Unit 3 Transducers Engineering (Instrumentation).pdf

Resistive Transducers

1. www.rejinpaul.com

2. Resistive Transducers
Thetransducerwhoseresistancevaries because of the
environmental effects such type oftransduceris known as
theresistive transducer.
Theresistive transduceris used for measuring the
physical quantities like temperature, displacement,
vibration etc.
Working Principle of Resistive TransducerThe resistive
transducer elementworks on the principle that the
resistance of the element is directly proportional to the
length of the conductor and inversely proportional to the
area of the conductor. Where R –resistance in ohms. A –
cross-section area of the conductor in meter square.www.rejinpaul.com

3. POTENTIOMETER
https://youtu.be/v3ag-
bGVt9Iwww.rejinpaul.com

4. Potentiometer & Loading Effect
Potentiometer is also called as pots and it one of the
most commonly used devise formeasuring the
displacement of the body. The potentiometer is the
electrical type of transducer or sensor and it is of
resistive type because it works on the principle of
change of resistance of the wire with its length.
Itconvertseitherlinearorangular
displacementintoanoutputvoltagebymovinga
slidingcontactalongthesurfacewww.rejinpaul.com

5. Potentiometer & Loading Effectwww.rejinpaul.com

6. Construction Of Potentiometer
A resistance potentiometer or a pot consist of resistive element provided with a
s
liding contact.
The sliding contact is called a wiper.
The motion of the sliding contact may be translatory or
rotational.
Some pot uses the combination of two motions i.e. translational as well as
r
otational
Working video: ht
tps://youtu.be/v3ag-bGVt9Iwww.rejinpaul.com

7. Loading Effect in Potentiometer
The output of a potentiometrictransducer is normally
connected to an amplifier or a recorder or a meter
which has a definite input impedance and hence a
current will be drawn by this meter or recorder or
amplifier.www.rejinpaul.com

8. www.rejinpaul.com

9. www.rejinpaul.com

10. Applications
Calibration of voltmeter
Calibration of ammeter
Measurement of resistance
Linear displacement measurement
Liquid level measurements using floats
Rotary displacement measurement
Brightness control
Volume controlwww.rejinpaul.com

11. ADVANTAGE:
They are cheap.
It is easy to use and useful in many applications where
requirements are not severe.
It gives sufficient output that does not require further
amplification.
Potentiometer efficiency is high.
They are useful for the measurement of large
displacement.

The resolution is infinite in cermetand metal film
potentiometers.www.rejinpaul.com

12. DISADVANTAGE:
Wear. Most potentiometers last only a few thousand ro
Noise. In new pots, this noise is inaudible, but it can
get worse with agetationsbefore the materials wear
out
Limited Power. Outofnecessity, most potentiometers
can dissipate only a few wattsofpower at most.
Inertia.Thefriction between the potentiometer’s wiper
and resistive element creates a drag or inertia that the
pot must overcome before it turns.www.rejinpaul.com

13. Excercise
Q. What are the factors while selecting a
potentiometer?
Ans.The important parameters that are important while
selecting a potentiometer are
Humidity
Operating temperature
Contamination and seals
Shock and vibration

lifecyclewww.rejinpaul.com

14. STRAINGAUGE
https://youtu.be/DB
AHlWfBopwwww.rejinpaul.com

15. STRAIN GAUGE
A strain gauge is a device used to measure strain on an
object.
As the object is deformed, the foil is deformed, causing
its electrical resistance to change.
This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.www.rejinpaul.com

16. PRINCIPLE
A strain gage (sometimes referred to as 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 measuredwww.rejinpaul.com

17. Types of Strain Gauge
Bonded and unbonded
metal type
Wire type
Foil type
Frame type
Sheath typewww.rejinpaul.com

18. Bonded type strain gaugewww.rejinpaul.com

19. APPLICATION
Strain Gauges are used for analysingthe dynamic
strains in complex structures like roads, bridges,
buildings, etc.
It is used for measuring tension, force, torque and
stresses in structures.

