Electrical measuring instruments introduction

EE 224
Electrical Measurements and Measuring
Instruments
Department of Electrical and Electronics Engineering
National Institute of Technology, Karnataka.
EEE Dept., NITK, Surathkal. No: 01

Course Code : EE224
Course Title : Electrical Measurements and Measuring Instruments
L-T-P : 3-1-3
Credits : 6
Objective : To gain knowledge of measuring electrical quantities and measuring instruments.
Outcome : At the end of the course students are expected to be equipped with skill to
choose a method of measurement suitable in an actual field and make
measurements and analyze.
Course Content
Introduction, Basics of measurements, Measurement of Resistance, Measurement of Inductance,
Capacitance, and Mutual Inductance, Study of measuring instruments, Megger & Earth Tester,
Locating cable faults, Potentiometers, Instrument Transformers, Energy meter.

References
Evaluation Plan
1. Assignments (2 No’s.) 5 %
2. Laboratory work 20 %
3. Mid Sem Exam 25 %
4. End Sem Exam 45 %
5. Quizzes 10 %
1. Golding and Widdis, “Electrical Measurements and Measuring Instruments” , Wheeler Publishing House, New
Delhi 1979.
2. A. K. Sawhney, “A Course in Electrical Measurement and Measuring Instruments” , Dhanpat Rai and Sons,
New Delhi 2007.
3. M. B. Stout , “Basic Electrical Measurements” , Prentice-Hall.
4. C.T. Baldwin , “Fundamental of Electrical Measurement”.
5. Related Indian and other standards.

Introduction to Measurement:
Measurement is the act or the result comparison between the whose quantity is not known and a
predefined standard.
 Is a process by which one can convert physical parameters to a meaningful number.
 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 unit for that particular property .
 Unit gives meaning to the measurements.
EEE Dept., NITK
EM & MI
In order that the results of the measurements are meaningful, there are two basic requirements:
1. The standard used for comparison must be accurately defined and should be commonly
accepted.
2. The apparatus used and the method adopted must be provable.

Significance of Measurements:
Measurement is required for
1. Design of equipment and processes
2. Proper operation and maintenance of equipment’s.
Because proper and economical design, operation and maintenance require a feedback of information.
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Methods of Measurements:
1. Direct method and
2. Indirect methods
 In direct method the unknown quantity is directly compared against a standard. The result is
expressed as a numerical number and a unit. The standard is a physical embodiment of a unit.
Ex: measurement of length, Mass, and Time
A human being can make direct length comparison with a preciseness of 0.25mm.
 In indirect method measurement using instruments.

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 Measurement by direct methods are not always possible, feasible and practicable.
 Direct method is most of the cases is in accurate as it involve human factors. Direct methods are not
preferable.
Instruments:
1. Mechanical
2. Electrical
3. Electronics
 The earliest instruments used by mankind were mechanical in nature and these earliest instruments
used the following components
 Detector
 Intermediate transfer device
 Indicator, recorder and storage device

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Mechanical Instruments:
 Reliable for static and stable conditions
Disadvantage:
 Unable to respond rapidly to measurements of dynamic and transient conditions.
 The reason is that mechanical instruments have moving parts. That are rigid, heavy and bulky
and consequently have large mass.
 Used for measuring slowly varying pressure
 They produces noise
Ex: Bourdon tube

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Electrical Instruments:
 More rapid than mechanical but it depends up on mechanical meter movement as indicating device.
These mechanical movement has some inertia and therefore these instruments have a limited time
response.
Electronic Instruments:
 Very fast response – for present days scientific and industrial measurements.
 Movement involves is only of electrons hence response is very fast
 Very weak signals can be detected using pre-amplifiers and amplifiers - results in high sensitivity.
 Important in case of biomedical instruments
 Used in detection of electromagnetically produced signals such as radio, video and microwaves.
These are light compact and have high degree of reliability and power consumption is very low.
8

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Classification of Instruments
There are many ways in which instruments can be classified. Broadly, instruments are classified into two
categories: (1) Absolute Instruments, and (2) Secondary Instruments.
Absolute Instruments:
These instruments give the magnitude of the quantity under measurements in terms of physical constants
of the instrument. The examples of this class of instruments are Tangent Galvanometer and Rayleigh's
current balance.
Secondary Instruments:
These instruments are so constructed that the quantity being measured can only be measured by
observing the output indicated by the instrument. These instruments are calibrated by comparison with an
absolute instrument or another secondary instrument which has already been calibrated against an absolute
instrument.

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Deflection and Null type instruments:
Depending upon the way they present the result of
measurement instruments are classified as deflection type and
null type.
Deflection Type:
In these types of instruments, pointer of the electrical
measuring instrument deflects to measure the quantity. The
value of the quantity can be measured by measuring the net
deflection of the pointer from its initial position. In order to
understand these types of instruments let us take an example of
deflection type permanent magnet moving coil ammeter which
is shown in Fig.

