Guide for the Use of the International System of Units (SI)

 
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Guide for the Use of the International System of Units (SI) 
 
30 
the quantity whose level is being reported, for example, electric field strength E or sound pressure p, and 
x
ref
is the value of the reference quantity, for example, 1 μV/m for E
0
, and 20 μPa for p
0
. Thus
L
E
 (re 1 μV/m) = − 0.58 Np or L
E/(1 μV/m)
= − 0.58 Np 
means that the level of a certain electric field strength is 0.58 Np below the reference electric field strength 
E
0
= 1 μV/m. Similarly
L
p
 (re 20 μPa) = 25 dB or L
p/(20 μPa) 
= 25 dB 
means that the level of a certain sound pressure is 25 dB above the reference pressure p
0
= 20 μPa.
Notes: 
1. When such data are presented in a table or in a figure, the following condensed notation may be 
used instead: − 0.58 Np (1 μV/m); 25 dB (20 μPa). 
2. When the same reference level applies repeatedly in a given context, it may be omitted if its value 
is clearly stated initially and if its planned omission is pointed out.
3. The rules of Ref. [5: IEC 60027-3] preclude, for example, the use of the symbol dBm to indicate a 
reference level of power of 1 mW. This restriction is based on the rule of Sec. 7.4, which does not 
permit attachments to unit symbols. 
8.8 Viscosity 
 
The proper SI units for expressing values of viscosity η (also called dynamic viscosity) and values of 
kinematic viscosity 
ν are, respectively, the pascal second (Pa·s) and the meter squared per second (m
2
/s) 
(and their decimal multiples and submultiples as appropriate). The CGS units commonly used to express 
values of these quantities, the poise (P) and the stoke (St), respectively [and their decimal submultiples the 
centipoise (cP) and the centistoke (cSt)], are not to be used; see Sec. 5.3.1 and Table 10, which gives the 
relations 1 P = 0.1 Pa
·s and 1 St = 10
−4
m
2
/s.