It is used for the measurement of force by the strain
produced in load rings.www.rejinpaul.com

20. EXERCISE
Q. What are Rosettes?
Ans.The cluster of strain gauges is called the Rosettes. Practically, stress can be applied in
any direction and it is not possible to orient the strain gauges along the direction of
principle stress. So with the help of a combination of strain gauges, it is possible to find
the strain and stress values without actually knowing their directions. It is used for
specific stress analysis or transducer applications.
What is the selection criterion of strain gauge?
Ans.Operating temperature, Stability requirements and Nature of the strain to be detected
are the three primary considerations in strain gauge selection. Also, choosing the right
carrier material, adhesive, grid alloy, and protective coating plays an important role in the
particular application.
Why are strain gauges called the piezoresistivestrain gauges?Ans.The resistance of conductor under strain is also changed due to change in resistivity of
the conductor. This property is known as piezo-resistive effect and hence strain gauges
are also called as piezoresistivestrain gauges.www.rejinpaul.com

21. LOAD CELL
https://www.youtub
e.com/watch?v=bI1V
v4b6mPowww.rejinpaul.com

22. PRINCIPLE
A load cell is a transducer that is used to create an electrical
signal whose magnitude is directly proportional to the
force being measured.
A load cell usually consists of four strain gauges in a
Wheatstone bridge. The change in resistance of the strain
gauge can be utilized to measure strain accurately when
connected to an appropriate measuring circuit
configuration. The electrical signal output is typically very
small in the order of a few millivolts. It is amplified by an
instrumentation amplifier before sending it to the
measurement system.
www.rejinpaul.com

23. CONSTRUCTIONwww.rejinpaul.com

24. ADVANTAGES
Rugged and compact construction
No moving parts
Can be used for static and dynamic loading
Highly Accurate
Wide range of measurement
Can be used for static and dynamic loadingwww.rejinpaul.com

25. DISADVANTAGES
Mounting is difficult.
Calibration is a tedious procedure.
It cannot be used for the pressure measurement of
highly reactive or corrosive
materials because they can damage the gauge.
It requires continuous electric energy for the
production and display of signals.
It cannot be used for the measurement of very high pressure if the diaphragm use is of plasticwww.rejinpaul.com

26. APPLICATIONS
DNA SYNTHESIS
COPTER PEDAL TESTING
BOLT FASTENING
CATHETAR FORCE MEASUREMENTwww.rejinpaul.com

27. Proving Ringwww.rejinpaul.com

28. The proving ring is a device used to measure force. It
consists of an elastic ring of known diameter with a
measuring device located in the center of the ring.

Proving rings can be designed to measure either
compression or tension forces. Some are designed to measure both.www.rejinpaul.com

29. CONSTRUCTION:www.rejinpaul.com

30. WORKING
The basic operation of the proving ring in tension is
the same as in compression. However, tension rings are
provided with threaded bosses and supplied with
pulling rods which are screwed onto the bosses.
Proving rings can be designed to measure either
compression or tension forces. Some are designed to
measure both. The basic operation of the proving ring
in tension is the same as in compression. However,
tension rings are provided with threaded bosses and
supplied with pulling rods which are screwed onto the
bosseswww.rejinpaul.com

31. APPLICATIONS
Proving-ring technology is used to measure thermally
induced displacements in large boreholes in rock

Proving ring is used to measure force with its elastic
ring of know diameterwww.rejinpaul.com

32. MERITS AND DEMERITS
MERITS –
It is easy to handle
It gives accurate values
Can be used in all condition
DEMERITS–
It gives a zero error
Parallel axes error may occurwww.rejinpaul.com

