The document outlines the lesson plan for a lecture on transducers and sensors, focusing primarily on strain gauges. It details various types of strain gauges, their construction, working principles, and factors influencing the choice of transducers. The document also includes objectives for student learning outcomes and quiz questions for assessment.

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Name of the Faculty : B.V.Ramadevi
Designation : Lecturer
Branch : Electrical
Institute : Govt. Polytechnic , Gudur
Year/Semester : III semester
Subject : Electrical Measurements and
Batteries
Code :EE-304
Topic : Transducers and Sensors
Sub topic : Transducers
Duration : 50 minutes
Teaching Aids : PPT
DEPARTMENT OF TECHNICAL EDUCATION
ANDHRA PRADESH

2. RECAP
Q) Name the devices used for measurement of
Temperature
Answer :
Thermistors and Thermocouples
Q) Which type of thermistors are most
commonly used ?
 a) NTC type
 b) PTC type
 c) Both
 d) None 2
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3. Objectives
After completion of this period you should be able to
know about
Strain transducers
Factors influencing the choice of transducers
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4. Strain gauge
Strain serves as an index of pressure
Strain gauge are classified as
Mechanical,
Optical and
Electrical
Strain transfers the from the specimen to the basic gauge
element
Electrical strain gauge measure the changes that occurs in
Resistance,
Inductance and
Capacitance
The most versatile type electrical strain gauge is the
Resistance type.
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5. High accuracy
Reliable data and known performance
Flat frequency response from steady state to several
thousand Hertz
Standardized signal conditioning device
Advantages of using Electrical Strain Gauge:
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6. Experimental stress analysis of machine structures
Construction of force , torque , pressure , flow and
acceleration transducers
Applications of Strain Gauges:
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7. Types of Strain Gauges:
 1. Unbonded resistance wire strain gauge
 2 . Bonded resistance wire strain gauge
 3. Bonded metal foil strain gauge
 4. Bonded semiconductor strain gauge
 5. Vacuum deposited thin metal film strain gauge
 6. Sputter deposited thin metal strain gauge
 7. Diffused metal strain gauge
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8. Strain Gauge Measurement Technique
A metal wire or the foil whose strain is to be measured
is attached to the element
The element deforms as soon as stress is applied.
As strain is a fractional change in resistance reflects
strain of gauge and element
Wheatstone bridge has very high sensitivity for
deflecting very small change in resistance
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9. Unbonded Resistance Strain Gauge
Construction :
It consists of a wire stretched between two points
in an insulating medium such as air
The diameter of the wire is about 25µm
This wire is kept under tension so that there is no
sag and no free vibration
Unbonded strain gauges are usually connected in
a bridge circuit
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10. Working :
apply load
Stress produced
strain changes resistance of one arm of the bridge
bridge gets unbalanced and produces O/p
O/p is proportional to strain
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11. Bonded Resistance Wire Strain gauge
The wires of resistive elements are exposed to air
and is cemented to the base
The base may be a thin sheet of Paper , Bakelite or Teflon
The resistance element may be in the form of wire (zigzag
shape ),foil or film of the material
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Bonded type
strain gauge
Base

12. Metallic Bonded Strain Gauge
Construction:
A fine wire element about 25 µ m or less in diameter is
looped back and forth on a base or mounting plate
The base (carrier ) is connected to the member
subjected to stress.
The grid of line wire is connected on a carrier
Carrier may be sheet of paper , Bakelite , Teflon
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13. Working :
 tensile strength is applied to the structure
 (to which gauge is fixed)
 Wire elongates
 wire length increases and diameter decreases
 resistance increases
 increase in a resistance is function of strain
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14. Gauge Factor
The measurement of sensitivity of a material to strain is
called gauge factor
 It is the change ratio of change in resistance ∆R ⁄ R to
the change in length ∆l ⁄ l
 ∆R ⁄ R
 K =
 ∆l ⁄ l
 where ∆R = change in resistance in Ω
 R = resistance of the gauge without strain in Ω
 ∆l = change in the length in metres
 l = initial length without strain in metres
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15. Strain is defined as the change in length divided
by the original length

