Measurements lecture 1

MEASUREMENTS
AND
INSTRUMENTATION

• MEASUREMENT
• A Method To Obtain The Information Regarding The
Physical Values Of A Variable.
• INSTRUMENTATION
• Device used for measurement system

Terminologies
• Physical quantity: variable such as pressure,
temperature, mass, length, etc.
• Data: Information obtained from the
instrumentation/measurement system as a result
of the measurements made of the physical
quantities
• Information: Data that has a calibrated numeric
relationship to the physical quantity.
• Parameter: Physical quantity within defined
(numeric) limits.

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Purpose of a measurement system
measurand Sensor, signal conditioning, display Man, tracking control etc

• Measurand: Physical quantity being measured.
• Calibration: Implies that there is a numeric relationship
throughout the whole instrumentation system and that it is
directly related to an approved national or international
standard.
• Test instrumentation: It is a branch of instrumentation and
most closely associated with the task of gathering data during
various development phases encountered in engineering, e.g.
flight test instrumentation for testing and approving aircraft.
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Terminology

• Transducer: A device that converts one form of energy to
another.
• Electronic transducer: It has an input or output that is
electrical in nature (e.g., voltage, current or resistance).
• Sensor: Electronic transducer that converts physical quantity
into an electrical signal.
• Actuator: Electronic transducer that converts electrical energy
into mechanical energy.
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Terminologies

• In the case of process industries and industrial
manufacturing…
– To improve the quality of the product
– To improve the efficiency of production
– To maintain the proper operation.
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Why measurement?

• To acquire data or information (hence data
acquisition) about parameters, in terms of:
– putting the numerical values to the physical quantities
– making measurements otherwise inaccessible.
– producing data agreeable to analysis (mostly in electrical
form)
• Data Acquisition Software (DAS) – data is acquired by
the instrumentation system.
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Why instrumentation?

Requirement of measuring instrument
• With the introduction of the instrument in the
circuit the circuit conditions should not alter
- Thus the quanity to be measured should not
get affected due to the instrument used.
• Power consumed by the instrument for their
operation should be small as possible.

• Direct comparison
– Easy to do but… less accurate
• e.g. to measure a steel bar
• Indirect comparison
– Calibrated system; consists of several devices to
convert, process (amplification or filtering) and
display the output
• e.g. to measure force from strain gages located in a
structure
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Types of measurements

 Stage 1: A detection-transducer or sensor-transducer, stage; e.g.
Bourdon tube
 Stage 2: A signal conditioning stage; e.g. gearing, filters, bridges
 Stage 3: A terminating or readout-recording stage; e.g. printers,
oscilloscope
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Generalised measuring system
General Structure of Measuring System

• Active Instruments
– the quantity being measured simply modulates (adapts to) the
magnitude of some external power source.
• Passive Instruments
– the instrument output is entirely produced by the quantity being
measured
• Difference between active & passive instruments is the level of
measurement resolution that can be obtained.
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Types of instruments in measurements

MEASUREMENT
• The process of measuring is essentially that of
comparing some unknown value with a value
which is assumed to be known.
• 1) latter one in standard.
• 2) And measuring system is instrument.

• Characteristics of INSTRUMENTS
• Calibration: All the static characteristics are obtained in
one form or another by a process called Calibration.
Calibration procedures involve a comparison of the
particular instrument with either
a) a primary standard.
b) a secondary standard with a higher accuracy than
the instruments be calibrated.
c) an instrument of known accuracy.
• Accuracy: Accuracy may be defined as the ability of a
device or a system to respond to a true value of a
measured variable under reference conditions.

• Precision: Precision is defined by the degree of exactness
for which an instrument is designed or intended to
perform.
• Repeatability: It is the closeness of agreement among a
number of consecutive measurements of the output for
the same value of the input, under the same operating
conditions.
• Reproducibility: It is the closeness of agreement among
repeated measurements of the output for the same value
of the input, made under the same operating conditions
over a period of time.
• Drift: It is an undesired change or a gradual variation in
output over a period of time that is unrelated to changes
in input and operating conditions.

• Span: If in a measuring instrument the highest point of
calibration is y units and the lowest point x units.
Then the instrument range y units
The instrument span is given by
Span=(x-y)units
• Sensitivity: Sensitivity can be defined as the ratio of a
change in output to the change in input which causes it.
• Resolution: The smallest increment in input (the quantity
being measured) which can be detected with certainty by
an instrument is its resolution.
• Dead zone: Dead zone is the largest range of values of a
measured variable to which the instrument does not
respond.

