Transducers

Transducer

One form Other Form

Transducer
of energy of energy

✘ Example :

Force Bellows LVDT Vo

 Primary and Secondary Transducers
✘ Primary Transducers
✗ If the quantity is in direct contact
with the transducer
✘ Secondary Transducer
✗ If the quantity to me measured is
not in direct contact with the
transducer is called secondary
transducer
 Active and Passive Transducers
✘ If the output signal is self generated by the transducer itself its
called an active transducer
✗ Thermocouple, PV cell, Piezoelectric transducer

✘ If the output electrical signal depends on an external source it is

called passive transducer
✗ R, L, C transducer
 Analog and Digital Transducer
✘ Analog transducers: They convert the input physical
phenomenon into an analog output that is a continuous function
of time. E.g. LVDT.

✘ Digital Transducers: They convert the input physical

phenomenon in discrete steps of electrical output i.e. in the form
of pulses. E.g. Rotary encoders.
 Requirement of the Transducer
✘ Sensitivity must be high
✘ Liner characteristics
✘ Reliable and stable
✘ Immune to atmospheric conditions
✘ Maintenance free
Measurement of
Displacement
Potentiometer
LVDT
Hall pickup
 Potentiometer Transducer – (POT)
Vo

✘ 𝑅𝑖 ∝ 𝑙𝑖
✘ 𝑅𝑜 ∝ 𝑙𝑜 ∝ 𝑥
𝑙𝑜 𝑥
✘ 𝑉𝑜 = 𝑉𝑖 = 𝑉𝑖 x
𝑙𝑖 𝑙𝑖
✘ Sensitivity
✘ Resolution
 Advantages & Disadvantages
✘ High sensitivity ✘ Friction between slidewire
✘ High resolution and contact causes damage

✘ Liner response ✘ The output voltmerer is

required to have high
✘ Low cost
sensitivity
✘ Simple construction
✘ Resistance of slide wire is
✘ Amplifier is not required affected by templarature
 LVDT
✘ LVDT works on the principle of Transformer consisting of single
primary winding and 2 secondary winding connected in series
opposition
 Principle of Operation and Working
✘ As the primary is connected to an AC source so alternating current and voltages
are produced in the secondary of the LVDT. The output in secondary S1 is e1 and
in the secondary S2 is e2. So the differential output is,
 LVDT
✘ Output VS Core Displacement A linear curve shows that output voltage varies linearly
with displacement of core.
 Advantage and Disadvantages
✘ High Sensitivity ✘ Affected by external
✘ Liner response magnetic field
✘ Accurate ✘ Output is affected due to
✘ Frictionless vibration
✘ No need of amplifier
✘ Measures wide range of
displacement in ‘mm’ range
✘ It can be used as primary
and secondary transducer
 Application of LVDT

✘ Overall sensitivity (So )

Vo Vo x
✘ So = = × V/N
F x F
✘ So = Ss × Sp
 Hall effect Transducer
✘ Current careering conductor of
metal or semiconductor is placed in
a magnetic field
✘ The magnetic field induces an
electrical field perpendicular to the
direction of current and Flux
density
✘ This phenomenon is called Hall
Effect
Hall pick-up
Hall pick-up
 Application of Hall Effect Transducer
✘ VH ∝ B
✘ VH ∝ BI
1 1
✘ VH ∝ ∝ = RH
ne p
p = ne = charge density
✘ σ = ne μ = pμ
✘ Semiconductor (n or p)
 Measurement of Displacement
✘ The hall effect transducer placed
between the poles of the
permanent magnet. The magnetic
field strength across the hall effect
element changes by changing the
position of the ferromagnetic field.
Q1 Assertion (A) : Piezoelectric transducers can be used for measurement of both
static and dynamic phenomena.
Reason (R) : Piezoelectric transducers have very good high frequency
response.
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true and R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true
Q2 Which displacement transducer is used for accurate and linear measurement ?
(a) LVDT
(b) Strain gauge
(c) Potentiometer
(d) Capacitive displacement transducer
Q3 Which of the following primary detector type transducers are employed for
displacement measurement?
1. Thermistor
2. Diaphragm
3. Thermocouple
4. Pivot torque
Select the correct answer using the code given below:
(a) 1 and 2 (b) 2 and 4
(c) 3 and 4 (d) 1 and 4
Q4 The principle of hall effect is made use of in the construction of which one of
the following?
(a) Ammeter
(b) Voltmeter
(c) Gaussmeter
(d) Galvanometer
Q5 What is a differential transformer ?
(a) Constant pressure transducer
(b) Variable pressure transducer
(c) Constant displacement transducer
(d) Variable inductance transducer
Q6 Sensitivity of LVDT is mainly due to
(a) Magnetic shielding of the core
(b) Permeability of the core
(c) Exact cancellation of secondary voltages
(d) Insulation used in the winding
Measurement of
Force/stress/wei
ght
 Strain Gauge
✘ A Strain gauge is a sensor whose resistance varies with applied
force; It converts force, pressure, tension, weight, etc., into a
change in electrical resistance which can then be measured.
 Strain Gauge
For F = 0
pl
R= = Vo = 0
A
f≠0 R → R + ∆R
Gauge Factor
Gauge Factor
 Unbonded Strain Gauge

