CHAPTER 1

INTRODUCTION

1.1 Definitions

Measurements: The process of determining the amount or quantity by compar-

ison (direct or indirect) with an accepted standard.
Instrument: A device or mechanism used to determine the present value of a
quantity under supervision.

1.2 Methods of meaurement

The methods of measurement may be broadly classified into two categories:

Direct methods
In these methods, the unknown quantity is directly compared against a standard.
The result is expressed as a numerical value and a unit. Direct methods are quite
common for measuring quantities like length, mass, and time.
Example: Suppose we want to measure the length of a bar. The unit of length is
meter. A bar is so many times long because that many units on our standard have
Measurements and Instrumentation, First Edition. 1
By Osama A. Alkishriwo Copyright c 2014 John Wiley & Sons, Inc.
2 INTRODUCTION

the same length as the bar. A human being can make direct length comparisons
with an accuracy of about 0.25 mm. Therefore, on account of human factors, it
is not possible to make very accurate measurements. the direct method for mea-
surement of length can used with a good degree of accuracy. However, when it
comes to measurement of mass, the problem becomes much more complicated.
It is not just possible for human beings to distinguish between wide margins of
mass.

Indirect methods
Measurement by direct methods are not always possible. These methods in
most of the cases are inaccurate. Hence, direct methods are not preferred and
are rarely used. In engineering applications measurement systems are used.
These measurement systems use indirect methods for measurement purposes.

Transducer Signal Signal Indicating

Processing Transmission Device

Figure 1.1 Elements of a measurement system.

1.3 Elements of a measurement system

In general, a measurement system is made up of the four elements shown in Fig. 1.1

Transducer: The transducer converts a nonelectrical signal into an electrical

signal. Therefore, a transducer is required only if the quantity to be measured
is nonelectrical (e.g . pressure).

Signal Processing: The signal processing is required to modify the incoming

electrical signal to make it suitable for application to signal transmission. The
signal may need to be amplified until it is of sufficient amplitude to cause any
appreciable change at the indicating device. Other types of signal processing
might be voltage dividers, which are designed to reduce (attenuate) the amount
of signal applied to the signal transmission, or wave shaping circuits such as
rectifiers or filters.

Signal transmission: When the elements of an instrument are physically sep-

arated, it becomes necessary to transmit data from one to another. The element
that performs this function is called a signal transmission element. For example,
space–crafts are physically separated from the earth where the control stations
guiding their movements are located. Therefore, control signals arc sent from
these stations to space–crafts by a complicated telemetry systems using radio
signals.
 UNITS AND STANDARDS 3

Indicating device: The indicating device is generally a deflection-type meter

for such general purpose instruments as voltmeters or current meters. Electronic
instruments may be used to measure current, voltage, resistance, temperature,
sound level, pressure, or many other physical quantities. However, regardless
of the units on the calibrated scale of the indicating meter, the pointer deflects
up scale because of the flow of electrical current.

1.4 Units and Standards

With the increase of world trade and the exchange of scientific information between
nations, it became necessary to establish a single system of units of measurement that
would be acceptable internationally. After several world conferences on the matter, a
metric system which uses the meter, kilogram, and second as fundamental units has
now been generally adopted around the world. This is known, from the French term
“systeme international,” as the SI or international system.

1.4.1 SI mechanical units

The three basic units in the SI system are:

Unit of length: the meter (m)
Unit of mass: the kilogram (kg)
Unit of time: the second (s)
These are known as fundamental units. Other units derived from the fundamental
units are called derived units. For example, the unit of area is meters squared (m2 ),
which is derived from meters.

Unit of force: The SI unit of force is the newton (N), defined as that force which
will give a mass of 1 kilogram an acceleration of 1 meter per second per second.
Unit of work: The SI unit of work is the joule (J), defined as the amount of
work done when a force of 1 newton acts through a distance of 1 meter.
Unit of energy: Energy is defined as the capacity for doing work where it is
measured in the same units as work.
unit of power: Power is the time rate of doing work. The SI unit of power is
the watt (W), defined as the power developed when 1 joule of work is done in 1
second.