• To define physical quantities in type and magnitude
• Units of measurement may be defined as the standard
measure of each kind of physical quantity.
• Efforts were made to standardise systems of measurement so
that instrument professionals and specialist in other
disciplines could communicate among themselves.
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Units Of Measurement

• Foot-pound-second (F.P.S.) used for:
– Length
– Mass
– Time
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English Systems Of Units

• As a physical representation of a unit of measurement
• It is used for obtaining the values of the physical properties of
other equipment by comparison methods; e.g.
– The fundamental unit of mass in the SI system is the
kilogram, defined as the mass of a cubic decimeter of
water at its temperature of maximum density of 4C.
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Standard Of Measurement

• International Organization for Standardization (ISO)
• International Electrotechnical Commission (IEC)
• American National Standards Institute (ANSI)
• Standards Council of Canada ( SCC)
• British Standards (BS)
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Examples of Standard Bodies

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Calibration
Calibration consists of comparing the output of the
instrument or sensor under test against the output of
an instrument of known accuracy (higher accuracy)
when the same input (the measured quantity is applied
to both instrument)
The procedure is carried out for a range of inputs covering
the whole measurement range of the instrument or sensor
Ensures that the measuring accuracy of all instruments and
sensors used in a measurement system is known over the
whole measurement range, provided that the calibrated
instruments and sensors are used in environmental conditions
that are the same as those under which they were calibrated

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Calibration involve a comparison of an
instrument with either:
1. A primary standard
2. A secondary standard
3. A known input
Example:
Weighing instrument
Standard weight
measurement facility
(for example: NIST)
Another weighing
instrument of higher
accuracy
Mass standard

• The method and apparatus for performing measurement
instrumentation calibrations vary widely.
• A rule that should be followed is that the calibration standard
should be at least 10 times as accurate as the instrument
being calibrated.
• By holding some inputs constant, varying others and recording
the output(s) develop the desired static input-output
relations. Many trial and runs are needed.
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Calibration

• Home
– Thermometer
– Barometer
– Watch
• Road vehicles
– speedometer
– fuel gauge
• Industry
– Automation
– Process control
– Boiler control
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Application

Types of Instruments
• Absolute Instruments:
Absolute instruments are those which give the value of the electrical
quantity to be measured, in terms of the constant of the instruments
and their deflection only, e.g. tangent galvanometer.
• Secondary Instruments:
Secondary instruments are those which have been precalibrated by
comparison with an absolute instrument. The value of the electrical
quantity to be measured in these instruments can be determined
from the deflection of the instrument.
Without calibration of such an instrument, the deflection is
meaningless.
• Different types of secondary instruments:
• 1)Indicating
• 2)Recording
• 3)Integrating

• 1)Indicating:
Indicating instruments are those which indicate the
instantaneous value of the electrical quantity being measured,
at the time at which it is being measured. Their indications are
given by pointers moving over calibrated dials(scale), e.g.
ammeters,voltmeters and wattmeters.
• 2)Recording:
Recording instruments are those which give a continuous
record of variations of the electrical quantity over a selected
period of time. The moving system of the instrument carries an
inked pen which rests tightly on a graph chart. E.g. recording
voltmeters used in supply station.
• 3)Integrating:
Integrating instruments are those which measure and register
the total quantity of electricity (in ampere-hour) or the total
amount of electrical energy(in watt-hours or kilowatt-hours)
supplied to a circuit over a period of time, e.g. ampere-hour
meters, energy meters.

Essentials of Indicating Instruments
• Deflecting Torque(Td):
It is the torque which deflects the pointer on a calibrated
scale according to the electrical quantity passing through
the instrument. The deflecting torque causes the moving
system and hence the pointer attached to it to move from
zero position to indicate on a graduated scale the value of
electrical quantity being measured.
• Controlling Torque(Tc):
It is the torque which controls the movement of the
pointer on a particular scale according to the quantity of
electricity passing through it. If deflecting torque were
acting alone, the pointer would continue to move
indefinitely and would swing over to the maximum
deflected position irrespective of the magnitude of current
(or voltage or power) to be measured.

1) Spring Control:
In the spring control method, a hair-spring, usually of phosphor-bronze,
attached to the moving system is used. With the deflection of the pointer,
the spring is twisted in the opposite direction. This twist in the spring
produces a restoring torque which is directly proportional to the angle of
deflection of the moving system. The pointer comes to a position of rest (or
equilibrium) when the deflecting torque (Td) and controlling torque (Tc) are
equal.
Tc ∞ θ
To give a controlling torque which is directly proportional to the angle of
deflection of the moving system, the number of turns of the spring should
be fairly large so that the deformation per unit length is small. The stress in
the spring must be limited to such a value that there is no permanent set.
Springs are made of materials which are
 Non magnetic
 Not subject to much fatigue
 Low in specific resistance
 Have low temperature coefficient of resistance.