✘ an unbounded strain gauge has a

resistance wire stretched between two
frames. The rigid pins of the two frames
are insulated. When the wire is stretched
due to an applied force, there occurs a
relative motion between the two frames
and thus a strain is produced, causing a
change in resistance value. This change
of resistance value will be equal to the
strain input.
 bonded Strain Gauge
✘ A bonded strain gauge will be either a wire type or a foil type
✘ It is connected to a paper or a thick plastic film support. The measuring leads
are soldered or welded to the gauge wire. The bonded strain gauge with the
paper backing is connected to the elastic member whose strain is to be
measured.
 Gauge Configurations
Biaxial Strain Gauges
✘ When the measurement of strain is to be
done in two directions (mostly at right
angles), this method is used.
✘ The basic structure for this is the two element
90 planar rosette or the 90 planar
shear/stacked foil rosette.
 Gauge Configurations
Three Element Rosettes
✘ It is divided into two types – three element
60delta rosette strain gauge
✘ three element 45planar rectangular rosette.
✘ They are used in applications where both the
magnitude and direction of the applied strains
are to be found out.
 Application
✘ It can be set in three different ways such as
✗ Full bridge
✗ Half bridge
✗ Quarter bridge.
✘ A full bridge will have all four of its gauges active. The half bridge will have two of its gauges
active and thus uses two precise value resistors. The quarter bridge will have only one gauge and
the rest of the resistors will be precise in value.
 Full bridge application
✘ A full bridge circuit is used in applications where
complimentary pair of strain gauges is to be
bounded to the test specimen.
✘ A full bridge circuit is said to be more linear than
other circuits.
✘ An external supply is given to the bridge as shown
in the diagram. Initially, when there is no application
of strain, the output measurement will be zero.
Thus, the bridge is said to be balanced. With the
application of a stress to the device, the bridge will
become unbalanced and produces an output
voltage that is proportional to the input stress.
 quarter bridge application
✘ A quarter bridge output corresponding to
the application of a force is shown
below. Initially, the circuit will be
balanced without the application of any
force. When a downward force is
applied, the length of the strain gauge
increases and thus a change in
resistance occurs. Thus an output is
produced in the bridge corresponding to
the strain
 Piezo electric Transducer
✘ A piezoelectric transducer (also known as a piezoelectric sensor) is a device that
uses the piezoelectric effect to measure changes in acceleration, pressure, strain,
temperature or force by converting this energy into an electrical charge.
 Piezoelectric Effect
✘ A piezoelectric transducer consists of quartz crystal
which is made from silicon and oxygen arranged in
crystalline structure (SiO2). Generally, unit cell (basic
repeating unit) of all crystal is symmetrical.
✘ Piezoelectric crystals are electrically neutral.
✘ The atoms inside them may not be symmetrically
arranged but their electrical charges are balanced
means positive charges cancel out negative charge. The
quartz crystal has the unique property of generating
electrical polarity when mechanical stress applied to it
along a certain plane. Basically, There are two types of
stress. One is compressive stress and the other is
tensile stress.
Parameters
 Applications
✘ As piezoelectric materials cannot measure static values these
are primarily used for measuring surface roughness, in
accelerometers and as a vibration pickup.
✘ They are used in seismographs to measure vibrations in rockets.
✘ In strain gauges to measure force, stress, vibrations etc…
✘ Used by automotive industries to measure detonations in
engines.
✘ These are used in ultrasonic imaging in medical applications.
Electrical Equivalent Circuit
 Practical Arrangement
Total Capacitance
CT = CP + CC + C A
Total Resistance
RT = RP ∥ RA
 Advantages & Disadvantages
1. No need for an external force 1. It is not suitable for measurement
in static condition
2. Easy to handle and use as it has
small dimensions 2. It is affected by temperatures
3. The output is low so some external
3. High-frequency response it means
circuit is attached to it
the parameters change very rapidly
4. It is very difficult to give the desired
shape to this material and also
desired strength
Q1 A strain gauge with a resistance of 250 ohms undergoes a change of 0.150
ohm during a test. The strain is 1.5 × 10–4. Then the gauge factor is
(a) 2.0
(b) 3.0
(c) 4.0
(d) 100
Q2 Which one of the following gives Gauge factor of a strain gauge?
∆𝐿 ∆𝑅
(a) /
𝐿 𝑅

