Referm
Book
Third Edition
Walt
Boyes
Editor


Instrumentation
Reference
Book

Instrumentation
Reference
Book
Third Edition
Edited
by
Walt
Boyes
With
specialist
contributors
~LJ
TT
E
R
w
o
R
T

H
EINEMANN
Boston Oxford Johannesburg
Melbourne New
Delhi
Singapore
Copyright
0
2003 by Elsevier Science
-@
A member of the Reed Elsevier group
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British Library Cataloguing-in-Publication Data
A catalogue record for this book is available from the British Library.
Library
of
Congress Cataloging-in-Publication Data
Instrumentation reference book 3rd ed. /edited by Walt Boyes, with specialist contributions.
p. cm.

Includes bibliographical references and index.
ISBN 0-7506-7123-8 (acid-free paper)
1.
Physical instruments-Handbooks, manuals, etc.
instruments-Handbooks; manuals, etc. I. Boyes, Walt
QC53.15742002
2.
Engineering
530’. 7-dc2
1
20020
18480
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Contents
Preface

to
the third edition
ix
Preface to the second edition
x
Preface
to
the
first
edition
xi
List
of
Contributors xii
In
t
ro d
LI
c
t
io
n
xiv
Part
1:
Mechanical
Measurements
1
Measurement
off

flow
3
Introduction. Basic principles of flow measurement.
Fluid flow in closed pipes. Flow in open channels. Point
velocity measurement. Flowmeter calibration methods.
References. Further reading.
2 Measurement
of
viscosity
45
Introduction. Newtonian and non-Newtonian
behavior. Measurement of the shear viscosity.
Shop-floor viscometers. Measurement of the extensional
viscosity. Measurement of viscosity under extremes of
temperature and pressure. On-line measurements.
Accuracy and range. References. Further reading.
3
Measurement
of
length
53
Introduction. The nature of length. Derived
measurements. Standards and calibration of length.
Practice
of
length measurement for industrial use.
Automatic gauging systems. References. Further reading.
4
Measurement
of

strain
71
Strain. Bonded resistance strain gauges. Gauge
characteristics. Installation. Circuits for strain gauges.
Vibrating wire strain gauge. Capacitive strain gauges.
Surveys of whole surfaces. Photo-elasticity. References.
5
Measurement
of
level and volume
81
Introduction. Practice of level measurement.
Calibration
of
level-measuring systems. Methods
providing full-range level measurement. Methods
providing short-range detection. References.
6
Vibration 92
Introduction. Amplitude calibration. Sensor practice.
Literature. References. Further reading.
7
Measurement
of
force
106
Basic concepts. Force measurement methods.
Lever-balance methods. Force-balance methods.
Hydraulic pressure measurement. Acceleration
measurement. Elastic elements. Further

developments. References.
8
Measurement
of
density
114
General. Measurement
of
density using weight.
Measurement of density using buoyancy. Measurement
of density using hydrostatic head. Measurement of
density using radiation. Measurement of density using
resonant elements. Further reading.
9 Measurement
of
pressure
123
What is pressure? Pressure measurement. Pressure
transmitters. References. Further reading.
10
Measurement
of
vacuum
144
Introduction. Absolute gauges. Non-absolute gauges.
References.
11
Particle sizing
154
Introduction. Characterization of particles. Terminal

velocity. Optical effects caused by particles. Particle
shape. Methods for characterizing
a
group of particles.
Analysis methods that measure size directly. Analysis
methods that measure terminal velocity. Analysis
methods that infer size from some other property.
References. Further reading.
12 Fiber optics in sensor
instrumentation
17
Introduction. Principles of optical fiber sensing.
Interferometric sensing approach. Doppler
anemometry. In-fiber sensing structures.
References.
13
Microprocessor-based
and intelligent transmitters
208
Introduction. Terminology. Background information.
Attributes and features of microprocessor-based and
intelligent transmitters. Microprocessor-based and
intelligent temperature transmitters. Microprocessor-
based and intelligent pressure and differential
transmitters. Microprocessor-based and intelligent
flowmeters. Other microprocessor-based and intelligent
transmitters. Other microprocessor-based and
intelligent measurement systems. Fieldbus. User
vi
Contents

