Guide for the Use of the International System of Units (SI)
Table 5. SI prefixes Factor
Prefix
10 24 10 21 10 18 10 15 10 12 10 9 10 6 10 3 10 2 10 1
= = = = = = = =
(10 3 ) 8 (10 3 ) 7 (10 3 ) 6 (10 3 ) 5 (10 3 ) 4 (10 3 ) 3 (10 3 ) 2 (10 3 ) 1
5
Units Outside the SI
Symbol
yotta zetta exa peta tera giga mega kilo hecto deka
Factor 1021 1022 1023 1026 1029 10212 10215 10218 10221 10224
Y Z E P T G M k h da
= = = = = = = =
Prefix deci centi milli micro nano pico femto atto zepto yocto
(10 3 ) 21 (10 3 ) 22 (10 3 ) 23 (10 3 ) 24 (10 3 ) 25 (10 3 ) 26 (10 3 ) 27 (10 3 ) 28
Symbol d c m m n p f a z y
Units that are outside the SI may be divided into three categories: — those units that are accepted for use with the SI; — those units that are temporarily accepted for use with the SI; and — those units that are not accepted for use with the SI and thus in the view of this Guide must strictly be avoided. 5.1
Units accepted for use with the SI The following four sections discuss in detail the units this Guide accepts for use with the SI.
5.1.1
Hour, degree, liter, and the like
Certain units that are not part of the SI are essential and used so widely that they are accepted by the CIPM, and thus by this Guide , for use with the SI [2, 3]. These units are given in Table 6. The combination of units of this table with SI units to form derived units should be restricted to special cases in order not to lose the advantages of the coherence of SI units. (The use of SI prefixes with the units of Table 6 is discussed in Sec. 6.2.8.) Additionally, this Guide recognizes that it may be necessary on occasion to use time-related units other than those given in Table 6; in particular, circumstances may require that intervals of time be expressed in weeks, months, or years. In such cases, if a standardized symbol for the unit is not available, the name of the unit should be written out in full. (See Sec. 8.1 for a suggestion regarding the symbol for year and Chapter 9 for the rules and style conventions for spelling unit names.) Table 6. Units accepted for use with the SI Name
Symbol
6 6
minute hour time day degree minute plane angle (a) second liter metric ton (c) (a) (b)
(c)
Value in SI units
min h d 8 ' " l, L (b) t
1 min 1h 1d 18 1' 1" 1L 1t
= = = = = = = =
60 s 60 min = 3600 s 24 h = 86 400 s (p/180) rad (1/60)8=(p/10 800) rad (1/60)'=(p/648 000) rad 1 dm3 = 1023 m3 10 3 kg
See also Sec. 7.2. The alternative symbol for the liter, L, was adopted by the CGPM in order to avoid the risk of confusion between the letter l and the number 1 (see Ref. [2] or [3]). Thus, although both l and L are internationally accepted symbols for the liter, to avoid this risk the symbol to be used in the United States is L (see Refs. [1] and [8]). The script letter , is not an approved symbol for the liter. This is the name to be used for this unit in the United States (see Refs. [1] and [8]); it is also used in some other Englishspeaking countries. However, this unit is called ‘‘tonne’’ in Ref. [2] and is the name used in many countries.
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