∆𝑅 ∆𝐿
(b) /
𝑅 𝐿

∆𝑅 ∆𝐷
(c) /
𝑅 𝐷

∆𝑅 ∆𝜌
(d) /
𝑅 𝜌
Q3 Why are dummy strain gauges employed ?
(a) For calibration of strain gauges
(b) For increasing the sensitivity of the bridge
(c) For compensation of temperature variations
(d) For neutralizing the influence of bridge voltage supply variations.
Q4 Why is strain gauge bridge sometimes excited with ac?
(a) It has stable performance with ac
(b) Its sensitivity is more with ac
(c) Power frequency pick-up can be avoided with ac
(d) Ac output can be easily amplified
Q5 The strain gauge with a resistance of 250 ohm undergoes a change of 0.15
ohm. During a test the strain is 1.5 × 10–4. what is the gauge factor ?
(a) 4.7
(b) 4.0
(c) 3.5
(d) 2.0
Q6 A strain gauge having a resistance of 500 ohm and a gauge factor 3.0 is
bonded on a member of structure undergoing tensile stress. If the change in
resistance of the gauge is accurately measured as 1.5 ohm, what is the value of
strain suffered by the member ?
(a) 0.01
(b) 0.001
(c) 0.1
(d) 0.003
Measurement of
Pressure
Mechanical Means
 Bourdon tube
✘ The Bourdon pressure gauge operates on
the principle that, when pressurized, a
flattened tube tends to straighten or regain
its circular form in cross-section.
 Diaphragm
✘ Diaphragm pressure gauges are used to measure
gases and liquids.
✘ They cover measuring spans from 10 mbar to 40
bar.
✘ The measuring element consists of one circular
diaphragm clamped between a pair of flanges.
The positive or negative pressure acting on these
diaphragms causes deformation of the measuring
element.
✘ The magnitude of the deformation is proportional
to the pressure to be measured, and it is coupled
to the pointer mechanism.
 Bellow
✘ When pressure is applied to the
closed end bellows will be
compressed.
✘ The closed end will move upwards
and the link, which is the rod in
between the closed end of the
bellows and the transmission
mechanism, will go up and rotate the
pointer.
Measurement of
Pressure
Electrical Means
 Inductive Pressure Transducer
✘ The inductance-type transducer consists of three
parts
✗ Coil
✗ Movable magnetic core
✗ Pressure sensing element
✘ The element is attached to the core, and, as
pressure varies, the element causes the core to
move inside the coil. An AC voltage is applied to the
coil, and, as the core moves, the inductance of the
coil changes.
✘ The current through the coil will increase as the
inductance decreases.
✘ For increased sensitivity, the coil can be separated
into two coils by utilizing a center tap
 Differential Inductive Pressure Transducer
✘ The primary coil is wound around the center of the tube. The
secondary coil is divided with one half wound around each end
of the tube.
✘ Each end is wound in the opposite direction, which causes the
voltages induced to oppose one another. A core, positioned by
a pressure element, is movable within the tube.
✘ When the core is in the lower position, the lower half of the
secondary coil provides the output.
✘ When the core is in the upper position, the upper half of the
secondary coil provides the output.
✘ The magnitude and direction of the output depends on the
amount the core is displaced from its center position. When the
core is in the mid-position, there is no secondary output.
 Capacitance Transducer
✘ A capacitor consists of two parallel conducting plates separated
by a small gap. The capacitance is defined by:

•εr is the dielectric constant of the material between the

plates (this is 1 for a vacuum)
•ε0 is the electric constant (equal to 8.854x1012 F/m),
•A is the area of the plates
•d is the distance between the plates
 Capacitance Transducer
✘ Changing any of the variables will cause a corresponding change in the capacitance. The easiest one
to control is the spacing. This can be done by making one or both of the plates a diaphragm that is
deflected by changes in pressure.
✘ Typically, one electrode is a pressure sensitive diaphragm and the other is fixed. An example of a
capacitive pressure sensor is shown to the right.
✘ An easy way of measuring the change in capacitance is to make it part of a tuned circuit, typically
consisting of the capacitive sensor plus an inductor. This can either change the frequency of an
oscillator or the AC coupling of a resonant circuit.
Photoelectric Transducer
 Photoelectric Transducer
✘ As the pressure increases amount of
light falling on the photo diode
reduces so that current and output
voltage is reduced and this voltage is
calibrated in term of pressure. It is
used for both static & dynamic
measurement.
Q1 Which one of the following transducers can be used for measurement of
pressures as high as 100000 atmosphere?
(a) Mcleod gauge
(b) Pirani gauge
(c) Bridgman gauge
(d) Knudsen gauge
Q2 A 1 cm piezoelectric transducer having a g-coefficient of 58 V/kg/m2 is
subjected to a constant pressure of 10–3 kg/m2 for about 15 minutes. The piezo
voltage developed by the transducer will be
(a) 116 mV
(b) 58 mV
(c) 29 mV
(d) 0 mV
Q3 Match list-I (Parameter) with List-II (Transducer) and select the correct
answer:
List-I List-II
A. Pressure 1. Thermistor
B. Temperature 2. Piezoelectric crystal
C. Displacement 3. Capacitance transducer
D. Stress 4. Ultrasonic waves
5. Ultrasonic waves
Codes:
A B C D
(a) 1 2 5 3
(b) 2 1 4 3
(c) 1 2 5 4
(d) 2 1 3 4
Q4 Match list-I (Transducer) with List-II and select the correct answer using the
codes given below:
List-I List-II
A. Piezo-electric crystal 1. Static pressure measurement
B. Manometer 2. Dynamic force measurement
C. Resistance bridge 3. Study of vibration of a cantilever
D. Resistance strain gauge 4. vibration in position of slider by an
external force changes the resistance
Codes:
A B C D
(a) 4 1 2 3
(b) 4 3 2 1
(c) 2 1 4 3
(d) 2 3 4 1
Q5 Which one of the following transducers is an active transducer?
(a) Piezo-electric pressure transducer
(b) Metallic strain gauge
(c) Semi-conductor strain gauge
(d) Platinum resistance thermometer
Measurement of
Vacuum
 Working Principle of Pirani Gauge
✘ A conducting wire gets heated when electric current flows
through it. The rate at which heat is dissipated from this wire
depends on the conductivity of the surrounding media.
✘ The conductivity of the surrounding media in-turn depends on the
density of the surrounding media (that is, lower pressure of the
surrounding media, lower will be its density). If the density of the
surrounding media is low, its conductivity also will be low causing
the wire to become hotter for a given current flow, and vice versa.
 Pirani Gauge
1. A pirani gauge chamber which encloses a platinum
filament.
2. A compensating cell to minimize variation caused due
to ambient temperature changes.
3. The pirani gauge chamber and the compensating cell
is housed on a wheat stone bridge circuit as shown in
diagram.
 Thermistor Vacuum Gauge
✘ R1 = R o 1 +∝ T1 − R 0
Thermistor Vacuum Gauge
 Thermistor Vacuum Gauge
✘ In Thermistor vacuum gauge the heater element is composed of
metal oxides like manganese, nickel, cobalt & semi-conductor
material.
✘ The sensitivity of the thermistor is very high, it measures up to
10-2 mm of Hg.
 Thermocouple Vacuum Gauge
✘ The thermocouple vacuum gauge consists of the heater element
and a thermocouple. Both the heater element and the
thermocouple are housed in a glass or metal envelope as shown
in figure 1. The thermocouple junction makes contact with the
heater element. The heater element is heated by a constant
current supplied by the source.
Thermocouple Vacuum Gauge
 Mc Leod Gauge
✘ McLeod gauge amplifies the low pressure and was developed to
extend the range of vacuum measurement significantly.
✘ The McLeod Gauge measures the vacuum pressure in the range
between 10-1 and 10-5 torr.
 Mc Leod Gauge
✘ McLeod gauge is essentially a mercury
manometer in which a volume of gas is
compressed before measurement.
✘ It operates by compressing a low-pressure gas
of known volume into a smaller volume so that
its pressure is sufficiently higher enough to be
read. The resultant final volume and pressure
provide the indication of applied low pressure.
 Bridgeman Gauge
✘ When a wire is subjected to pressure from all sides its electrical resistance
changes. This principle can be utilized to obtain a primary type resistive
pressure sensor and is called as a Bridgeman pressure sensor. The distortion
produced in the crystal lattice due to the external pressure causes the change
in resistance. In most common metal wires, the resistance decreases with
increase in pressure, while for antimony, bismuth, lithium, and manganin, it
increases. In cesium, it initially decreases for small values of pressure
changes and reaches a minimum, beyond which it increases with increase in
pressure.
Bridgeman Gauge
Temperature
Measurement
 RTD/ Thermometer
Material Range Relative Cost
Platinum 0.00391 -250 to 900 150
Copper 0.00425 -200 to 150 1
Nickel 0.0066 -70 to 150 10