experience with microprocessor-based and intelligent
transmitters. Fieldbus function and benefits. References.
Part
2:
Measurement
of
Temperature and Chemical
Composition
14 Temperature measurement 239
Temperature and heat. Temperature scales.
Measurement techniques: direct effects. Measurement
techniques: electrical. Measurement techniques:
thermocouples. Measurement techniques: radiation
thermometers. Temperature measurement
considerations. References. Further reading.
15 Chemical analysis:
introduction 303
Introduction to chemical analysis. Chromatography,
Polarography and anodic stripping voltammetry.
Thermal analysis. Further reading.
16 Chemical analysis:
spectroscopy 31
8
Absorption and reflection techniques. Atomic
techniques: emission, absorption, and fluorescence.
X-ray spectroscopy. Photo-acoustic spectroscopy.
Microwave spectroscopy. Neutron activation. Mass
spectrometers. References. Further reading.
17 Chemical analysis:
electrochemical techniques 343

Acids and alkalis. Ionization of water. Electrical
conductivity. The concept
of
pH. Electrode potentials.
Ion-selective electrodes. Potentiometry and specific ion
measurement. Common electrochemical analyzers.
Further reading.
18 Chemical analysis: gas
analysis 382
Introduction. Separation of gaseous mixtures. Detectors.
Process chromatography. Special gas analyzers.
Calibration
of
gas analyzers. Further reading.
19 Chemical analysis: moisture
measurement 41 4
Introduction. Definitions. Measurement techniques
Calibration. References.
Part
3:
Electrical and Radiation
Measurements
20 Electrical measurements 425
Units and standards of electrical measurement.
Measurement of d.c. and a.c. current and voltage using
indicating instruments. Digital voltmeters and digital
multimeters. Power measurement. Measurement of
electrical energy. Power-factor measurement. The
measurement of resistance, capacitance, and inductance.
Digital frequency and periodhie-interval

measurement. Frequency and phase measurement using
an oscilloscope. References. Further reading.
21 Optical measurements 492
Introduction. Light sources. Detectors. Detector
techniques. Intensity measurement. Wavelength and
color. Measurement of optical properties. Thermal
imaging techniques. References.
22 Nuclear instrumentation
technology 517
Introduction. Detectors. Electronics. References.
Further reading.
23 Measurements employing
nuclear techniques 547
Introduction. Materials analysis. Mechanical
measurements. Miscellaneous measurements.
References.
24 Non-destructive testing 566
Introduction. Visual examination. Surface-inspection
methods. Ultrasonics. Radiography. Underwater
non-destructive testing. Developments. Certification
of
personnel. References. Further reading.
25 Noise measurement 596
Sound and sound fields. Instrumentation for the
measurement of sound-pressure level. Frequency
analyzers. Recorders. Sound-intensity analyzers.
Calibration
of
measuring instruments. The measurement
of sound-pressure level and sound level. Effect of

environmental conditions on measurements. References.
Further reading.
Part
4:
Instrumentation Systems
26 Design and construction
of
instruments 625
Introduction. Instrument design. Elements of
construction. Construction of electronic instruments.
Mechanical instruments. References.
27 Instrument installation and
commissioning 642
Introduction. General requirements. Storage and
protection. Mounting and accessibility. Piping systems.
Cabling. Grounding. Testing and pre-commissioning.
Plant commissioning. References.
28 Sampling 648
Introduction. Sample system components. Typical
sample systems. References.
Contents
vii
29
Telemetry 666
Part
5:
Further Scientific
Introduction. Communication channels. Signal
and Technical Information
multiolexing. Pulse encoding. Carrier wave modulation.

