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| 1. |
Only units of the SI and those units recognized for use with
the SI are used to express the values of quantities. Equivalent values in other
units are given in parentheses following values in acceptable units only
when deemed necessary for the intended audience. (See
Chapter 2.)
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|
| 2. |
Abbreviations such as sec (for either s or second), cc (for
either cm3 or cubic centimeter), or mps (for either m/s or meter per
second) are avoided and only standard unit symbols, SI prefix symbols, unit
names, and SI prefix names are used. (See
Sec. 6.1.8.)
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| 3. |
The combinations of letters "ppm," "ppb,"
and "ppt," and the terms part per million, part per billion, and part
per trillion, and the like, are not used to express the values of quantities.
The following forms, for example, are used instead: 2.0 µL/L or
2.0 × 10-6 V, 4.3 nm/m or
4.3 × 10-9 l, 7 ps/s or
7 × 10-12 t, where V, l, and
t are, respectively, the quantity symbols for volume, length, and time.
(See Sec. 7.10.3.) |
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|
4. |
Unit symbols (or names) are not modified by the addition of
subscripts or other information. The following forms, for example, are used
instead. (See Sec. 7.4 and
Sec. 7.10.2.) |
| Vmax = 1000 V |
but not: |
V = 1000 Vmax |
| a mass fraction of 10 % |
but not: |
10 % (m/m) or 10 % (by weight) |
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|
5. |
Statements such as "the length l1
exceeds the length l2 by 0.2 %" are avoided because
it is recognized that the symbol % represents simply the number 0.01.
Instead, forms such as "l1 =
l2(1 + 0.2 %)" or
"Δ = 0.2 %" are used, where Δ
is defined by the relation Δ = (l1 -
l2)/l2. (See
Sec. 7.10.2.) |
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|
6. |
Information is not mixed with unit symbols (or names). For
example, the form "the water content is 20 mL/kg" is used and
not "20 mL H2O/kg" or "20 mL of
water/kg." (See Sec. 7.5.) |
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|
7. |
It is clear to which unit symbol a numerical value belongs and
which mathematical operation applies to the value of a quantity because forms
such as the following are used. (See
Sec. 7.7.) |
| 35 cm × 48 cm |
but not: |
35 × 48 cm |
| 1 MHz to 10 MHz or
(1 to 10) MHz |
but not: |
1 MHz-10 MHz or 1 to 10 MHz |
| 20 °C to 30 °C or (20 to 30) °C |
but not: |
20 °C - 30 °C or 20 to 30 °C |
| 123 g ± 2 g or (123 ± 2) g |
but not: |
123 ± 2 g |
| 70 % ± 5 % or (70 ± 5) % |
but not: |
70 ± 5 % |
| 240 × (1 ± 10 %) V |
but not: |
240 V ± 10 % (one cannot add 240 V and 10 %) |
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| 8. |
Unit symbols and unit names are not mixed and mathematical
operations are not applied to unit names. For example, only forms such as
kg/m3, kg · m-3, or kilogram per cubic
meter are used and not forms such as kilogram/m3,
kg/cubic meter, kilogram/cubic meter, kg per m3, or
kilogram per meter3. (See
Sec. 6.1.7,
Sec. 9.5, and
Secs 9.8.) |
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|
9. |
Values of quantities are expressed in acceptable units using
Arabic numerals and symbols for units. (See
Sec. 7.6.) |
| m = 5 kg |
but not: |
m = five kilograms |
| the current was 15 A |
but not: |
the current was 15 amperes |
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|
10. |
There is a space between the numerical value and unit symbol,
even when the value is used in an adjectival sense, except in the case of
superscript units for plane angle. (See
Sec. 7.2.) |
| a 25 kg sphere |
but not: |
a 25-kg sphere |
| an angle of 2°3′4″ |
but not: |
an angle of 2 ° 3 ′ 4 ″ |
| If the spelled-out name of a unit is used, the normal rules of
English are applied: "a roll of 35-millimeter film." (See
Sec. 7.6, note 3.) |
|
| 11. |
The digits of numerical values having more than four digits on
either side of the decimal marker are separated into groups of three using a
thin, fixed space counting from both the left and right of the decimal marker.
For example, 15 739.012 53 is highly preferred to
15739.01253. Commas are not used to separate digits into groups of
three. (See Sec. 10.5.3.) |
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| 12. |
Equations between quantities are used in preference to
equations between numerical values, and symbols representing numerical values
are different from symbols representing the corresponding quantities. When a
numerical-value equation is used, it is properly written and the corresponding
quantity equation is given where possible. (See
Sec. 7.11.) |
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| 13. |
Standardized quantity symbols such as those given in
Refs. [6] and [7]
are used, for example, R for resistance and Ar for
relative atomic mass, and not words, acronyms, or ad hoc groups of
letters. Similarly, standardized mathematical signs and symbols such as are
given in Ref. [6: ISO 31-11]
are used, for example, "tan x" and not
"tg x." More specifically, the base of "log" in
equations is specified when required by writing loga
x (meaning log to the base a of x),
lb x (meaning log2 x),
ln x (meaning loge x), or
lg x (meaning log10 x). (See
Sec. 10.1.1 and
Sec. 10.1.2.) |
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| 14. |
Unit symbols are in roman type, and quantity symbols are in
italic type with superscripts and subscripts in roman or italic type as
appropriate. (See Sec. 10.2 and
Sec. 10.2.1 to
Sec. 10.2.4.) |
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| 15. |
When the word "weight" is used, the intended meaning
is clear. (In science and technology, weight is a force, for which the SI unit
is the newton; in commerce and everyday use, weight is usually a synonym for
mass, for which the SI unit is the kilogram.) (See
Sec. 8.3.) |
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| 16. |
A quotient quantity, for example, mass density, is written
"mass divided by volume" rather than "mass per unit volume."
(See Sec. 7.12.) |
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| 17. |
An object and any quantity describing the object are
distinguished. (Note the difference between "surface" and
"area," "body" and "mass," "resistor"
and "resistance," "coil" and "inductance.") (See
Sec. 7.13.) |
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| 18. |
The obsolete terms normality and the symbol N, and
the obsolete term molarity and the symbol M,
are not used, but the quantity amount-of-substance concentration of B (more
commonly called concentration of B), and its symbol cB
and SI unit mol/m3 (or a related acceptable unit), are used instead.
Similarly, the obsolete term molal and the symbol m are not used, but the
quantity molality of solute B, and its symbol bB or
mB and SI unit mol/kg (or a related unit of the SI), are used
instead. (See Sec. 8.6.5 and
Sec. 8.6.8.) |