Copper and platinum produces liner response compared to nickel

 Thermistor
1 1
R1 = R o e β −
T1 T0

±1000 C
= R = 0.5Ω to 0.75 Ω
Sensitivity
dR1 /R1 −β
S= =
dT1 T21
 Thermistor
✘ Thermistor has negative temperature coefficient of resistance
✘ This is used to measure small change in temperature.
✘ The size in mm range & hence to measure the temperature when
RTD & Thermocouple are not suitable.
✘ It has higher sensitivity.
 Thermocouple

Vo = a T1 − T2 + a2 (T1 − T2 )2 +. . . .

Type +ve -ve Temp. Voltage Lead metal Lead metal

metal Metal range swing +ve -ve
K Chromel Alumel -200 to 1300 56 1 rm Cu-Ni
T Copper Constantan -200 to 350 26 Cu Constantan
J 1 rm Constantan -150 to 1000 50 1rm Constantan
S Pt + 10% Rh Pt 0 to 1500 16 Cu Cu – Ni
R Pt = 13% Rh Pt 0 to 1600 19 Cu Cu – Ni
B Pt + 30% Rh Pt + 6% ph 30 to 1800 14 Cu Cu – Ni
E Chromel Constantan 0 to 980 75 1rm Constantan
 Thermocouple

Constantan = 55% Cu + 45% Ni

Chromel = 90% Ni + 10 % Chromium
Alumel = 94% Ni + 3% Mn + 2% Al + 1% Si
Pt = Platinum,
Rh (Rhodium)
 Thermocouple
✘ Thermocouples are normally specified type name.
✘ Thermocouples S, R type used to measure higher temperatures
which produce stable output
✘ It is less affected by atmospheric conditions & oxidation is also
less.
✘ The output swing voltage is very small for these combination so
that sensitivity is minimum.
✘ The type E has highest sensitivity.
 Cold junction Compensation
✘ The cold junction is normally exposed to atmosphere which is
Continuously varying parameter of the temperature so that output
voltage produces error.
✘ For Compensating this cold junction variation
✗ Cold junction is maintained at 0°C by playing the metals in
the ice tub which is not practical