33. RESISTANCE
TEMPERATURE
DETECTOR
https://youtu.be/0
_jRsNcOH0c
https://youtu.be/a
qrvhaV-19wwww.rejinpaul.com

34. Working principle
A resistance temperature detector (RTD) can also be
called a resistance thermometer as the temperature
measurement will be a measure of the output
resistance.
Resistance temperature detectors (RTDs), are sensors
used to measure temperature
The main principle of operation of an RTD is that
when the temperature of an object increases or
decreases, the resistance also increases or decreases
proportionallywww.rejinpaul.com

35. CONSTRUCTION:www.rejinpaul.com

36. Applications-
Air conditioning and refrigeration servicing
Food Processing
Stoves and grills
Textile production
Plastics processing
Petrochemical processing
Micro electronics
Air, gas and liquid temperature measurement
Exhaust gas temperature measurementwww.rejinpaul.com

37. Advantages
Very stable output
Linear and predictable
Easy to verify and recalibrate
High accuracy
No special wire required for installationwww.rejinpaul.com

38. Disadvantages-
More limited temperature range (-200 deg C to 500
deg C)
High initial price

Slower response time than a thermocouplewww.rejinpaul.com

39. EXERCISE
Q. Why is platinum considered most suitable material for RTDs for
most laboratory work and for industrial material measurements
of high accuracy?
Ans.Platinum is the most suitable material for RTDs for most laboratory
work and for industrial measurements of high accuracy because
Platinum has optimum characteristics for service over a wide
temperature range.
Platinum is available in pure form and is stable under varying
environmental conditions.
Platinum’s resistance-temperature curve is simple and holds over a
wide range of temperature (-263 °C to 545 °C) with high precision.
Its resistivity tends to increase less rapidly at a higher temperature
than for the other metals.

Platinum can withstand high temperature while maintaining stability.www.rejinpaul.com

40. THERMISTOR
https://youtu.be/R
5rs78Sp28swww.rejinpaul.com

41. PRINCIPLE
A thermistoris a type of resistor whose resistance strongly
depends on temperature.Theword thermistoris a
combination of word ‘thermal’ and ‘resistor’www.rejinpaul.com

42. Advantages
High sensitivity
Can be used at normal room temperature.
Measurements uptohigh temperatures.
Small in size
Low costwww.rejinpaul.com

43. Disadvantages
High sensitivity allows the thermistorto work at low
temperature range.
Need to be sterilized
Require form of power like batterieswww.rejinpaul.com

44. Applications
Applications include temperature measurements ,
compensation and control.

Used In detection of fire alarms.www.rejinpaul.com

45. EXERCISE:
Q. Why is thermistor well suited to precision temperature measurement, control
and compensation?
Ans.Thermistorsare well suited to precision temperature measurement, control and
compensation because they are rugged in construction, cheap and highly sensitive
devices.
Q. What is the application of PTC thermistors?
Ans.PTC thermistorscan be used as heating elements in small temperature controlled
ovens.
Q. What are the application of NTC type thermistors?
Ans.NTC thermistorscan be used as inrush current limiting devices in power supply
circuits. It is also used for temperature compensation i.e. for cancelling the effect of
temperature on other electronic devices. It is also used for measurement and control of
liquid flow, temperature, gas flow, etc.

Q. Why most of the thermistorshave negative temperature coefficient?
Ans.The reason is that when thermistors temperature is increased, the concentration of
charge carriers increases resulting in a decrease in resistance.www.rejinpaul.com

46. GYROSCOPE
https://www.yo
utube.com/wat
ch?v=ti4HEgd4
Fgowww.rejinpaul.com

47. PRINCIPLE
A gyroscope is a spinning wheel or disc in which the
axis of rotation is free to assume any orientation by
itself. When rotating, the orientation of this axis is
unaffected by tilting or rotation of the mounting,
according to the conservation of angular momentum.
Because of this, gyroscopes are useful for measuring or
maintaining orientationwww.rejinpaul.com