Spring Control method
1. Two phosphor bronze hair
springs of spiral shapes are
attached to the spindle of
the moving system of the
instrument.
2. They are wound in
opposite direction
3. Pointer is attached to the
spindle of the moving
system

Working of Spring Control Method
1. When the moving system
deflected, one spring gets
wounded and the other one gets
unwounded. This results in
controlling torque whose
magnitude is directly
proportional to angle of
deflection.
2. Td is directly proportional to
current I and Tc is directly
proportional to deflection angle,
at final steady state Td = Tc,
deflection is directly proportional
to current, hence scale is linear

Gravity Control Method
1. In gravity control method, a small
weight is attached to the spindle of
the moving system. Due to the
gravitational pull, a control torque
(acting in opposite direction to the
deflecting torque) is produced
whenever the pointer tends to move
away from its initial position.
2. In this case, Td is directly
proportional to current I and Tc is
directly proportional to sine of the
deflection angle, since Td = Tc, sine
of the deflection is directly
proportional to current, hence scale is
non linear i.e. cramped scale.

2) Gravity Control:
Gravity control is obtained by attaching a small weight to the moving
system in such a way that it produces a restoring or controlling torque
when the system is deflected.
Tc ∞ Sinθ
Thus, controlling torque in a gravity control system is proportional to the
sine of the angle of deflection.
The degree of control is adjusted by screwing the weight up or down on the
carrying system.
Advantages:
The advantages of gravity control system, as compared to spring control,
are given below:
 It is cheap
 It is unaffected by temperature
 It is not subjected to fatigue or distortion, with time.
Disadvantages:
The disadvantages of gravity control system, as compared to spring control,
are given below:
 It gives a cramped scale.
 The instrument has to be kept vertical.

• Damping Torque:
If the moving system is acted upon by deflecting and controlling torques alone, then
pointer, due to inertia, will oscillate about its final deflected position for quite sometime
before coming to rest. This is often undesirable because it makes difficult to obtain
quick and accurate readings. In order to avoid these oscillations of the pointer and to
bring it quickly to its final deflected position, a damping torque is provided in the
indicating instruments.
There are three types of damping:
 Air friction damping:
air friction damping uses either aluminium piston or vane, which is attached to or
mounted on the moving system and moves in an air chamber at one end.
 Fluid friction damping:
In fluid friction damping, a light vane (attached to the moving system) is dipped into a
pot of damping oil. The fluid produces the necessary opposing (or damping) force to the
vane. The vane should be completely submereged in the oil.
The disadvantage of this type of damping is that it can only be used in the vertical position.
 Eddy Current Damping:
Eddy-current damping uses a conducting material which moves in a magnetic field so as
to cut through the lines of force, thus setting up eddy currents. Force always exists
between the eddy current and magnetic field which is always opposite to the direction
of motion. This is most efficient type of damping and is largely used in permanent
magnet moving coil instruments.

Damping torque
• If the moving system is acted upon by deflecting and controlling torques
alone, then pointer, due to inertia, will oscillate about its final deflected
position for quite sometime before coming to rest. This is often
undesirable because it makes difficult to obtain quick and accurate
readings. Damping torque minimizes the oscillations of the pointer about
the final steady state deflection and makes it steady.. In the absence of this
torque, pointer continues oscillating to its final position after reaching to its
final position.
• Depending on the magnitude of damping, it can be classified as under
damped, over damped and critical damp

Damping Methods
Air Friction Damping
Eddy current Damping
Fluid Friction Damping

Air friction damping:
air friction damping uses either
aluminium piston or vane, which is
attached to or mounted on the moving
system and moves in an air chamber at one
end.

Fluid friction damping:
In fluid friction damping, a light vane (attached to the
moving system) is dipped into a pot of damping oil. The fluid
produces the necessary opposing (or damping) force to the
vane. The vane should be completely submereged in the oil.
The disadvantage of this type of damping is that it can only be
used in the vertical position.

Eddy Current Damping:
Eddy-current damping uses a conducting
material which moves in a magnetic field so as
to cut through the lines of force, thus setting
up eddy currents. Force always exists between
the eddy current and magnetic field which is
always opposite to the direction of motion.
This is most efficient type of damping and is
largely used in permanent magnet moving coil
instruments.

