobiological quantities in terms of the result of optical radiation on the associated chemical or biological receptors.
In the field of metrology, the only photobiological quantity which has been formally defined for measurement in the SI is for the interaction of light with the human eye in vision. An SI base unit, the candela, has been defined for this important photobiological quantity. Several other photometric quantities with units derived from the candela have also been defined (such as the lumen and the lux, see Table 3 in Chapter 2).