 strain ( ) = ∆l ⁄ l
 Then K = {∆R ⁄ R}
Gauge factor K = 2 for most strain gauges
Initial resistance R = 120 Ω
Strain Gauge is normally formed as one arm of the
bridge. The bridge may be a.c. or d.c. actuated
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16. Foil Strain Gauge
Strain is detected by using foil
Nichrome Constantan Isoelastic, Nickel and
Platinum metals are used for the foil and wire
Foil can be etched on the base
Foils are thin(2 mm thickness) and flexible
Resistance value of the foil gauges vary
50 to 1000 Ω
Foil type
Strain gauge
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17. Due to large surface area foil gauges have a much
greater dissipation capacity
Foil strain gauges can be used at a higher
operating temperature range
The characteristic of wire type and foil type strain
gauge are similar
Strain gauge factor is also same for wire type and
foil type strain gauges
These strain gauges can be mounted in remote
and restricted places and especially on curved
surfaces
They can be fabricated on a large scale and in any
shape
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18. Semiconductor Strain Gauge
The basic principle of operation is Piezoresistive
effect
When strain applied, resistivity of the semiconductor
changes corresponding to it resistance changes
( In metallic gauges the change in resistance
is mainly due to dimension strained)
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19. Construction
Germanium and silicon are used as semiconductor
material
Thickness of semiconductor wafer of filaments is
0.05 mm
They are bonded on insulating material Teflon
Gold leads are used for making contacts
These strain gauges are fabricated along with an
IC OP-AMP which act as a pressure transducer
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20. semiconduct
or
Gold leads
Electrodes
Base
Diagram of Semiconductor Strain
Gauge
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21. Semiconductor strain gauges are used when
a very high value of gauge factor is desired
The gauge factor of these are about130+10%
for a unit of 350 Ω
The gauge is stable and can be operated with
conventional indicating and recording systems
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22. Applications of Semiconductor Strain Gauges
Measurement of strain and the associated stress in
experimental stress analysis
Measurement of pressure, acceleration and
Temperature
Investigate strain in many structural materials over a
wide range of environmental conditions
In medical field investigation
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23. Advantages :
High gauge factor
Long life (more than 10 X 106
operations )
Excellent frequency response
Very small in size ranging from 0.7 to 7 mm
Disadvantages:
1. Very sensitive to change in temperature
2. The gauge is more expensive
3. The linearity of the gauge is poor
4. The gauge factor varies with strain
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24. Selection of a transducer
or
Factors influencing the Choice of
Transducer
Selection of an appropriate transducer is an important
decision to be taken for obtaining accurate results
This is because ,it is transducer through which the I/p
signal is transformed to a proportional electrical signal
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25. The following points are to be considered
while selecting a particular transducer
1. The nature of the physical quantity to be measured
 Operating principle
 Operating range
 Sensitivity
 Accuracy
 Electrical parameters
 Transient and frequency response
 Environmental considerations
 Usage and ruggedness
 Errors
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26. 11. Loading effects
12. Stability and Reliability
13. Cost and availability
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27. SUMMARY
In this period , we have discussed about
• Types of strain tranducers
• Factors influencing the choice of tranducers

28. Quiz
Q) What is the most versatile type of Electrical
Transducer
a) Resistance type
b) Inductance type
c) Capacitance type
d) None
Q) Identify the insulating materials used for base
in Strain gauges
a)Paper
b)Bakelite
c)Teflon
d)All the above
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29. Questions
1. What is a Strain Gauge ?
2. List various types of Strain Gauges.
3 . What are the factors to be considered while
selecting the Transducers ?
4. Explain briefly about unbonded and bonded
resistance wire strain gauge.
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