48. WORKING
Gyroscope works on the principle that Angular momentum changes in the direction of Torque.
M=mass of flywheel
W=Mg
N=W=Mg
while flywheel is rotating with angular velocity ωs, in anticlockwise direction. Now the torque due to weight of
flywheel is in positive y direction.
Since angular momentum goes in the direction of torque so flywheel will go to y axis changing direction of Ls
from x axis to y axis.
so, dLs=lW.dt
Where dLs/dtis torque by weight of flywheel.
We can now find the angular frequency of pivot Ω.
ΩLs=lW (balancing torques)
We get
Ω=lWIsωs
where Is ismoment of inertia of flywheel.
That is the reason why rotating gyroscope does not behave like
pendulum when it is in normal gravitational field.
Same concept will be applied when flywheel moves in clockwise direction; now flywheel will move in negative y
direction, as angular momentum points toward pivot.www.rejinpaul.com

49. Advantages and Disadvantages of
Gyroscope
ADVANTAGES :
They really make smaller stabilized system.
They impart greater Stabilization.
They are Accurate and Easy to understand.
`DISADVANTAGES :
The Gyroscopes are really expensive, but not in the terms of
camera stabilization.
They are noisy if you are concerned about sound.
Pan and tilt speed is limited.
They take too much time to get up the speed.

They require another cable, battery and an inverter to work.www.rejinpaul.com

50. Application of Gyroscope
CoriolisForce Gyros
Lateral movement induced by rotation.
TorsionalResonator
Miniature Gyroscope in a chip
Wine glass Gyroscope
Conservation of momentum-Basic Gyroscope and
Basic Rate Gyroscope.

Centripetal Force-Accelerator used for Rotation
Sensing.www.rejinpaul.com

51. HOT WIRE
ANEMOMETER
https://youtu.be/
OrzoTqBvnRUwww.rejinpaul.com

52. HOT WIRE ANEMOMETER
It is used for measurement of velocity of flow
Resistance wire is used as basic sensor which
heated initially by passing an electric current
These resistance wire are mounted on probe is
cooled because of fanning effect

The amount of cooling depends on the velocity variation how velocitywww.rejinpaul.com

53. TYPE OF HOT WIRE ANEMOMETER
1.CONSTANT
TEMPTYPE (CCT)
2.CONSTANT CURRENT TYPE
(CTT)
Principle:
Sensor resistance is kept
constant by servo motor
Advantages:
Easy to use
Accepted standard
Disadvantages:

More complex circuit
Principle:
Current through sensor is
kept constant
Advantages:
High frequency response
Disadvantage:
Difficult to use

Risk of probe burnoutwww.rejinpaul.com

54. CONSTANT CURRENT TYPEwww.rejinpaul.com

55. A fine resistance wire carries fixed current is exposed to the
flow velocity
The wire attains an equilibrium temp when I2r heat
generated in it is just balanced by the convective heat loss
from its surface and it is essentially constant
Therefore the wire temp must adjust itself to change the
convective heat until equilibrium is reached

This wire temp is measure of the flow velocity which is measured in terms of electrical resistancewww.rejinpaul.com

56. CONSTANT TEMPERATURE TYPEwww.rejinpaul.com

57. Current through the wire is adjusted to keep the wire temp
constant .the current required to keep temp constant is a
measure of the velocity
It is most widely used method for measurement of
velocity
Steady flow
It can only be used for steady flow not for large fluctuation in
the velocity of flow
It can be used for measuring both average and fluctuating
component of velocity by balancing the bridge operation
automatically

With zeroflowvelocity the bridge excuitionis shut off
(iw=0),the hot wire assume the fluid tempwww.rejinpaul.com

58. R3 is manually adjusted so that R3>Rwto unbalance the
bridge
If excitation current is turned on unbalanced bridge
produces an unbalanced voltage be which is applied to high
gain current amp
So the current flow through Rwincrease its temp thus its
resistance Rwincreases ,Eedecreases .
Since the amp has limited voltage so voltage cannot
become zero
Frequency response =17000 cps (cycle per second )
Average flow velocity = 9 m/s
30000cps for 30 m/s

50000cps for 90m/swww.rejinpaul.com