Instrument Technology. Measurement of Pressure, Level, Flow and Temperature

406 Pages·1974·10.197 MB·English

by  E. B. Jones (Auth.)

Dedicated to
P.R.H.
Instrument Technology
Volume 1
MEASUREMENT OF PRESSURE,
LEVEL, FLOW AND TEMPERATURE
E. B. JONES
B.Sc, F.Inst.P., F.Inst.M.C.
BUTTERWORTHS
London-Boston-Durban-Singapore-Sydney-Toronto-Wellington
All rights reserved. No part of this publication may be reproduced or
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system of any nature.
This book is sold subject to the Standard Conditions of Sale of Net Books and
may not be re-sold in the UK below the net price given by the Publishers in
their current price list.
First published 1953
Revised/reprinted 1960
Reprinted 1961, 1964
Second edition 1965
Reprinted 1970, 1972
Third edition 1974
Reprinted 1976, 1979, 1981, 1983
© Butterworth & Co (Publishers) Ltd, 1974
ISBN 0 408 70535 3
Printed and bound in Great Britain by
Redwood Burn Limited, Trowbridge, Wiltshire
PREFACE TO THE THIRD EDITION
The enormous and rapid increase in the measurement, recording and control
of critical variables in industrial processes has led to a corresponding expansion
of the staffs responsible for the installation and maintenance of instruments
which, incidentally, represent considerable investment of capital.
To gain the full advantages of instrumentation, the instruments should be
installed and maintained by those who bring understanding as well as manual
skill to their work. This book is written with the object of helping the reader to
understand the 'why' as well as the 'how' of his work. It is presumed that he
has some acquaintance with physics, but to help those whose knowledge of
physics is small, some basic principles are stated at the beginning of each
section.
The mathematics have been kept as simple as possible to avoid embarrassing
students whose attainments in mathematical subjects are limited.
The selection of material has been a difficult problem as the subject is so
wide, but the aim has been to give as complete a picture as possible while
emphasising the more important and the more common types of instruments.
It is hoped that this volume together with its companion volumes will cover
the requirements of students studying both craftsmen and technician courses
of the City and Guilds of London, and instrument personnel studying the
courses established by the various industrial training boards.
The usefulness of the book is not, however, limited to candidates for the
examination courses mentioned above, but it is expected that instrument and
chemical engineers and others, will find a great deal of useful information
within its covers which will help them solve the instrument problems which
may occur during their training and their daily work.
The introduction of SI units, development of new instruments, and the new
standards required for the use of electrical instruments where an explosive
hazard exists, has made it essential to produce this third edition. Many
changes are taking place as we seek to align ourselves with the practices
followed on the continent of Europe. Many of our existing instrument
standards will change as the European influence increases, but although much
of our practice may change, the fundamental principles described in this book
will remain valid.
The author cannot adequately express his appreciation of the considerable
help received from his colleagues past and present, and manufacturers and
users of industrial instruments, in bringing this work into being.
The instrument documents of the British Standards Institution and of
B.A.S.E.E.F.A. have been a valuable source of information and due acknow­
ledgement is made to these bodies. The documents referred to are too numerous
to mention here but extracts are noted in the text.
In conclusion, the writer acknowledges with gratitude the assistance of his
wife in undertaking the typing and the multitude of other tasks associated with
this work.
Holywell E. B. JONES
INTRODUCTION
Metric units
The system of units of measurement used in all three volumes is the interna­
tionally agreed 'Systeme International d'Unites' usually abbreviated to SI
units. This system is closely associated with the metric system and the starting
point is six base units, each already precisely defined, from which all other
units are derived. As the SI system is a coherent system any derived unit is
the result of the product or quotient of two or more base units, e.g. unit area
results when unit length is multiplied by unit length, unit velocity when unit
length is divided by unit time and unit force when unit mass is multiplied by
unit acceleration. The base units are given in Table 1.
Table 1
Quantity Unit Symbol
Length metre m
Mass kilogramme kg
Time second s
Electric current ampere A
Thermodynamic temperature kelvin K
Luminous intensity candela cd
In addition there are supplementary units for plane angle, the radian (rad)
and solid angle, the steradian (sr). The units Kelvin (K) and degree Celsius
(°C) of temperature interval are identical and the International Practical
Temperature Scale is used for all practical purposes. A temperature expressed
in degrees Celsius is equal to the temperature expressed in Kelvin less 273-15.
Derived units
The derived unit of force is the kilogramme metre per second per second and
is known as the newton (N). The newton is that force which when applied to
a body having a mass of 1 kg gives it an acceleration of one metre per second
per second and is therefore independent of the gravitational force.
The derived unit of pressure is the pressure produced when a force of one
newton acts over an area of a square metre, i.e. N/m 2. Because this unit is
very small, 100 000 N/m2 being equal to approximately 1 atmosphere, the
bar = 105 N/m2 and m bar are used as the practical units of pressure. In
2 INTRODUCTION
order to avoid errors in calculations it is advisable to use the basic coherent SI
units.