~L
-
Error detection and correction codes. Direct analog
signal transmission. Frequency transmission. Digital
953
36
Trigonometric
functions
and
signal transmission. References. Further reading.
general formulae
Mathematical signs and symbols. Trigonometric
formulae. Trigonometric values. Approximations for
small angles. Solution of triangles. Spherical triangle.
Exponential form. De Moivre's theorem. Euler's
relation.
Hyperbolic
functions,
Complex
variable,
Cauchy-Riemann equations. Cauchy's theorem. Zeros;
poles, and residues. Some standard forms. Coordinate
systems. Transformation of integrals. Laplace's
equation. Solution of equations. Method
of
least
squares. Relation between decibels, current and voltage
ratio, and power ratio.
30 Display and recording
691

Introduction. Indicating devices. Light-emitting diodes
(LEDs). Liquid crystal displays (LCDs). Plasma
displays. Cathode ray tubes (CRTs). Graphical
recorders. Magnetic recording. Transienthaveform
recorders. Data loggers. References.
31 Pneumatic instrumentation 71
0
Basic characteristics. Pneumatic measurement and
control svstems. Princioal measurements. Pneumatic
37 Statistics
96
Introduction. Data presentation. Averages. Dispersion
from
the
average,
Skewness,
Combinations
and
transmission. Pneumatic controllers. Signal
conditioning. Electropneumatic interface. References.
oermutations. Regression and correlation. Probabilitv.
v
Probability distributions. Sampling. Tests of
significance. Further reading.
32 Reliability in instrumentation
and control 734
Reliability principles and terminology. Reliability
assessmenl
. System design. Building high-reliability
systems. The human operator in control and

instrumentation. Safety monitoring. Software
reliability. Electronic and avionic systems. Nuclear
reactor control systems. Process and plant control.
References.
33 Safety
Introduction. Electrocution risk. Flammable
atmospheres. Other safety aspects. Conclusion.
References. Further reading.
793
34 History
of
instrumentation 802
Introduction. Parts of the system. Microsoft Windows.
Traditional and graphical programming languages.
Personal computers for data acquisition (and
instrument control). Other parts of the system. Machine
vision. Industrial networks and fieldbuses. Personal
computer buses. New serial buses. Firewire"
:
IEEE-1394. Older PC buses. The VXI bus. HS488:
38 Quantities and units 97
International unit system. Universal constants in
SI
units. Metric to Imperial conversion factors. Symbols
and abbreviations. References.
39 Electricity
79
Introduction. Molecules, atoms, and electrons. Atomic
structure. Wave mechanics. Electrons in atoms. Energy
levels. Electrons in metals. Conductivity. Electron

emission. Electrons in crystals. Insulators.
Semiconductors. Magnetism. Simplified electrical
theories.
40
Light 98
Introduction. The optical spectrum. Basic concepts of
optical radiation. Radiometry and photometry. Units
of measurement. Practical measurements. Interaction
of light with matter. References. Further reading.
94
enhancing the IEEE488.1 Specification. Ethernet.
Virtual Instrument Software Architecture (VISA). Inter-
changeable Virtual Instruments (IVI). Real-time program-
ming.
The
luture
of
virtual instrumentation. Further reading.
41
Radiation
Electromagnetic radiation. Nuclear radiation. Further
reading.
35
EMC
861
42
Connectors 999
Introduction. Interference coupling mechanisms.
Circuits, layout, and grounding. Interfaces. filtering,
and shielding. The regulatory framework. Further

reading. References. Further reading.
Connector housings. Connector contacts. Connector
terminations. Tooling. Mass termination connectors.
Fiber optics connectors. Radio frequency connectors.
viii
Contents
43
Noise
and
communication
1007
B
Professional Societies and
Interference and noise in communication systems.
Associations 1021
Man-made noise. Natural sources of noise. Random
noise. Electronic vacuum tube noise. Transistor noise.
C The Institute of Measurement
Noise figure. Measurement of noise. Methods of
improving signal-to-noise ratio. References.
and Control
D
The Instrument Society
of
America 1029
Appendices
A
General Instrumentation
Books
1017

