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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

continued...

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Parent category: Life