Measurement Technology - The Electronic Nose Artificial Olfaction Technology - page 6

imprecise, varying as they do from one person to the next. Therefore, there has
been a progressive movement toward measurement units that are defined much
more accurately.
In the history of measurement, units of measurement occupied an important role.
Units are labels which distinguish one type of measurable quantity from other types.
Temperatures, mass, time, length, and volume are diverse physical quantities, and
therefore have different unit names, degrees, kilograms, seconds, meters, and liters [ 1 ].
Many different units of common physical quantities have been used throughout
history. Units are necessary so that one person can correctly interpret a mea-
surement another has made, and measuring instruments must be correctly cali-
brated to give readings in the appropriate units.
Measurement is the process by which one can convert physical parameters into
meaningful numbers. The measuring process is one in which the property of an
object or system under consideration is compared to an accepted standard unit, a
standard defined for that particular property.

1.4.1 Base Quantities and Units

In the SI system of units, there are seven base quantities and corresponding units as
shown in Table 1.2 [ 2 ].

1.4.2 Derived Quantities and Units

These are physical quantities which are resulting from the seven base quantities by
mathematical operations such as multiplication, division. Their units are similarly
derived as products or quotients of the seven base units as shown in Table 1.3 [ 3 ].
Since physical quantities can take a wide range of values, prefixes such as kilo,
centi, and milli are used together with units to simplify the expressions for both very
large and very small quantities. Table 1.4 lists the decimal multiples and

submultiples.

Table 1.2 Unit (base quantity and unit)
Base quantity SI Base Unit

Name Usual symbol Name Symbol

Length l Meter M

Mass m Kilogram Kg

Time t Second s

Amount of substance n Mole mol

Electric current I Ampere A

Temperature T Kelvin K

Luminous intensity l v Candela cd