Index
1031
1025
This edition
is
not completely new. The second
edition built on the first, and
SO
does this edition.
This work
has
been almost entirely one of “inter-
nationalizing”
a
work mainly written for the
UK.
New matter has been added, especially in the
areas of analyzers, level and flowmeters, and
fieldbus. References
to
standards are various,
ard British Standards are often referenced. Inter-
national standards are in
flux,
and most stand-
ards bodies are striving to have equivalent
standards throughout the world. The reader
is
encouraged to refer to
IEC,

ANSI, or other stand-
ards when only a British Standard
is
shown. The
ubiquity of the World Wide Web has made it
possible for any standard anywhere to be located
Preface
to
the
third edition
and purchased, or
in
some cases, read online free,
so
it has not been necessary to cross-reference
standards liberally
in
this work.
The Editor wishes to thank all the new contri-
butors, attributed and not. for their advice, sug-
gestions, and corrections. He fondly wishes that
he
has
caught all the typographical errors? but
knows that
is
unlikely. Last, the Editor wishes
to
thank his several editors at Butterworth-
Heinemann for their patience, as well

as
Michael
Forster, the publisher.
Walt
Boyes
Maple
Valley,
Washington
2002
E.
B. Jones’s writings on instrument technology go
back at least to 1953. He was something of a
pioneer in producing high level material that
could guide those studying his subjects. He had
had both practical experience of his subject and
had taught it at college, and this enabled him to
lay down a foundation that could be built on for
more than forty years. I must express my thanks
that the first edition of the
Instrumentation Refer-
ence
Book,
which
E.
B. Jones’s work was molded
into, has sold well from 1988 to 1994.
This book has been accepted as one of the
Butterworth-Heinemann series of reference
books-a goodly number of volumes covering
much of technology. Such books need updating

to keep abreast of developments, and this first
updating calls for celebration!
There were several aspects that needed enlar-
ging and several completely new chapters were
Preface to the
second edition
needed. It might be remarked that
a
number of
new books, relevant
to the whole field of instru-
mentation, have appeared recently, and these
have been added to the list. Does this signify
a growing recognition of the place of instrumenta-
tion?
Many people should be thanked for their work
which has brought together this new edition. Col-
laboration with the Institute of Measurement and
Control has been established, and this means that
the book is now produced under their sponsor-
ship. Of course, those who have written, or
revised what they had written before, deserve
my gratitude for their response. I would also like
to say thank you to the Butterworth-Heinemann
staff for their cooperation.
B.
E.
N.
Dorking
Preface to the

Instrumentation is not a clearly defined subject,
having what might be called a “fuzzy frontier”
with many other subjects. Look for books about
it, and in most libraries you are liable to find
them widely separated along the shelves, classi-
fied under several dirferent headings. Instrumen-
tation is barely recognized as a science or
technology in its own right. That raises some
difficulties for writers in the field and indeed for
would-be readers. We hope that what we are
offering here will prove to have helped with
clarification.
A reference book should of course be there for
people to refer to for the information they need.
The spectrum
is
wide: students, instrument engin-
eers; instrument users, and potential users who
just want
to
explore possibilities. And the informa-
tion
needed in real life is a mixture of technical
and commercial matters.
So
while the major part
of the
Instrumentation Reference
Book
is a tech-

nical introduction to many facets of the subject,
there is also a commercial part where manufac-
turers etc. are listed. Instrumentation is evolving,
perhaps even faster than most technologies,
emphasizing the importance of relevant researcli;
we have tried
to
recognize that by facilitating
first
edit
contact with universities and other places spear-
heading development.
One need for information
is
to ascertain where
more information can be gained. We have catered
for this with references at the ends of chapters to
more specialized books.
Many agents have come together
to
produce
the
Instrumentation Reference
Book
and
to whom
thanks are due: those who have written, those
who have drawn, and those who have painstak-
ingly checked facts.
I

should especially thank
Caroline Mallinder and Elizabeth Alderton who
produced order out of chaos in the compilation
of
long lists of names and addresses. Thanks should
also go elsewhere in the Butterworth hierarchy
for the original germ of the idea that this could
be a good addition
to
their family of reference
books. In a familiar tradition,
I
thank my wife for
her tolerance and patience about time-consuming
activities such as telephoning, typing, and travel-
ing-or at the least for limiting her natural intol-
erance and impatience of my excessive indulgence
in them!
B.
E.
N.
Dorking
List
of
Contributors
C.
S.
Bahra,
BSc, MSc, CEng, MIMechE, was
formerly Development Manager at Transducer