Methods of damping:
1. Air friction or pneumatic damping
2. Eddy current or electromagnetic damping
3. Fluid friction damping
• Air friction or pneumatic damping:

Air Friction or Pneumatic Damping:
• In this system a light aluminium piston is attached to the
spindle of the instrument and is arranged to move in a fix
air chamber closed at one end. The cross section of the
chamber may be either circular or rectangular and the
clearance between the piston and the side of the
chamber is small and uniform. Compression and suction
action of the piston on the air in the chamber damp the
possible oscillations of moving system, because the
motion of the piston in either direction is oppose by the
air.
• In second case a thin aluminium vane, mounted on the
spindle, moves with very small clearance in a sector
shaped box. Any tendency of the moving system to
oscillate is damped by the action of the air on vane.

Eddy current damping:
• It is the most efficient type of the damping
• In this a thin disc usually of copper or aluminium is
mounted on the spindle. When this disc moves in the
magnetic field of permanent magnet, line of force are cut
and eddy current are set up in it.
• The force that exists between these current and
magnetic field is always in the direction opposing the
motion and therefore, provide necessary damping.
• The magnitude of the induce current and therefore of
the damping force which is dependent on it, is directly
proportional to the velocity of moving system.

Fluid friction damping:
• In this method of damping, a light disc is attached to the
spindle of the moving system and completely submerge
in the damping oil in a pot.
• The motion of the disc is always opposed by a frictional
drag on the disc. This frictional drag is zero when the disc
is stationary and increases with the speed of the rotation
of the disc.
• For increase damping, vanes in a vertical planes, carried
on a spindle and immersed in oil are used.
• Fluid friction damping can only be used in the
instruments which are use in the vertical position.

Error
• Types of Error
• Static Error : the numerical difference between the true value of a
quantity and its value as obtained by measurement.
• Mistakes: These are errors due to human mistakes such as
careless reading, mistakes in observations, incorrect application of
a correction, improper application of instruments and
computational errors.
• Systematic Error:
• 1)Instrumental Error: Instrumental errors are the errors inherent
in measuring instruments because of their mechanical structure,
such as friction in bearings of various moving components,
irregular spring tension.

• 2)Environmental Error: it is due to conditions
external to the measuring device including
conditions in the area surrounding the
instrument such as the effect of change in
temp , humidity, barometric pressure , or
magnetic or electrostatic fields.
• Random Error: the cause of such error is
unknown or not determinable in the ordinary
process of making measurements. Such errors
are normally small and follows the laws of
chance.

Sources of error
• Insufficient knowledge of process parameters
and design conditions.
• Poor design.
• Poor maintenance.
• Error caused by people who operate
instrument equipment.
• Certain design limitations.

Introduction - Ammeter & Voltmeter:
Direct measurement of voltage and current is done with the use of
voltmeter and ammeter respectively. In construction and principle of operation,
both the instruments are very much same.
Connecting voltmeter and ammeter in a Circuit:

• Voltmeter is used to measure the
potential difference across a load.
• So the resistance of the voltmeter shall
be very high compared to circuit
resistance.
• In contrast to this, if the voltmeter
resistance is small, and connecting the
voltmeter across load will be equal to
parallel connecting another load.
• Then the circuit conditions are changed
very much and no worth-while reading
will be measured.
• So the resistance of voltmeter must be
very high.
• If the meter's moving system has only a
small resistance, additional resistance
will be included in series with the
moving coil.
• This additional resistance is known as
swamping resistance.

• An ammeter is used to
measure the current flows
through the circuit.
• So the resistance must be
very small.
• If the ammeter resistance is
very high, it will introduce
high resistance in series
with the existing circuit an
disturb the circuit
conditions.
• Ammeter should never
connect across a load or
voltage source.

Classification of Ammeters and
Voltmeters
Types of Ammeters, Voltmeters are discussed in the below
section:
Ammeters and voltmeters are classified as follows:
• Moving Iron Type
(a) Attraction type
(b) Repulsion type
• Moving Coil
(a) Permanent magnet type
(b) Dynamometer type
• Hot wire
• Induction type
• Electrostatic (voltmeter only)

Types of indicating instruments
• PMMC(permanent magnet moving coil)
• MI(moving iron)
Only DC instruments
In any instrument, if the deflection is directly proportional to the voltage or
current to be measured, then they are suitable for dc measurement only. If we use
such kind instrument in ac circuit, the moving system will try to move forward and
backward several times within a second. So it will simply vibrate and no reading will be
shown. The permanent Magnet Moving Coil meter (PMMC) is the one suited for dc
measurement only.
Only AC Instruments
The instruments which utilize the electromagnetic induced current for
producing deflection are used for ac measurement only. We know that
electromagnetic induction is not possible with DC current. Therefore, induction type
instruments (Voltmeter, ammeter and wattmeter) are used only for AC
measurements.

