Geoengineering Geotechnical § Geoenvironmental § Geological § Geophysical Presentation of Numerical and Physical Magnitudes:
Conventions for Expressing Values of Quantities
Draft versions of technical documents within our research groups often contain an easily
correctable set of errors involving consistency, convention, and ambiguity in expressing values
of quantities. Accordingly, the following rules and style conventions should be followed for
dissemination of our research unless the sponsor or journal office specifically requires a nonstandard format. These rules synthesize several internationally recognized conventions,
including the following:
•
Chicago Manual of Style (University of Chicago Press 2003),
•
The Elements of Style (Strunk et al. 2000),
•
Guide to the SI (National Institute of Science & Technology 2009),
•
IEEE/ASTM SI 10™, American National Standard for Use of the International System of
Units (SI): The Modern Metric System (ASTM 2002), and
•
ASTM Style Guide (ASTM 2008).
Rule 1–Spaces Between Numerical Values and Physical Units
The physical unit symbol is placed after the numerical value, and a space is left between the
numerical value and the unit:
•
•
The length of the probe was 35 mm.
The mean temperature was 30.2 °C.
Exceptions:
α = 20°24’12”
m/s
$500
µg/L
Incorrect: …the probe was 35mm.
Incorrect: …temperature was 30.2°C.
Incorrect: …temperature was 30.2° C.
25%
mL/2g
Rule 2–Spacing in Mathematical Operations
Spacing is required in mathematical expressions. Use one space separation on either side of
mathematical symbols (+, -, ±, ×, <, >, ≤, ≥), on each side of an equal sign, and between two
quantities:
•
•
•
•
29 ± 0.4
2.9 × 102
a+b=c-d
T = 30.2 °C
Rule 3–Units Assigned to Physical Magnitudes
Clearly assign the physical unit symbol to the numerical value to avoid ambiguity:
•
23 cm × 46 cm
Incorrect: 23 × 46 cm
•
23 g ± 2 g or (23 ± 2) g
Incorrect: 23 ± 2 g
UW–MADISON GEOENGINEERING TECHNICAL WRITING STYLE GUIDE Rules and Style Conventions for Expressing Values of Quantities The Write Stuff–No. 2 UW–MADISON GEOENGINEERING Conventions for Expressing Values of Quantities TECHNICAL WRITING STYLE GUIDE The Write Stuff–No. 2 Rule 4–Avoiding Range Anxiety
To avoid confusion between mathematical expressions and ranges, use the word “to” whenever
possible instead of a range dash (i.e., an en dash):
•
0 °C to 100 °C
Incorrect: 0 °C – 100 °C, 0–100 °C
•
(0 to 100) °C
Incorrect: (0–100) °C
•
1 Hz to 500 kHz
Incorrect: 1 to 500 kHz
Rule 5–Only and Always Applying SI Units and Unit Symbols
Our program has one accepted system of units–the International System of Units (one ring to
rule them all!). Just below you will find five tables that provide guidance on acceptable base
units, derived units, and SI prefixes in addition to the only acceptable non-SI units. When in
doubt, refer to IEEE/ASTM SI 10, Standard for Use of the International System of Units (SI):
The Modern Metric System.
Table 1. SI Base Units
Quantity
Unit
Symbol
meter
m
Mass
kilogram
kg
Time
second
s
Electric current
ampere
A
Thermodynamic temperature
kelvin
K
Amount of substance
mole
mol
Intensity of luminosity
candela
cd
Length
Table 2. Examples of Common and Acceptable Derived Units
Derived Quantity
Symbol
Area
m
2
Volume
m
3
Speed/velocity
m/s
Density
kg/m
3
Table 3. Examples of SI-Derived Units with Special Names and Symbols
In Terms of
Other SI Units
Derived Quantity
Name
Symbol
Frequency
hertz
Hz
s
Force
newton
N
kg ⋅ m ⋅ s
Pressure, stress
pascal
Pa
N/m
2
kg ⋅ m ⋅ s
Energy, work
joule
J
N⋅m
kg ⋅ m ⋅ s
Power, radiant flux
watt
W
J/s
kg ⋅ m ⋅ s
Electric potential
volt
V
W/A
kg ⋅ m ⋅ s A
becquerel
Bq
Activity (radionuclide)
Version 2.1–October 8, 2013 SI Base Units
-1
-1
-2
2
-2
2
-3
2
s
-2
-3
-1
-1
page 2 of 4 UW–MADISON GEOENGINEERING Conventions for Expressing Values of Quantities TECHNICAL WRITING STYLE GUIDE The Write Stuff–No. 2 Table 4. SI Prefixes
Quantity
Name
Symbol
10
9
giga
G
10
6
mega
M
10
3
kilo
k
10
-3
milli
m
10
-6
micro
µ
10
-9
nano
n
Table 5. Only Acceptable Non-SI Units
Quantity
Time*
Volume
Mass
Unit
Symbol
minute
min
hour
h
day
d
liter
L (not
metric ton
)
3
t (t = 1 Mg = 10 kg)
*Note: The SI unit of time is the second (s), which should be used in technical calculations. However,
where time relates to life customs or calendar cycles, the minute, hour, day, and other calendar units
may be used.
The following terms and units should NEVER be used:
•
•
•
•
ångstrom, Å
standard atmosphere, atm
bar, bar (note, 1 bar = 100 kPa)
millimeter of mercury, mmHg
Rule 6–Arabic Numerals and Words
Values of quantities are expressed in Arabic numerals, with the SI symbols for the units:
Correct: power = 5 MW
Incorrect: power = five MW
Incorrect: power = 5 megawatts
Correct: The current was 15 A.
Incorrect: The current was fifteen amperes.
However, spell out a number that precedes a numeral used in a unit modifier:
• Three 10-cm3 vials
• Four 2.5-km segments
In addition, spell out numbers that are ten or under which do not refer to measured quantities
and numbers that start a sentence:
•
•
•
Three alternative sites
Two vehicles
…12 vehicles
Version 2.1–October 8, 2013 page 3 of 4 UW–MADISON GEOENGINEERING Conventions for Expressing Values of Quantities TECHNICAL WRITING STYLE GUIDE The Write Stuff–No. 2 Rule 7–Correct Hyphenation
Hyphenate compound adjectives:
•
•
50-mm gauge
low-alloy steel
Write expressions with a suspended hyphen after the first word to maintain parallelism:
•
“high- and low-temperature tests”
except where numerals are involved:
•
50, 100, and 150-mm specimens
Use a hyphen to join numbers (whether numerals or spelled out) that are part of a unit modifier
for the named noun:
•
•
•
…15-dB acoustical impact
Ten-story building
Three-phase study
Rule 8–Unit Symbols
The symbol for a compound unit that is the product of two or more units is indicated either by a
raised dot (preferred) or by a space; thus, for newton meter, use one of the following options:
•
•
20.6 N ⋅ m
20.6 N m
The symbol for a compound unit that is a quotient of two or more units is indicated in one of the
following ways:
m
m/s
m ⋅ s-1
s
Rule 9–Mathematical Copy
Write all mathematical copy in sentence form so that the mathematical content reads as clearly
and grammatically correct as any other text, as noted in the example below:
The ideal gas law is
P V =n R T
(3.4)
where P is the vapor pressure of the gas (Pa), V is the volume of the gas (m3), n is the
amount of substance of the gas (mol), R is the universal gas constant (8.314 J K-1 mol-1),
and T is temperature (K).
Version 2.1–October 8, 2013 page 4 of 4