Systems Ltd.
J. Barron,
BA, MA(Cantab), is a Lecturer at the
University of Cambridge.
Jonas Berge,
Senior Engineer, SMAR Corp.,
Singapore. Mr. Berge is a recipient of ISA’s
Excellence In Documentation Award, and has
a
volume on Foundation Fieldbus in preparation
for publication.
Walt H. Boyes,
Principal, Marketing Practice
Consultants, Maple Valley, Wash.,
is
Contributing
Editor of
Flow
Control
magazine, and a recognized
industry analyst and consultant. He has over
25
years experience in sales, marketing, technical
support, new product development, and manage-
ment in the instrumentation industries.
G. Burns,
BSc, PhD, AMIEE, Glasgow College of
Technology.
J. C. Cluley,
MSc, CEng, MIEE, FBCS, was

formerly a Senior Lecturer in the Department of
Electronic and Electrical Engineering, University
of Birmingham.
R.
Cumming,
BSc, FIQA, Scottish School of
Non-destructive Testing.
W.
G.
Cummings,
BSc, CChem, FRSC, MInstE,
MinstMC, former Head of the Analytical Chem-
istry Section at Central Electricity Research
Laboratories.
A.
Danielsson,
CEng, FIMechE, FInstMC, is
with Wimpey Engineering Ltd. He was a member
of
the BS working party developing the Code of
Practice for Instrumentation in Process Control
Systems: Installation-Design.
C.
I.
Daykin,
MA, is Director
of
Research and
Development at Automatic Systems Laboratories
Ltd.

Dr. Stanley
Doh,
Scientist, Omega Engineering,
Stamford, Conn., is an expert on the measure-
ment of temperature.
T.
Fountain,
BEng, AMIEE, is the Technical
Manager for National Instruments UK Corp.,
where he has worked since 1989. Before that he
was a design engineer for Control Universal,
interfacing computers to real-world applications.
G.
Fowles
was formerly a Senior Development
Engineer with the Severn-Trent Water Authority
after some time as a Section Leader in the
Instrumentation Group of the Water Research
Centre.
J.
G.
Giles,
TEng, has been with Ludlam Sysco
Ltd for a number
of
years.
Sir Claud Hagart-Alexander
Bt, BA, MInstMC,
DL, formerly worked in instrumentation with IC1
Ltd. He was then a director

of
Instrumentation
Systems Ltd. He
is
now retired.
D.
R.
Heath,
BSc, PhD, is with Rank Xerox Ltd.
E.
H.
Higham,
MA, CEng, FIEE, MIMechE,
MInstMC, is a Senior Research Fellow in the
School of Engineering at the University of
Sus-
sex, after a long career with Foxboro Great Brit-
ain Ltd.
W.
M.
Jones,
BSc, DPhil, FInstP, is a Reader in
the Physics Department at the University College
of Wales.
D. Aliaga Kelly,
BSc, CPhys, MInstP, MAmPhys-
Soc, MSRP,
FSAS,
is now retired after working
for many years as Chief Physicist with Nuclear

Enterprises Ltd.
C. Kindell
is with AMP of Great Britain Ltd
E.
G.
Kingham,
CEng, FIEE, was formerly at the
Central Electricity Research Laboratories.
T.
Kingham,
is with AMP of Great Britain Ltd.
J.
Kuehn,
FInst Accoust, is Managing Director of
Bruel
&
Kjaer
(UK)
Ltd.
C.
K.
Laird,
BSc, PhD, CCheni, MRSC, works
in the Chemistry Branch at Central Electricity
Research Laboratories.
List
of
Contributors
xiii
F. F.