1.4.2 SI electrical units

The coulomb was originally selected as the fundamental electrical unit from which
all other units were derived. However, since it is much easier to measure current
4 INTRODUCTION

accurately than it is to measure charge, the unit of current is now the fundamental
electrical unit in the SI system. Thus, the coulomb is a derived unit, defined in terms
of the unit of electric current.

Unit of current: The ampere (A) is the unit of electric current where it is
defined as that constant current which when flowing in each of two infinitely
long parallel conductors 1 meter apart, exerts a force of 2 × 10−7 newton per
meter of length on each conductor.

Unit of charge: The coulomb (C) is the unit of electrical charge or quantity
of electricity which is defined as that charge which passes a given point in a
conductor each second, when a current of 1 ampere flows.

Unit of potential difference and electromotive force: The volt (V) is the unit
of electromotive force (emf) and potential difference. It is defined as the poten-
tial difference between two points on a conductor carrying a constant current of
1 ampere when the power dissipated between these points is 1 watt.

Unit of resistance and conductance: The ohm (Ω) is the unit of resistance.
The ohm is defined as that resistance which permits a current flow of 1 ampere
when a potential difference of 1 volt is applied to the resistance. The term
conductance (G) is applied to the reciprocal of resistance. The siemens (S) is
the unit of conductance.

Unit of magnetic flux and flux density: The weber (Wb) is the SI unit of
magnetic flux. It is defined as the magnetic flux which linking a single turn coil
produces an emf of 1 V when the flux is reduced to zero at a constant rate in
1 s. The tesla (T) is the SI unit of magnetic flux density. The tesla is the flux
density in a magnetic field when 1 weber of flux occurs in a plane of 1 square
meter, that is, the tesla can be described as 1 Wb/m2 .

Unit of inductance: The SI unit of inductance is the henry (H). The inductance
of a circuit is 1 henry, when an emf of 1 volt is induced by the current changing
at the rate of 1 A/s.

Unit of capacitance: The farad (F) is the SI unit of capacitance. The farad
is the capacitance of a capacitor that contains a charge of 1 coulomb when the
potential difference between its terminals is 1 volt.

1.4.3 Temperature scales

There are two SI temperature scales, the Celsius scale and the Kelvin scale. The
Celsius scale has 100 equal divisions (or degrees) between the freezing temperature
and the boiling temperature of water. At normal atmospheric pressure, water freezes
at 0o C (zero degrees Celsius) and boils at 100o C.
The Kelvin temperature scale, also known as the absolute scale, starts at absolute
zero of temperature, which corresponds to −273.15o C. Therefore, 0o C is equal to
 UNITS AND STANDARDS 5

273.15 K, and 100o C is the same temperature as 373.15 K. A temperature difference

of 1 K is the same as a temperature difference of 1o C.

1.4.4 Scientific notation and metric prefixes

Scientific notation: Very large or very small numbers are conveniently written
as a number multiplied by 10 raised to a power:

100 = 1 × 10 × 10
= 1 × 102
10000 = 1 × 10 × 10 × 10 × 10
= 1 × 104

Note that in the SI system of units, spaces are used instead of commas when
writing large numbers. Four–numeral numbers are an exception.
Metric prefixes: Metric prefixes and the letter symbols for the various multi-
ples and submultiples of 10 are listed in Table 1.1, with those most commonly
used with electrical units shown in bold type.