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Null type instruments:
In opposite to deflection type of instruments, the null or zero type electrical measuring instruments tend
to maintain the position of pointer stationary. They maintain the position of the pointer stationary by
producing opposing effect.
Thus for the operation of null type instruments following steps are required:
Value of opposite effect should be known in order to calculate the value of unknown quantity.
Detector shows the balance and the unbalance condition accurately.
The detector should also have the means for restoring force.

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Advantages and disadvantages of deflection and null type of measuring instruments:
1. Deflection type of instruments is less accurate than the null type of instruments. It is because, in the null
deflecting instruments the opposing effect is calibrated with the high degree of accuracy while the
calibration of the deflection type instruments depends on the value of instrument constant hence usually
not having high degree of accuracy.
2. Null point type instruments are more sensitive than the Deflection type instruments.
3. Deflection type instruments are more suitable under dynamic conditions than null type of instruments as
the intrinsic responses of the null type instruments are slower than deflection type instruments.

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Applications of Measurement Systems:
The way the instruments and measurement systems are used for different applications are as under:
1. Monitoring of processes and operations.
2. Control of processes and operations.
3. Experimental Engineering analysis.

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Components of automatic Control System:
1. Input
2. Comparator
3. Feed Forward Element (Amplifier)
4. Actuator
5. Feed Back (Measuring Instrument / Transducer)

Electrical Measurements and Measuring Instruments
Elements of measuring system, Measurement System
Performances and Error

Elements of Generalized Measurement System
It is possible and desirable to describe the operation of a measuring instrument or a system in a generalized
manner without resorting to intricate details of the physical aspects of a specific instrument or a system.
The whole operation can be described in terms of functional elements. Most of the measurement systems
contain three main functional elements. They are:
 Primary Sensing Element,
 Variable Conversion Element, and
 Data Presentation Element.

Primary Sensing Element:
 The quantity under measurement makes its first contact with the primary sensing element of a measurement
system. In other words, the measurand is first detected by primary sensor.
 This act is then immediately followed by the conversion of measurand into an analogous electrical signal. This is
done by a transducer.
 A transducer in general, is defined as a device which converts energy from one form to another. But in
measurement systems, this definition is limited in scope. A transducer is defined as a device which converts a
physical quantity into an electrical quantity. The physical quantity to be measured, in the first place is sensed and
detected by 'an element which gives the output in a different analogous form.
 This output is then converted into an electrical signal by a transducer, This is true of most of the cases but is not
true for all cases. In many cases the physical quantity is directly converted into an electrical quantity by a
transducer. However, the first stage of a measurement system is known as a detector transducer stage.

Variable Conversion Element:
 The output of the primary sensing element may be any kind of electrical signal. It may be a voltage, a frequency
or some other electrical parameter. Sometimes this output is not suited to the system.
 For the instrument to perform the desired function, it may be necessary to convert this output to some other
suitable form while preserving the information content of the original signal information content of the original
signal.
Variable Manipulation Element:
 The function of this element is to manipulate the signal presented to it preserving the original nature of the signal.
Manipulation here means a change in numerical value of the signal.
 For example, an electronic amplifier accepts a small voltage signal is input and produces an output signal which is
also voltage but of greater magnitude. Thus, voltage amplifier acts as a variable manipulation element. It is not
necessary that a variable manipulation element should follow the variable conversion element.

Data Transmission Element:
 When the elements of an instrument are physically separated, it becomes necessary to transmit data from one to
another. The element that performs this function is called a Data Transmission Element.
 For example, space crafts are physically separated from the earth where the control stations guiding their
movements are located. Therefore, control signals are sent from these stations to spacecrafts by a complicated
telemetry system using radio signals.
Data Presentation Element:
 The information about the quantity under measurement has to be conveyed to the personnel handling the
instrument or the system for monitoring, control, or analysis purposes.
 The information conveyed must be in a form intelligible to the personnel. This function is done by data
presentation element. In case data is to be monitored, visual display devices are needed.
 These devices may be analogue or digital indicating instruments like ammeters. voltmeters etc.

Measurement System Performance
Measurement System Performance:
The treatment of instrument and measurement system characteristics can be divided into two distinct categories viz:
 Static characteristics, and
 Dynamic characteristics.
Some applications involve the measurement of quantities that are either constant or vary very slowly with
time. Under these circumstances it is possible to define a set of criteria that gives a meaningful description of quality
of measurement without/interfering with dynamic descriptions that involve the use of differential equations. These
criteria are called Static Characteristics.
Normally static characteristics of a measurement system are, in general, those that must be considered when
the system or instrument is used to a condition not varying with time.

Static Calibration:
 All the static performance characteristics are obtained in one form or another by a process called static calibration.
The calibration of all instruments is important since it affords the opportunity to check the instrument against
a known standard and subsequently to errors in accuracy. Calibration procedure involve a comparison of the particular
instrument with either
 a primary standard,
 a secondary standard with a higher accuracy than the instrument to be calibrated, or an instrument of know
accuracy.
 Actually all working instruments, i.e., those instruments which arc actually used for measurement work must be
calibrated against some reference instruments which have a higher accuracy. The reference instruments in turn
must be calibrated against instrument of still higher grade of accuracy or against primary standard, or against other
standards of known accuracy. It is essential that a measurement made must ultimately be traceable to the relevant
primary standards.