Mazda,
DFH, MPhil, CEng, MIEE, MBIM,
is
with Rank Xerox Ltd.
W.
McEwan,
BSc, CEng, MIMechE, FweldInst,
Director Scottish School
of
Non-destructive Testing.
A.
McNab,
BSc, PhD, University
of
Strathclyde.
D. B. Meadowcroft,
BSc, PhD, CPhys, FInstP,
FICorrST works
in
the Chemistry Branch at Cen-
tral Electricity Research Laboratories.
B.
T. Meggitt,
BSc, MSc, PhD:
is
Development
Manager of LM Technology Ltd. and Visiting
Professor in the Department of Electronic and
Electrical Engineering, City University, London.
Alan Montgomery,

Sales Manager, Lumberg
Canada Ltd.,
is
a long-time sales and marketing
expert in the instrumentation field, and is an
expert on modern industrial connectors.
William L. Mostia,
Principal, WLM Engineering,
Kemah. Tex. Mr. Mostia is an independent con-
sulting engineer and an expert on pneumatic
instrumentation, among other specialties.
uir,
BSc,
MSc,
MIM, MInstNDT, MWel-
dInst, CEng,
FIQA,
Scottish School of Non-
destructive Testing.
B.
E.
Noltingk,
BSc, PhD, CEng, FIEE, FInstP, is
now a Consultant after some time
as
Head of the
Instrumentation Section at the Central Electricity
Research Laboratories.
Eoin O’Riain,
Publisher,

Readout
Magazine.
D.
J.
Pacey,
BSc, FInst P, was, until recently, a
Senior Lecturer in the Physics Department at
Brunel University.
Dr.
Jerry Paros,
President, Paroscientific Corp.,
Redmond, Wash., is founder of Paroscientific,
a
leading-edge pressure sensor manufacturer, and
one
of
the leading experts
on
pressure measurement.
J.
Riley
is with AMP
of
Great Britain Ltd.
M.
L.
Sanderson,
BSc, PhD, is Director of the
Centre for Fluid Instrumentation
at

Cranfield
Institute of Technology.
M.
6.
Say,
MSc, PhD, CEng, ACGI, DIC, FIEE,
FRSE,
is
Professor Emeritus
of
Electrical Engi-
neering at Heriot-Watt University.
R. Service,
MSc, FInstNDT, MWeldInst, MIM,
MICP, CEng, FIQA, Scottish School of Non-
destructive Testing.
A.
C.
Smith,
BSc, CChem, FRSC, MInstP, former
Head of the Analytical Chemistry Section at Cen-
tral Electricity Research Laboratories.
W.
L.
Snowsill,
BSc, was formerly a Research
Officer in the Control and Instrumentation
Branch of the Central Electricity Research
Laboratories.
K.

R.
Sturley,
BSc, PhD, FIEE, FIEEE,
is
a Tele-
communications Consultant.
P.
H.
Sydenham,
ME, PhD, FInstMC, FIIC.
AMIAust: is Head of and Professor at the School
of Electronic Engineering in the
South
Australian
Institute of Technology.
A.
W.
S.
Tarrant,
BSc, PhD, CPhys. FInstP,
FCIBSE, is Director of the Engineering Optics
Research Group at the University
of
Surrey.
M. Tooley,
BA, is Dean
of
the Technology Depart-
ment at Brooklands College and the author
of

numerous electronics and computing books.
K.
Torrance,
BSc, PhD, is
in
the Materials Branc!i
at Central Electricity Research Laboratories.
L.
C.
Towle,
BSc, CEng, MIMechE, MIEE,
MInstMC,
is
a Director of the MTL Instruments
Group Ltd.
L.
W. Turner,
CEng, FIEE, FRTS, is a Consultant
Engineer.
Ian Verhappen,
Instrumentation Engineer, Syn-
crude Canada Ltd., is an expert
on
process analy-
zers
of
all types, as well as a noted authority on
Foundation Fieldbus.
K.
Walters,