Ammeter and voltmeter
Types of ammeter and voltmeter
1. Permanent magnet moving coil type (P.M.M.C.)
2. Moving iron (M.I.)
Attraction type Repulsion type

Permanent magnet moving coil ammeter and
voltmeter

PMMC………….
• Principle of Operation: When a current carrying
conductor is placed in a magnetic field, it experiences a
force and tends to move in the direction as per Fleming’s
left hand rule.
Fleming left hand rule: If the first and the second finger and
the thumb of the left hand are held so that they are at right
angle to each other, then the thumb shows the direction of
the force on the conductor, the first finger points towards the
direction of the magnetic field and the second finger shows
the direction of the current in the wire.

Construction:
• A coil of thin wire is mounted on an aluminum frame
(spindle) positioned between the poles of a U shaped
permanent magnet which is made up of magnetic alloys
like alnico.
• The coil is pivoted on the jewelled bearing and thus the
coil is free to rotate. The current is fed to the coil through
spiral springs which are two in numbers. The coil which
carries a current, which is to be measured, moves in a
strong magnetic field produced by a permanent magnet
and a pointer is attached to the spindle which shows the
measured value.

PMMC
CONSTRUCTION:
• It consists of a powerful U-shaped
permanent magnet made of Alnico, and soft
iron pole pieces bored out cylindrically.
• A soft iron core is fixed between the
magnetic poles whose functions are
i) to make the field uniform
ii) to decrease the reluctance of the air path
between the poles and hence increase the
magnetic flux.
• Surrounding the core is a rectangular coil of
many turns wound on a light aluminum or
copper frame, supported by delicate bearings
• A light pointer fixed to the frame moves on
the calibrated scale according to the amount
of electricity passed through the coil.
• The aluminum frame provides not only
support for the coil but also a damping torque
by the eddy currents induce in it.

A movable coil in a magnetic field
(no current).
A movable coil in a magnetic
field (current).

The coil of wire is wound on an aluminum
frame, or bobbin, and the bobbin is supported
by jeweled bearings which allow it to move
freely.
permanent-magnet moving-coil device as a meter, two
problems must be solved.
• First, a way must be found to return the coil to its original
position when there is no current through the coil.
•Second, a method is needed to indicate the amount of coil
movement.
The first problem is solved by the use of hairsprings
attached to each end of the coil. These hairsprings can also
be used to make the electrical connections to the coil.
With the use of hairsprings, the coil will return to its initial
position when there is no current. The springs will also
tend to resist the movement of the coil when there is
current through the coil. When the attraction between the
magnetic fields (from the permanent magnet and the coil)
is exactly equal to the force of the hairsprings, the coil will
stop moving toward the magnet.

As the current through the coil increases, the
magnetic field generated around the coil
increases. The stronger the magnetic field
around the coil, the farther the coil will move.
This is a good basis for a meter.
But, how will you know how far the coil moves?
If a pointer is attached to the coil and extended
out to a scale, the pointer will move as the coil
moves, and the scale can be marked to indicate
the amount of current through the coil.

Two other features are used to increase the
accuracy and efficiency of this meter movement.
First, an iron core is placed inside the coil to
concentrate the magnetic fields. Second, curved
pole pieces are attached to the magnet to ensure
that the turning force on the coil increases
steadily as the current increases.
This permanent-magnet moving-coil
meter movement is the basic movement
in most measuring instruments. It is
commonly called the d'Arsonval
movement because it was first employed
by the Frenchman d'Arsonval in making
electrical measurements.

PMMC instruments internal structure

Working:
• When a current flow through the coil, it generates a
magnetic field which is proportional to the current in
case of an ammeter. The deflecting torque is produced by
the electromagnetic action of the current in the coil and
the magnetic field.
• The controlling torque is provided by two phosphorous
bronze flat coiled helical springs. These springs serve as a
flexible connection to the coil conductors.
• Damping is caused by the eddy current set up in the
aluminum coil which prevents the oscillation of the coil.