Table 1.1 Scientific notation and metric prefixes

Value Scientific notation Prefix Symbol
12
1 000 000 000 000 10 tera T
1 000 000 000 109 giga G
1 000 000 106 miga M
1000 103 kilo K
2
100 10 hecto h
10 10 deca da
0.1 10−1 deci d
−2
0.01 10 centi c
0.001 10−3 milli m
0.000 001 10−6 micro µ
0.000 000 0001 10−9 nano n
−12
0.000 000 000 0001 10 pico p

1.4.5 Other unit systems

In the traditional systems of measurements, the fundamental mechanical units are

the foot for length, the pound for mass, and the second for time. Other mechanical
units derived from these are similar in both systems, with the exception of the units
for liquid measure.
6 INTRODUCTION

Before the SI system was adopted, CGS systems using the centimeter, gram, and sec-
ond as fundamental mechanical units were employed for scientific purposes. There
were two CGS systems: an electrostatic system and a magnetic system. Many CGS
units were too small or too large for practical engineering applications, so practical
units were also used.
When solving problems, it is sometimes necessary to convert from the traditional
unit systems to SI units.

1.4.6 Standard classification

Measurement standards are classified in four levels: international standards, primary
standards, secondary standards, and working standards.

International standards are defined by international agreements, and are main-

tained at the International Bureau of Weights and Measures in France. These
are as accurate as it is scientifically possible to achieve. They may be used
for comparison with primary standards, but are otherwise unavailable for any
application.

Primary standards are maintained at institutions in various countries around

the world such as the National Bureau of Standards in Washington. They are
also constructed for the greatest possible accuracy, and their main function is
checking the accuracy of secondary standards.

Secondary standards are employed in industry as references for calibrating

high accuracy equipment and components, and for verifying the accuracy of
working standards. Secondary standards are periodically checked at the institu-
tions that maintain primary standards.

Working standards are the standard resistors, capacitors, and inductors which
usually found in an electronics laboratory. Working standard resistors are nor-
mally constructed of manganin or a similar material, which has a very low tem-
perature coefficient.

1.5 Electrical and electronic instruments

The history of development of instruments encompasses three phases of instruments:

Mechanical instruments

Electrical instruments

electronic instruments
 QUESTIONS 7

1.5.1 Mechanical instruments

These instruments are very reliable for static and stable conditions. But they suffer
from a very major disadvantage. They are unable to respond rapidly to measurements
of dynamic and transient conditions. This is due to the fact that these instruments
have moving parts that are rigid, heavy and bulky and consequently have a large
mass. Mass presents inertia problems and hence these instruments cannot faithfully
follow the rapid changes which are involved in dynamic measurements. Thus it
would be virtually impossible to measure a 50 Hz voltage by a mechanical method,
but it is relatively easy to measure a slowly varying pressure. Another disadvantage
of mechanical instruments is that most of them are a potential source of poise and
cause pollution of silence.

1.5.2 Electrical instruments

Electrical methods of indicating the output of detectors are more rapid than mechan-
ical methods. It is unfortunate that electrical system normally depends upon a me-
chanical meter movement as indicating device. This mechanical movement has some
inertia and therefore these instruments have a limited time (and hence, frequency) re-
sponse. For example, some electrical recorders can give full scale response in 0.2
s, the majority of industrial recorders have responses of 0.5 to 24 s. Some gal-
vanometers can follow 50 Hz variations but even these are too slow for present day
requirements of fast measurement.

1.5.3 Electronic instruments

These days most of the scientific and industrial measurements require very fast re-
sponses. The mechanical and electrical instruments and systems cannot cope up with
these requirements which have led to the design of today’s electronic instruments.
These instruments require semiconductor devices.

1.6 Questions

1.1 Define the following:

(a) Measurement.
(b) Instrument.
1.2 State the methods of measurements.
1.3 List the three fundamental SI mechanical units and unit symbols.
1.4 State the SI units and unit symbols for energy and power. and define each unit.
1.5 List the names of the various metric prefixes and the corresponding symbols.
Also, list the value represented by each prefix in scientific notation.
8 INTRODUCTION

1.6 State the SI units and unit symbols for electric current and charge, and define
each unit.

1.7 State the SI units and unit symbols for electrical resistance and conductance,
and define each unit.
1.8 List the various levels of measurement standards, and discuss the application of
each classification.

1.9 What are the advantages and disadvantages of electrical instruments?

1.10 Plot the general block diagram of a measurement system and state its elements.