Owing to the convenience and ease of use of the simple manometer, pres­
sures on industrial plant will still be measured where appropriate in terms of
the height of a column of liquid. The column of liquid, however, can be
calibrated in terms of SI units although it will be some time before the use of
lengths of liquid column will cease to be used to express the values of pressures
which can be conveniently measured by manometers.
The unit of energy in all forms is the joule, and of power is the watt. Thus
the variously defined units such as the calorie, British Thermal Unit and
horsepower are all superseded.
For some of the derived SI units special names and symbols exist, these are
shown in Table 2.
Table 2
Expressed in terms
Name of
Quantity Symbol of SI base units or
SI unit
derived units
Frequency hertz Hz 1 Hz = 1/s
Force newton N 1 N = 1 kg m/s2
Work, energy, quantity
of heat joule J U = 1 Nm
Power watt W 1 W = U/s
Quantity of electricity coulomb C 1 C = 1 As
Electric potential, potential
difference, tension,
electromotive force volt V 1 V = 1 W/A
Electric capacitance farad F 1 F = 1 As/V
Electric resistance ohm Ω 1 = 1 V/A
Flux of magnetic induction,
magnetic flux weber Wb 1 Wb = 1 Vs
Magnetic flux density,
magnetic induction tesla T 1 T = 1 Wb/m2
Inductance henry H 1 H = 1 Vs/a
Luminous flux lumen lm 1 lm = 1 cd sr
Illumination lux Ix 1 lx = 1 lm/m2
The SI definition of other quantities will be dealt with as they arise in the
text but students wishing to study the system of units further are advised to
study PD 5686, 'The use of SI units' available from British Standards Institu­
tion. All dimensions used in the Figures are in millimetres unless otherwise
stated. Some instruments illustrated have Imperial Scales and these will be
revised in subsequent editions when such instruments having SI scales are
available from the manufacturers. Likewise the section on instruments for
hazardous areas is based on the latest acceptable practice but when the new
British Codes based on the International Electrotechnical Commission's
recommendations become available this may be revised.
In describing the performance of an instrument four terms are frequently
used, viz. 'accuracy', 'precision', 'sensitivity' and 'rangeability'.
Accuracy. The 'accuracy' of a reading made with an instrument may be
defined as 'the closeness with which the reading approaches the true value'.
INTRODUCTION 3
Precision. The 'precision' of the readings is the 'agreement of the readings
among themselves'. If the same value of the measured variable is measured
many times and all the results agree very closely then the instrument is said
to have a high degree of precision, or reproducibihty. A high degree of
reproducibihty means that the instrument has no 'drift'; i.e. the calibration
of the instrument does not gradually shift over a period of time. Drift occurs
in flowmeters because of wear of the diiferential-pressure-producing element,
and may occur in a thermo-couple or a resistance-thermometer owing to
changes in the metals brought about by contamination or other causes. As
drift often occurs very slowly it is likely to be unnoticed and can only be
detected by a periodic check of the instrument calibration.
A high degree of precision is, however, no indication that the value of the
measured variable has been accurately determined. A manometer may give
a reading of 1-000 bar for a certain pressure to within 0-001 bar, but it may be
up to 0-2 bar in error, and the true value of the pressure will be unknown until
the instrument has been calibrated. It is accurate calibration that makes
accurate measurement possible.
All measurement is comparison. When a length is measured it is compared
with a fixed length, or 'standard of length', and the number of times the un­
known length is greater than the standard is found. The standard is chosen
so that the 'number of times' or 'numeric' is not too large or too small.
To ensure that the length, or any other measured quantity, as measured by
one person, shall agree with that as measured by a second person, the standards
must be absolutely fixed and reproducible with precision. Once a standard
of length has been established, all measuring instruments based on this
standard are made to agree with the standard. In this way, the length of an
object as measured by one instrument will agree with the length as measured
by any other instrument.
Accuracy can be obtained only if measuring instruments are periodically
compared with standards which are known to be constant; i.e. from time to
time the instruments are calibrated.
In some cases the standard is reproducible on the spot so that comparison
is easy, while other standards are not so easy to reproduce. In the latter case,
it is sometimes necessary to send the instrument back to the maker for cali­
bration. The maker compares the instrument with a 'standard' which has
been compared with an absolute standard in the National Physical Labora­
tory. In this way the measured quantity is being indirectly compared with
the absolute standard.
If all users of instruments do this then the results obtained will agree among
themselves, for two measurements which are each equal to a third must be
equal to each other. In this way precision is obtained.
The aim of all instrument users should be to obtain as high a degree of
accuracy as possible without involving unreasonable labour and expense,
but the relative sizes of errors should always be kept in mind. An error of
1 metre in the measurement of the length of a room is a serious error, but the
same error in the measurement of a distance of several kilometres is negligible.