Static Characteristics:
The main static characteristics discussed here are:
 Accuracy,
 Sensitivity,
 Reproducibility,
 Drift,
 Static error, and
 Dead Zone·

Accuracy:
 It is the closeness with which an instrument reading approaches the true value of the quantity being measured.
Sensitivity:
 It is a measure of the change in instrument output that occurs when the quantity being measured changes by a
given amount.
 Resolution is the smallest change in input signal which can be detected by the instrument.
Precision:
 It is a measure of the reproducibility of the measurements, i.e. given a fixed value of a quantity, precision is a
measure of the degree of agreement within a group of measurements i.e., it is a measure of the reproducibility of
the measurement. The term 'Precise' means clearly or sharply defined.

Reproducibility:
 It is the degree of closeness with which a given value may be repeatedly measured. It may be specified in terms of
units for a given period of time.
Drift:
 Perfect reproducibility means that the instrument has no drift. No drift means that with a given input the measured
values do not vary with time.
Zero Drift:
 If whole calibration gradually shifts due to slippage,
permanent set, or due to undue warning up of electronic tube
circuits, zero drift sets in. it can be prevented by zero setting.

Spam Drift:
 If there is proportional change in the indication all along the upward
scale, then the drift is called span drift or sensitivity drift.
Zonal drift:
 In case the drift occurs over a portion of span of an instrument, it is called
zonal drift.
Cause of drift:
 Stray electric and magnetic fields
 Thermal emfs,
 Changes in temperature,
 Mechanical Vibrations,
 Wear, tear and some high mechanical stresses developed in some parts of the instruments and systems

Dead Zone:
 Dead zone is defined as the largest change of input quantity for which there is no output of the instrument. It is
basically range of input value for which output is zero. Dead zone is also known as Dead band or dead space or
neutral zone.
 Any instrument that exhibits hysteresis also displays dead zone.

Reproducibility:
It is the degree of closeness with which a given value may be repeatedly measured. It may be specified in terms of
units for a given period of time. Perfect reproducibility means that the instrument has no drift. No drift means that
with a given input the measured values do not vary with time.
Repeatability:
Reproducibility and Repeatability are a measure of closeness with which a given input may be measured over and
over again. The two terms cause confusion. Therefore, a distinction is made between the two terms. Reproducibility is
specified in terms of scale readings over a given period of time. On the other hand, Repeatability is defined as the
variation of scale reading and is random in nature.

Errors in Measurement
 Measurements done in a laboratory or at some other place always involve errors. No measurement is free from
error.
 If the precision of the equipment is adequate, no matter what its accuracy is, a discrepancy will always be
observed between two measured results.
True value:
 The true value of quantity to be measured may be defined as the average of an infinite number of measured
values when the average deviation due to the various contributing factors tends to zero.
 Such an ideal situation is impossible to realise in practice and hence it is not possible to determine the “True
value” of a quantity by experimental means.
 The reason for this is that the positive deviations from the true value do not equal the negative deviations and
hence do not cancel each other.

Static Error:
 The most important characteristic of an instrument or measurement system is its accuracy, which is the agreement
of the instrument reading with the true value of quantity being measured. The accuracy of an instrument is
measured in terms of its error.
 Static error is defined as the difference between the measured value and the true value of the quantity.
Then: 𝛿A = Am - At
𝛿A is also called as absolute static error of quantity ’A’, So ɛo = 𝛿A
The absolute value of 𝛿A doesn’t indicate precisely the accuracy of measurement

Numerical Problems:
1. A meter reads 127.50 V and the true value of the voltage is 127.43 V. Determine: (a) the static error, and (b) the
static correction for this instrument.
Sol:
a. Static error = measured value – true value = +0.07 V.
b. Static correction = - static error = -0.07 V.
2. A thermometer reads 95.45° C and the static correction given in the correction curve is -0·08° C. Determine the true
value of the temperature.
Sol:
True value of temperature = measured value + static correction
= 95.37° C

3. A voltage has a true value of 1.5 V. An analog indicating instrument with a scale range of (0-2.5) V shows a voltage
of 1.46 V. What are the values of absolute error and correction? Express the error as a fraction of the true value and
the full-scale deflection (f.s.d).
t m
C A A
  
Sol: Absolute error = 1.46 – 1.50 = -0.04
Absolute correction 0.04
C A
 
   
Relative error
0.04
2.66%
1.50
r
t
A
A
 
    
0.04
*100 1.60%
2.5

 
Relative error (expressed as a percentage of f.s.d.) =
4. The measured value of a capacitor is 205.3 micro farad. Whereas its true value is 201.4 micro farad. Determine the relative
error.
Sol:
Relative error = 0.0194

5. A moving coil ammeter has a uniform scale with 50 divisions and gives full-scale reading of 5 A. the instrument
can read up to 1/4th of a scale division with a fair degree of certainty. Determine the resolution of the instrument in
mA.
Sol:
Full scale reading = 5 A.
No. of divisions on scale = 50
1 scale division = (5/50) * 1000 = 100 mA.
Resolution = 1/4th of division = (1/4)* 100 = 25 mA.

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