✗ By using an electronic circuit which will produce a

compensating voltage corresponding to changes in cold
junction temperature.
 Thermo electric Laws
1. Law of Intermediate Temperature
✘ Law of Intermediate Metals
 Pyrometer
q0 = 𝜎𝑇4
✘ Materials Used in Pyrometer [1000 °𝐶 to 3500°𝐶]
✗ Quartz
✗ Pyrex
✗ Fluorite
✗ Furred silica
 Optical Pyrometer
✘ An optical pyrometer uses this radiation to
measure the temperature of the object. The
brightness of the measured object is compared
with the brightness of the lamp at a reference
temperature.
✘ The reference temperature is produced by the
lamp whose brightness can be adjusted using
rheostat.
 Infrared/Radiation Pyrometer
✘ The heat energy is transferred through the
optical lens to the mirror. The mirror focuses
this energy on the detector.
✘ The detector may be either thermopile or
photomultiplier tubes. The photomultiplier
tubes are much faster than the thermocouple.
Thus temperature measurement is much
faster when used photomultiplier tubes. The
detector converts the collected heat energy
into an electrical current signal and drives it to
the control unit or temperature display.
 Bimetallic Strip
✘ A bimetallic strip consists of two different materials with different expansion
coefficients that are bonded together. For example, for brass and steel, the
coefficients of linear expansion are:
✗ Brass: 19 x 10-6 /°C Steel: 11 x 10-6 /°C

✘ When this bimetallic strip is heated, the brass expands more than the steel
and the strip curves with the brass on the outside. If the strip is cooled, it
curves with the steel on the outside.
✘ Bimetallic strips are used as switches in thermostats.
Bimetallic Strip

𝜋(+𝑣𝑒)

𝜋 −𝑣𝑒 mathmatically
2t
π=
∝A −∝B T2 − T1
Q1 Consider the following statements in connection with measurement of temperature:
1. A thermistor is highly sensitive as compared with platinum resistance
thermometer
2. The resistance of a thermistor is solely a function of its absolute temperature
whether the source of heat is external, internal or both.
3. A thermistor has linear resistance temperature characteristics
4. Most thermistors exhibits negative resistance temperature coefficient.
Which of these statements are correct?
(a) 1, 2 and 3
(b) 1, 2 and 4
(c) 2, 3 and 4
(d) 1, 3 and 4
Q2 Which curve in the given figure represents resistance temperature characteristics of
a thermistor?