To ensure that this idea of relative size of an error is kept in mind, the error is
usually expressed in terms of the true value of the quantity being measured,
as a fraction, or, more conveniently, as a percentage: e.g., if a pyrometer
is used to measure a furnace temperature at say 1,650° C and it is accurate
4 INTRODUCTION
to + 5° C, then the percentage error is
5 10
+ x 100 per cent = +— per cent = +0-3 per cent
"1650 y ~33F ^
If, however, an error of ±5° C occurs in the measurement of the temperature
of boiling water at 100° G, then the percentage error is
5
+ x 100 = +5 per cent
~100 ~ H
a much more serious error.
The accuracy of an instrument is usually expressed in terms of its inaccuracy,
i.e. it is expressed in terms of the percentage error.
The accuracy of an instrument may be expressed in a number of ways.
Makers usually state the 'intrinsic accuracy' of the instrument. This is the
accuracy of the instrument when calibrated in the laboratory in the absence
of vibration, ambient temperature changes etc. This is not necessarily the
same as the practical accuracy obtained with the instrument under plant
conditions. To ensure that the instrument shall give in use an accuracy which
approaches its intrinsic accuracy it must be carefully sited, maintained, and
standardised and readjusted at intervals.
The instrument must also be suited to the plant on which it is used. It
must not be corroded by the atmosphere in which it is placed, and should
as far as possible be protected from vibration and wide variations of ambient
temperature. In some cases it is necessary to sacrifice intrinsic accuracy in
order to obtain an instrument which has a more robust construction, and
which is more likely to maintain a higher degree of practical accuracy under
difficult plant conditions.
Point Accuracy. The 'limits of error' of an instrument may be expressed in
a number of ways. In some cases the 'point accuracy' is given. This is the
accuracy of the instrument at one point on its scale only, and does not give
any information on the general accuracy of the instrument. The general
accuracy may be given, however, by a table giving the point accuracy at
a number of points throughout its range. For example, the accuracy of a
pyrometer may be given at the fixed points on the International Scale of
Temperature which happen to be within the range of the pyrometer.
Accuracy as 'Per cent of Scale Range'. When an instrument has a uniform scale
its accuracy is often expressed in terms of the scale range, i.e. 'accurate to
within +0-5 per cent of the scale range'. Accuracy expressed in this way
may be misleading, for an error of 0-5 of the scale range at the upper end
of the scale may be negligible, but at 10 per cent of the full scale this is an
error of 5 per cent of the true value.
Accuracy as 'Per cent of the True Value'. Probably a better conception of the
accuracy of an instrument is obtained when the error is expressed in terms of
the true value, i.e. accurate to within ±0-5 per cent of the true value. Such
a statement means that as the reading gets less so does the size of the error,
so that at 10 per cent of the full scale the accuracy of the instrument would be
ten times better than that of an instrument which is accurate to within ±0-5
per cent of the scale range. When an instrument consists of several units
(such as, say, the orifice plate and the manometer in a flow meter), each unit
INTRODUCTION 5
will have its own limits of error. Suppose an instrument consists of three units,
the limit of error for the units being + a, +b, +c respectively. Then the
maximum possible error will be ±(a + b + c). It is unlikely that all the units
will have the maximum possible error at the same time, so the accuracy is
often expressed in terms of the root square error ±J(a* + b* + c*).
A Complete Statement of Accuracy. If it is required to give a complete picture
of the accuracy of an instrument, a graph should be drawn showing the error
at several points on the scale plotted against the true value as measured by
some reliable standard. The instrument should be calibrated as the measured
variable is increased in steps, and the process repeated as the measured
variable is decreased by the same steps. The error at each reading can then
be plotted against the true value at each point. In this way, two curves are
obtained which give the error and the 'hysteresis' at each reading. The
'hysteresis' is the difference between the readings obtained when a given
value of the measured variable is approached from below, and when the
same value is approached from above, and is usually caused by friction or
backlash in the instrument movement, or by changes in the controlling spring.
Curves of this nature give a complete statement of the accuracy of the instru­
ment, and may be used for correcting the instrument reading. Where the
curves coincide there is no hysteresis error.
Sensitivity. The sensitivity of an instrument is usually taken to be 'the size
of the deflection produced by the instrument for a given change in the measured
variable'. It is, however, quite frequently used to denote 'the smallest change
in the measured quantity to which the instrument responds'. The largest
change in the measured variable to which the instrument does not respond is
called the 'dead zone'.
Rangeability. The rangeability of a measuring instrument is usually taken
to mean the ratio of the maximum meter reading to the minimum meter
reading for which the error is less than a stated value. For example: In a
positive displacement flow meter a certain quantity of liquid passes through
the meter without being registered because of the leakage between the fixed
and moving parts. The calibration curve is therefore of the form shown in
Figure 1. The maximum quantity which can be measured by the meter is
2 0
S
u
v.
i
Γ 20 40 60 60
Per cent of meter maximum
Figure 1 Graph relating error to meter reading
usually fixed by the meter size. Increasing the flow above the meter maximum
will shorten the life of the meter owing to greatly increased wear and is there­
fore highly undesirable. It will be seen from the graph that the minimum flow
for which the meter accuracy is plus or minus 0-5 per cent is 10 per cent of the