Deflecting torque equation:-
The magnetic field in the air gap is radial due to the presence of soft iron core. Thus, the
conductors of the coil will move at right angles to the field. When the current is passed through
the coil, forces act on its both sides which produce the deflecting torque.
Let, B = flux density, Wb/m2
L = length or depth of coil, m
b = breadth of the coil.
N = no. of turns of the coil.
If a current of ‘I’Amperes flows in the coil, then the force acting on each coil side is given by,
Force on each coil side, F = BILN Newtons.
Deflecting torque, Td = Force × perpendicular distance
= (BILN) × b
Td = BINA Newton metre.
Where, A =L× b, the area of the coil in m2.
Thus, Td α I
The instrument is spring controlled so that, Tc α θ
The pointer will comes to rest at a position, where Td =Tc
Therefore, θ α I
Thus, the deflection is directly proportional to the operating current. Hence, such instruments
have uniform scale.

Applications:
The PMMC has a variety of uses onboard ship. It can be
used as:
1) Ammeter:
• When PMMC is used as an ammeter, except for a very
small current range, the moving coil is connected across
a suitable low resistance shunt, so that only small part of
the main current flows through the coil.
• The shunt consists of a number of thin plates made up of
alloy metal, which is usually magnetic and has a low
temperature coefficient of resistance, fixed between two
massive blocks of copper. A resistor of same alloy is also
placed in series with the coil to reduce errors due to
temperature variation.

Applications………..
• Voltmeter:
When PMMC is used as a voltmeter, the coil is connected in
series with high resistance. Rest of the function is same as
above. The same moving coil can be used as an ammeter or
voltmeter with an interchange of above arrangement

Applications………..
• Galvanometer:
Galvanometer is used to measure small value of current
along with its direction and strength. It is mainly used
onboard to detect and compare different circuits in a
system

• ADVANTAGE:
• Low power consumption.
• Uniform scale extendable over an arc of 270º or so.
• High torque weight ratio.
• No hysteresis loss.
• Very effective and efficient eddy current damping.
• Not effected much by stray and magnetic fields due to strong operating field.
• DISADVANTAGE:
• Costlier compared to moving iron instruments, due to delicate construction and
accurate machining and assembly of various parts.
• Some error arise due to the ageing of control springs and the permanent magnet.
• Use limited to d.c. only.
• Scale length of meter can be increased from 120º and 240º or even 270º or 300º.
• APPLICATION:
• PMMC instruments can be used as dc ammeter. And its range can be increased by using
a large number of turns in parallel with the instrument.
• The range of this instrument, when used as a dc voltmeter, can be increased by using a
high resistance in series with it.

Moving Iron Instruments – Voltmeter and
Ammeter
Construction and basic principle operation of moving-iron
instruments
Moving-iron instruments are generally used to measure
alternating voltages and currents. In moving-iron
instruments the movable system consists of one or more
pieces of specially-shaped soft iron, which are so pivoted as
to be acted upon by the magnetic field produced by the
current in coil.
There are two general types of moving-iron instruments
namely:
1. Repulsion (or double iron) type
2. Attraction (or single-iron) type

The brief description of different components of
a moving-iron instrument is given below:
• Moving element: a small piece of soft iron in the form of
a vane or rod.
• Coil: to produce the magnetic field due to current
flowing through it and also to magnetize the iron pieces.
• In repulsion type, a fixed vane or rod is also used and
magnetized with the same polarity.
• Control torque is provided by spring or weight (gravity).
• Damping torque is normally pneumatic, the damping
device consisting of an air chamber and a moving vane
attached to the instrument spindle.
• Deflecting torque produces a movement on an
aluminum pointer over a graduated scale.

Working:
Current in the coil induces both vanes to become
magnetized and repulsion between the similarly
magnetized vanes produces a proportional rotation. The
deflecting torque is proportional to the square of the
current in the coil, making the instrument reading is a true
‘RMS’ quantity Rotation is opposed by a hairspring that
produces the restoring torque. Only the fixed coil carries
load current, and it is constructed so as to withstand high
transient current.
Moving iron instruments having scales that are nonlinear
and somewhat crowded in the lower range of calibration

Application:
Measurement of Electric Voltage and Current
• Moving iron instruments are used as Voltmeter and
Ammeter only.
• Both can work on AC as well as on DC.
Ammeter:
• Instrument used to measure current in the circuit.
• Always connected in series with the circuit and carries
the current to be measured.
• This current flowing through the coil produces the
desired deflecting torque.
• It should have low resistance as it is to be connected in
series.

Application:
Voltmeter
• Instrument used to measure voltage between two points
in a circuit.
• Always connected in parallel.
• Current flowing through the operating coil of the meter
produces deflecting torque.
• It should have high resistance. Thus a high resistance of
order of kilo ohms is connected in series with the coil of
the instrument

Advantages:
• The instruments are suitable for use in AC and DC
circuits.
• The instruments are robust, owing to the simple
construction of the moving parts.
• The stationary parts of the instruments are also simple.
• Instrument is low cost compared to moving coil
instrument.
• Torque/weight ratio is high, thus less frictional error.

