# Units And Unit Conversion

Skills Refresher: Units and Unit Conversion
Every time something is measured there is a unit attached to that measurement. This guide will review the basics
about units and working with units in calculation.
There have been many different unit systems used by different cultures throughout history. In Canada we are most
familiar with two: the Imperial system (where feet measure distance) and the metric system (where meters measure
distance). Le Systeme international d'Unites (SI) is a modern form of the metric system used in scientific work.
Le Systeme international (SI) d'Unites
The SI system is based on seven primary units of measure:
Unit
Abbrev-
Name
iation
Measures:
Definition
Metre
m
Length
the distance light travels, in a vacuum, in 1/299792458th of a second
Kilogram
kg
Mass
the mass of a platinum-iridium cylinder at Sevres in France
Second
s
Time
the time required for 9192631770 vibration periods of the caesium-133 atom to occur
Ampere
A
Electric
the current which produces a specified force between two parallel wires which are 1
current
metre apart in a vacuum
Kelvin
K
Temperature
1/273.16 of the thermodynamic temperature of the triple point of water
mole
mol
Amount of
the amount of substance that contains as many elementary units as there are atoms in
substance
0.012 kg of carbon-12
candela
cd
Light
the intensity of a source of light of a specified frequency, which gives a specified amount
intensity
of power in a given direction
2
All other SI units are derived from this set of units. For instance, area is length x length (e.g., m
), volume is length
3
x length x length (e.g., m
), flow rate is length per unit time (e.g., m/s), and concentration is mass per volume (e.g.,
3
kg/m
) or mass per mass (e.g., kg substance / kg organism).
Unit Prefixes
Prefix
Symbol
Value
Multiple
To easily express with very large and very small
15
Peta
P
1,000,000,000,000,000
10
numbers, unit prefixes are used (see table at
12
Tera
T
1,000,000,000,000
10
right). As one reads down the table, the prefixes
9
Giga
G
1,000,000,000
10
describe smaller units.
6
Mega
M
1,000,000
10
3
Any of these prefixes can be utilized with any SI
Kilo
K
1,000
10
2
Hecto
h
100
10
unit to make numbers easier to conceptualize.
1
Deca
da
10
10
For instance, meters without a prefix might be
-1
Deci
d
0.1
10
fine to describe the length of a room (e.g., 3.3
-2
Centi
c
0.01
10
m), but to describe the distance between the
-3
Milli
m
0.001
10
Toronto and Peterborough, kilometres might be
-6
Micro
µ
0.000 001
10
more appropriate (e.g., 130 km).
-9
Nano
n
0.000 000 001
10
-12
pico
p
0.000 000 000 0001
10
Concentration Units
In environmental work, units of substance concentration are used frequently, and can be expressed in several ways:
Mass substance per unit mass medium
This concentration relationship expresses the concentration of a substance (solute) as a proportion of the medium it
was found it. This expression can be used for any medium, but is most commonly used to describe concentrations
in solids, such as soils and plants. Examples: 5 mg Hg/kg blubber, 1.7 µg Cd/kg soil, or 227 mg Ca/kg leaf matter.
Mass substance per unit volume medium
3
This unit is often used for expressing concentrations in liquid (water; e.g. mg/L) and air (e.g., mg/m
). Water has
the unique property where 1 mL of water weighs 1 g, allowing for easy conversion for aqueous (water-based)
measurements between mass per unit mass and mass per unit volume. E.g., 5 mg Pb/L water = 5 mg Pb/kg water.
Percent by mass
Related to mass per unit mass, percent mass (also called weight percent or percent by weight) =
(solute mass)/(medium mass)*100. The total mass of the solution is equal to the mass of the solute plus the mass of
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