MSc, PhD, is a Professor in the
Department
of
Mathematics at the University
College
of
Wales.
T.
Williams,
BSc,
CEng,
MIEE,
formerly with
Rosemount, is a consultant in electromagnetic
compatability design and training with Elmac
Services, Chichester.
Introduction
1
Techniques and applications
We can look at instrumentation work in two
ways, by
techniques
or by
applications.
When we
consider instrumentation by technique, we survey
one scientific field, such as radioactivity or ultra-
sonics, and look at all the ways in which it can be
used to make useful measurements. When we
study instrumentation by application, we cover the

different techniques to measure a particular quan-
tity. Under flowmetering, for instance, we look at
many methods, including tracers, ultrasonics, or
pressure measurement. This book is mainly appli-
cations oriented, but in a few cases, notably pneu-
matics and the employment of nuclear technology,
the technique has been the primary unifying theme.
2
Accuracy
The most important question in instrumentation
is the accuracy with which the measurement is
made. It is such a universal issue that we will talk
about it now, as well as in the individual chapters
to follow. Instrument engineers should be skepti-
cal of accuracy claims, and they should hesitate to
accept their own reasoning about the systems
they have assembled. They should demand evi-
dence, and preferably proof. Above all, they
should be clear in their own minds about the level
of accuracy needed to perform a job. Too much
accuracy will unnecessarily increase costs, while
too little may cause performance errors that make
the project unworkable.
Accuracy is important but complex. We must
first distinguish between “systematic” and “ran-
dom” errors in an instrument. “Systematic” error
is the error inherent in the operation of the instru-
ment, and calibrating can eliminate it. We will
discuss calibration in several later chapters. Cali-
bration is the comparison of the reading of the

instrument in question to a known “standard”
and the maintenance
of
the evidentiary chain
from that standard. We call this “traceability.”
The phrase
random errors
implies the action
of
probability. Some variations in readings, though
clearly observed, are difficult to explain, but most
random errors can be treated statistically without
knowing their cause.
In
most cases it is assumed
that the probability
of
error is such that errors in
individual measurements have a normal distribu-
tion about the mean, which is zero if there is
no
systematic error.
This implies that we should quote errors based
on
a certain probability of the whereabouts of
the true value. The probability grows steadily
wider as the range where it might be also grows
wider.
When we consider a measurement chain with
several links, the two approaches give increas-

ingly different figures. For if we think of possibil-
ities/impossibilities then we must allow that the
errors in each link can be extreme and in the same
direction, calling for a simple addition when cal-
culating the possible total error. On the other
hand, this is
improbable,
so
the “chain error” that
corresponds to a given probability,
e,,
is appreci-
ably smaller.
In
fact, statistically,
e,
=
de:
+
e:
+
.
. .
where
el, e2,
etc. are the errors in the different
links, each corresponding to the same probability
as
e,.
We can think of “influence quantities” as the

causes of random errors. Most devices that measure
a physical quantity are
influenced
by other quanti-
ties. Even in the simple case of a tape measure, the
tape itself is
influenced
by temperature. Thus, a tape
measure will give a false reading unless the influence
is allowed for. Instruments should be as insensitive
as possible to influence quantities, and users should
be aware of them. The effects of these influence
quantities can often be reduced by calibrating under
conditions as close as possible to the live measure-
ment application. Influence quantities can often be
quite complicated. It might not only be the tempera-
ture than can affect the instrument, but the change
in temperature. Even the
rate
of change
of
the
temperature can be the critical component of this
influence quantity. To make it even more complex,
we must also consider the differential between the
temperatures of the various instruments that make
up the system.
One particular factor that could be thought
of
as an influence quantity is the direction in which