Errors:
• Error due to variation in temperature.
• Error due to friction is quite small as torque-weight ratio
is high in moving coil instruments.
• Stray fields cause relatively low values of magnetizing
force produced by the coil. Efficient magnetic screening is
essential to reduce this effect.
• Error due to variation of frequency causes change of
reactance of the coil and also changes the eddy currents
induced in neighbouring metal.
• Deflecting torque is not exactly proportional to the
square of the current due to non-linear characteristics of
iron material.

MI
• The basic principle of operation is that when a current
carrying conductor is brought in a magnetic field (they should
not be parallel to each other) a torque on the conductor is
produced.
• The Permanent Magnet Moving Coil instrument consists of a
rectangular coil pivoted so that its sides lie in the air gap between
the two poles of a permanent magnet and a soft-iron cylinder.
• The air gap between the magnet poles and iron core is small and
the flux density is uniform and is in a radial direction, so that the
flux lines are always at right angle to the current carrying conductor
and hence when current passes through the coil, a deflecting
torque is produced owing to the interaction between the two
fluxes, one due to permanent magnet and the other due to the
magnetic field of the coil.

Moving Iron instruments
Moving Iron instruments depend for their action
upon the magnetic effect of current, and are
widely used as indicating instruments. In this
type of instrument , the coil is stationery and
the deflection is caused by a soft-iron piece
moving in the field produced by the coil.
There are two types of moving iron instruments:
i) Attraction type
ii) Repulsion type

Attraction type:
Construction:
Fig shows the constructional
details of an attraction
type MI instrument. It
consists of a cylindrical
coil or solenoid which is
kept fixed. An oval-shaped
soft-iron piece. The
controlling torque is
provided by an aluminium
vane, attached to the
spindle, which moves in a
closed air chamber.

Working:
When the instrument is connected in the circuit to
measure current or voltage, the operating current
flowing through the coil sets up a magnetic field. In
other words, the coil behaves like a magnet and
therefore it attracts the soft iron piece towards it. The
result is that the pointer attached to the moving system
moves from zero position. The pointer will come to rest
at a position where deflecting torque is equal to the
controlling torque. If current in the coil is reversed, the
direction of magnetic field also reverses and so does the
magnetism produce in the soft iron piece. Hence, the
direction of the deflecting torque remains unchanged.
For this reason, such instruments can be used for both
d.c. and a.c. measurements.

Repulsion type
 Construction:
Fig shows the constructional details of
repulsion type moving-iron
instrument. It consists of two soft-iron
pieces or vanes surrounded by a fixed
cylindrical hollow coil which carries
the operating current. One of these
vanes is fixed and the other is free to
move. The movable vane is cylindrical
shape and is mounted axially on a
spindle to which a pointer is attached.
The fixed vane, which is wedge-shaped
and has a larger radius, is attached to
the stationery coil. The controlling
torque is provided by one spiral spring
at the top of the instrument. It may be
noted that in this instrument, springs
do not provide the electrical
connections. Damping is provided by
air friction due to the motion of a
piston in an air chamber.

Working:
When current to be measured or current proportional to
the voltage to be measured flows through the coil, a
magnetic field is set up by the coil. This magnetic field
magnetises the two vanes in the same direction i.e.
similar polarities are developed at the same ends of the
vanes. Since the adjacent edges of the vanes are of the
same polarity, the two vanes repel each other. As the
fixed vane cannot move, the movable vane deflects and
causes the pointer to move from zero position. The
pointer will come to rest at a position where deflecting
torque is equal to controlling torque provided by the
spring. If the current in the coil is reversed, the direction
of deflection remains unchanged. It is because reversal of
the field of the coil reverses the magnetisation of both
iron vanes so that they repel each other regardless which
way current flows through the coil. For this reason, such
instruments can be used for both d.c. and a.c.
applications.

o Advantages:
i) Cheap, robust and give reliable service.
ii) Usable in both a.c. and d.c. circuits.
o Disadvantages:
i) Have non-linear scale.
ii) Cannot be calibrate with high degree of precision for d.c.
on account of the affect of hysteresis in the iron vanes.
iii) Deflection up to 240º only may be obtained with this
instrument.
iv) This instrument will always have to be put in the vertical
position if it uses gravity control.
o Errors with MI instruments:
i) Due to hysteresis when used in a.c. and d.c.
ii) Due to stray magnetic fields when used both in a.c. and
d.c.
iii) Due to frequency variation when used in a.c.
iv) Due to waveforms effect when used in a.c.

