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SI brochure, Table 8 ( Section 4.1)

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. 