the quantity to be measured is changing. Many
instruments give slightly different readings
according to whether, as it changes, the particular
value
of
interest is approached from above or
below. This phenomenon
is
called “hysteresis.”
Introduction
xv
If we assume that the instrument output is
exactly proportional to a quantity, and we find
discrepancies, this is called “non-linearity error.”
Non-linearity error is the maximum departure
of
the true input/output curve from the idealized
straight line approximating it.
It may be noted that this does not cover
changes in “incremental gain,” the term used
for the local slope of the input/output curve.
Special cases of the accuracy of conversion
from digital to analog signals and vice versa
are discussed in Sections 29.3.1 and 29.4.5 of
Part 4. Calibration at sufficient intermediate
points in the range of an instrument can cover
systematic non-linearity.
Microprocessor-based instrumentation has
reduced the problem of systematic non-linearity
to a simple issue. Most modern instruments have

the internal processing capability to do at least a
multipoirit breakpoint linearization. Many can
even host and process complex linearization
equations of third order or higher.
Special terms used in the discussion above are
defined
ir,
BS
5233; several ANSI standards, and
in
the ISA
Dictionary
of
Instrumentation,
along
with numerous others.
The general approach to errors that we have
outlined follows a statistical approach to a static
situation.
Communications theory emphasizes working
frequencies and time available, and this approach
to
error
is
gaining importance in instrumentation
technology as instruments become more intelli-
gent. Sensors connected
to
digital electronics
have little or no error from electronic noise, but

most accurate results can still be expected from
longer measurement times.
Instrument engineers must be very wary of
measuring the wrong thing! Even a highly accur-
ate measurement
of the wrong quantity may
cause serious process upsets. Significantly for
instruments used for control, Heisenberg’s Law
applies
on
the macro level
as
well as on the
sub-
atomic. The operation of measurement can often
disturb the quantity measured.
This can happen in most fields: a flowmeter can
obstruct flow and reduce the velocity to be meas-
ured, an ‘over-large temperature seiisor can cool
the material studied, or a low-impedance volt-
meter can reduce the potential it is monitoring.
Part
of
th’e instrument engineer’s task is to foresee
and avoid errors resulting from the effect his
instrument has
on
the system it is being used to
study.
3

Environment
Instrument engineers must select their devices
based on the environment where they will be
installed. In plants there will be extremes of tem-
perature, vibration, dust, chemicals, and abuse.
Instruments for use in plants are very different
from those that are designed for laboratory use.
Two kinds of ill effects arise from badly
selected instruments: false readings from excep-
tional values of influence quantities, and the irre-
versible failure of the instrument itself.
Sometimes manufacturers specify limits to
working conditions. Sometimes instrument engin-
eers must make their own judgments. When
working close to the limits
of
the working condi-
tions of the equipment, a wise engineer derates
the performance of the system or designs environ-
mental mitigation.
Because instrumentation engineering
is
a prac-
tical discipline, a key feature of any system design
must be the reliability of the equipment. Reliabil-
ity is the likelihood of the instrument: or the
system, continuing
to
work satisfactorily over
long periods. We will discass reliability deeply in

Part 4. It must always be taken into account when
selecting instruments and designing systems for
any application.
4
Units
The introductory chapters to some books have
discussed the theme of what system of units
is
used there. Fortunately the question
is
becoming
obsolete because
SI
units are adopted nearly
everywhere, and certainly in this book. In the
United States, and a few other areas, where other
units still have some usage, we have listed the
relations for the benefit of those who are still
more at home with the older expressions.
5
References
British Standards
Institution.
Glossary
of
terms used in
Metrology,
BS
5233
(1975)

Dietrich,
D.
F.,
Uncertainty, Calibration and Probabil-
ity: the Statistics
of
Scientific and Industrial Measure-
ment,
Adam Hilger, London
(1973)
Instrumentation, Systems
and
Automation
Society
(ISA)
The ISA Comprehensive Dictionary
of
Measure-
ment and ControMrd edition.
online edition
http:llwww. isa. org
Chapman
and
Hall. London
(1972)
Topping.
J.,
Errors
of
Observation and their Treatment.


Instrumentation
Reference
Book

Part
I
Mechanical
Measurements