Applications of MI instruments:
As an ammeter:
It may be constructed for full-scale deflection of 0.1 to 30A
with out the use of shunts or current transformers. To
obtain full-scale deflection with currents less than 0.1A,
it requires a coil with a large number of fine wire turns,
which results in an ammeter with a high impedance.
As an voltmeter:
The MI voltmeter is a fairly low impedance instrument,
typically, 50Ω/V for a 100V instrument. The lowest full
scale is of the order of 50V.The range of the instrument,
when used as a voltmeter, can be extended by using a
high non-inductive resistance R connected in series with
it. This series resistance is known as ‘multiplier’.

90
Ranges of Ammeters and Voltmeters
For a given moving-iron instrument the ampere-turns necessary to produce full-
scale deflection are constant. One can alter the range of ammeters by providing a
shunt coil with the moving coil.
Shunts and Multipliers for MI instruments:
For moving-iron ammeters: For the circuit shown in Fig.42.9, let Rm and Lm are
respectively the resistance and inductance of the coil and Rsh and Lsh the
corresponding values for shunt.

91
The ratio of currents in two parallel branches is
It is difficult to design a shunt with the appropriate inductance, and shunts are rarely
incorporated in moving iron ammeters. Thus the multiple ranges can effectively be obtained by
winding the instrument coil in sections which may be connected in series, parallel or series-
parallel combination which in turn changing the total ampere-turns in the magnetizing coil.

For moving-iron voltmeters:
•Voltmeter range may be altered connecting a resistance in series with the coil. Hence
the same coil winding specification may be employed for a number of ranges.
•Let us consider a high resistance Rse is connected in series with the moving coil and it
is shown below.

93
• DC Ammeter
– is always connected in series
– low internal resistance
– maximum pointer deflection is produced by a very
small current
– For a large currents, the instrument must be
modified by connecting a very low shunt resister
– Extension of Ranges of Ammeter
• Single Shunt Type of Ammeter

94
In most circuits, Ish >> Im
Fig. 1-2 D’Ársonval meter movement
used in ammeter circuit
Basic DC Ammeter Circuit
Ammeter
Where
Rsh = resistance of the shunt
Rm = internal resistance of the meter
movement (resistance of the moving
coil)
Ish = current through the shunt
Im = full-scale deflection current of the
meter movement
I = full-scale deflection current for the
ammeter

96
Example 1-1
Calculate the value of the shunt resistance required
to convert a 1-mA meter movement, with a 100-ohm
internal resistance, into a 0- to 10-mA ammeter.
m
mm
sh
msh
sh
mm
sh
mmshsh
msh
II
RI
R
III
I
RI
R
RIRI
VV







97
Solution:
VmARIV mmm 1.01001 
VVV msh 1.0
mAmAmAIII msh 9110 
 11.11
9
1.0
mA
V
I
V
R
sh
sh
sh

98
Example 4.1: An ammeter as shown in Figure 3-9 has a PMMC
instrument with a coil resistance of Rm = 99 and FSD current
of 0.1 mA. Shunt resistance Rs = 1. Determine the total
current passing through the ammeter at (a) FSD, (b) 0.5 FSD,
and 0.25 FSD

99
Solution
(a) At FSD
mA10
mA0.1mA9.9IIIcurrenttotal
mA9.9
Ω1
mV9.9
R
V
I
VRIand
Ω99mA0.1
RIVvoltagemeter
ms
s
m
s
mss
mmm






(b) At 0.5 FSD
mA5
mA4.95
Ω1
mV4.95
R
V
I
mV4.95Ω99mA0.05RIV
mA0.05mA0.10.5I
ms
s
m
s
mmm
m





(b) At 0.25 FSD
mA2.5
mA2.475
Ω1
mV2.475
R
V
I
mV2.475Ω99mA0.025RIV
mA0.025mA0.10.25I
ms
s
m
s
mmm
m






100
Example 4.2: A PMMC instrument has FSD of 100 A and a coil
resistance of 1 k. Calculate the required shunt resistance value to
convert the instrument into an ammeter with (a) FSD = 100 mA and
(b) FSD = 1 A.
Solution
(a) FSD = 100 mA
Ω1.001
mA99.9
mV100
I
V
R
mA99.9Aμ100mA100III
III
mV100kΩ1Aμ100RIV
s
m
s
ms
ms
mmm




(b) FSD = 1 A
Ω0.1001
mA999.9
mV100
I
V
R
mA999.9Aμ100A1III
mV100RIV
s
m
s
ms
mmm




Measurements lecture 1

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