(a) Curve A
(b) Curve B
(c) Curve C
(d) Curve D
Q3 Which one of the following thermocouples has the highest temperature measuring
range?
(a) Copper-Constantan
(b) Iron-Constantan
(c) Alumel-Chromel
(d) Platinum-Rhodium-Platinum
Q4 Thermistors are essentially semiconductors
(a) Well suited to precision measurement of temperature
(b) Widely used in the lower temperature range of - 1000 C to 3000 C
(c) Which behave as resistors with a high negative temperature coefficient of
resistance
(d) All of the above
 Flow
Measurement
Mechanical
Orifice plate
Venturi Tube
Rotameter
 Flow
Measurement
Electrical
 Hot wire ammeter
✘ When a fluid flows over a heated surface temperature will be
reduced because of absorption of heat by fluid.
✘ This change in temperature causes change in resistance of hot
wire which creates unbalance in bridge circuit which is calibrated
in terms of liquid flow.
 Thermistor Flow meter
✘ The hot wire is usually made of platinum or tungsten which have
positive 𝛼, but in thermistor flow meter hot wire is made of
thermistor materials which has negative 𝛼.
✘ Works on the same principle as hot wire but has higher
sensitivity.
 Turbine flow meter
✘ The main purpose of turbine
flowmeter is to measure the
volumetric flow rate through flow
velocity analysis. This principle is
based on tracking multi bladed rotor
rotation speed.
 Electromagnetic Flow meter
✘ Magnetic flow meters works based on
Faraday’s Law of Electromagnetic Induction.
According to this principle, when a conductive
medium passes through a magnetic field B, a
voltage E is generated which is proportional to
the velocity v of the medium, the density of the
magnetic field and the length of the conductor.
Q1 Statement (I): Electromagnetic flow meter is preferred for flow velocity
measurement of slurries in pipes as long as the slurry has adequate electrical
conductivity.
Statement (II): Electromagnetic flow meter does not insert any instrument parts
into the body of the fluid flow to cause obstruction as in most of other flow-meters.
A. Both Statement (I) and Statement (II) are individually true and Statement (II) is
the correct explanation of Statement (I).
B. Both Statement (I) and Statement (II) are individually true but Statement (II) is
not the correct explanation of Statement (I).
C. Statement (I) is true but Statement (II) is false.
D. Statement (1) is false but Statement (II) is true.
Q2 Match List-I (Instrument) with List-II (Measured) and select the correct answer:
List-I List-II
A. McLeod gauge 1. Temperature
B. Turbine meter 2. Pressure
C. Pyrometer 3. Flow
D. Synchros 4. Displacement
Codes:
A B C D
(a) 1 4 2 3
(b) 2 3 1 4
(c) 1 3 2 4
(d) 2 4 1 3
Q3 Measurement of flow, thermal conductivity and liquid level using thermistors make
use of
(a) Resistance decrease with temperature
(b) Resistance increase with temperature
(c) Self-heating phenomenon
(d) Change of resistivity
Q4 Match List-I with List-II and select the correct answer using the code given below the lists:
List-I
A. Variable capacitance device
B. Orifice meter
C. Thermistors
List-II
1. Flow measurement
2. Temperature measurement
3. Pressure transducer
4. Force and torque measurement
Codes:
A B C
(a) 3 1 2
(b) 2 4 3
(c) 2 1 3
(d) 3 4 2
Q5 Consider the following statements:
1. Electromagnetic flowmeter is independent of liquid density.
2. Electromagnetic flowmeter cannot be employed for measuring flow of non-
conducting fluids.
Which of these statements is/are correct?
(a) 1 only
(b) Both 1 and 2
(c) 2 only
(d) Neither 1 nor 2
Angular Speed
 Tacho generator
✘ It works on the principle of relative motion between the magnetic
field and shaft of the coupled device. The relative motion induces
the EMF in the coil which is placed between the constant
magnetic field of the permanent magnet. The develops EMF is
directly proportional to the speed of the shaft.
 DC Tacho generator
✘ The DC tachometer works on the
principle that when the closed
conductor moves in the magnetic field,
EMF induces in the conductor. The
magnitude of the induces emf depends
on the flux link with the conductor and
the speed of the shaft.
AC Tacho generator
 Inductive Reluctance
✘ The flux produced by the magnet is changing due to change in
the reluctance which induces emf in the pick up coil in the form of
pulses applied to digital counter and calibrated in RPM.

No.of pulses/sec
Speed in RPM = N =
no.of teeth
 Photo Electric Tachometer
✘ The tachometer consists the opaque disc which is mounted
on the shaft whose speed needs to be measured.
✘ When the disc rotates their holes and the opaque portion
comes alternatively between the light source and light
sensor.
✘ When the holes come in the line of the light source and the
light sensor, then the light passes through the holes and
collapse to the sensor. Hence the pulse is generated.
These pulses are measured through the electric counter.
 Stroboscope
✘ Speed in RPM
𝐹
𝑁=
𝑛
Stroboscope is counting flash controller so that
the number of flashes can be adjusted until the
marking disc looks stationery. This corresponds to
speed of motor which is to be measured.
It is used widely to measure speeds upto 2000
rpm
Q1 In a stroboscopic method of rotational speed measurement of a machine shaft,
N = the machine shaft speed of rotation of the shaft in revolutions/min
n = No. of points on the circuit pattern
F = No. of flash per min.
The speed of rotation N will be
(a) N = F + n
(b) N = F – n
(c) N = f/n
(d) N = F.n
Q2 Which of the following transducers is classified as an active transducer?
(a) Metallic strain gauge
(b) Capacituve microphone
(c) LVDT
(d) Piezoelectric transducer
Q3 An imperfect capacitor is represented by a capacitance C in parallel with a
resistance R. the value of its dissipation factor tan δ is
(a) ω CR
(b) ω2 CR
(c) 1/ω2 CR
(d) 1/ω CR
Q4 Pair of active transducers is
(a) Thermistor, solar cell
(b) Thermocouple, thermistor
(c) Thermocouple, solar cell
(d) Solar cell, LVDT
Q5 Piezo-electric crystal is generally employed for the measurement of which one
of the following?
(a) Flow
(b) Velocity
(c) Acceleration
(d) Temperature