The document discusses various types of sensors and transducers. It defines sensors as devices that measure physical quantities and produce a corresponding signal, while transducers are elements that experience a related change when subject to some input change. Common physical quantities that can be measured include temperature, pressure, light, current, and weight. Performance characteristics of sensors like range, error, accuracy, sensitivity, hysteresis, nonlinearity, repeatability, and resolution are also described. The document then discusses different types of displacement, position, velocity and motion sensors like potentiometers, strain gauges, capacitive, inductive, Hall effect, incremental encoders and tachogenerators.

1. Chapter 2
Sensors and Transducers
1

2. 2.1 Sensors and
Transducers

Sensors are electronics devices that
measure the physical quantity or
produces a signal relating to the
quantity being measured.

Physical quantities can be
temperature, pressure, light,
current, weight etc.
2

3. 2.1 Sensors and
Transducers


Transducers are defined as
elements that when subject to
some change experience a related
change.
Thus we can say sensors are
transducers, but a measurement
system may use transducers in
addition to the sensors.
3

4. 2.2 Performance
Terminology
The following terms are associated with
the performance of transducers and/or
measurement system as a whole.
 Range of a transducer is the limits with
in which the input can vary.
Thus a load cell having lower limit of 0
KN and higher limit of 50 KN has a
range of 0 to 50 KN.
 Span is the maximum value of the input
minus the minimum value. For the
above load cell the span is 50 KN.

4

5. 2.2 Performance
Terminology

Error is the difference between the
result of the measurement and the true
value of the quantity.
Error = Measured value - True value
e.g. if measurement system gives a
temperature reading of 25ºC when
actual is 24ºC, then the error is +1ºC. If
actual temperature had been 26ºC then
error would have been -1ºC.
5

6. 2.2 Performance
Terminology

Accuracy is the extent to which the
value indicated by a measurement
system might be wrong.
e.g. a temperature measuring
instrument might be specified as having
an accuracy of ±2 ºC. This would mean
that the reading given by the
instrument can be expected to lie with
in + or -2ºC of the true value.
6

7. 2.2 Performance
Terminology
Sensitivity is the relationship indicating
how much output we will get per unit
input i.e. output/input.
e.g. a resistance thermometer may
have a sensitivity of 0.5 Ω/ºC.
 Hysteresis Error Transducers can give
different outputs from the same value
of quantity being measured according
to whether that value has been reached
by a continuously increasing change or
a continuously decreasing change.

7

8. 2.2 Performance
Terminology
Hysteresis Curve is shown in figure
below.
8

9. 2.2 Performance
Terminology

Non-Linearity Error: For many
transducers a linear relationship
between the input and output is
assumed over the working range. But
only few of transducers have a truly
linear relationship and hence errors
occur as a result of the assumption of
linearity. The error is the maximum
difference from straight line.
9

10. 2.2 Performance
Terminology
Fig. Non-Linearity Errors using
a) End Range Values
b) Best Straight Line for all values
c) Best Straight Line through zero point
10

11. 2.2 Performance
Terminology

Repeatability/Reproducibility
This term is used to describe the
ability of a transducer to give the
same output for repeated
applications of the same input
value.
Repeatability = (Max. – Min. Values given)/(Full Range) *
100
11

12. 2.2 Performance
Terminology

Stability
The stability is the ability of a
transducer to give the same output
when used to measure a constant
input over a period of time.
The term drift is often used to
describe the change in output that
occurs over time.
12

13. 2.2 Performance
Terminology

Dead band/time
The dead band or dead space of a
transducer is the range of input
values for which there is no output.
The dead time is the length of time
from the application of an input
until the output begins to respond
and change.
13

14. 2.2 Performance
Terminology

Resolution
The resolution is the smallest
change in the input value that will
produce an observable change in
the output.
14

15. 2.2.1 Static and Dynamic
Characteristics


The Static characteristics are the values
given when steady-state conditions occur,
i.e. the values given when the transducer
has settled down after having received
some input.
The Dynamic characteristics refer to the
behaviour between the time that the input
value changes and the time that the value
given by the transducer settles down to the
steady-state value.
15

16. 2.2.1 Static and Dynamic
Characteristics
1-Response Time
This is the time which elapses after a constant
input is applied to the transducer up to the
point at which the transducer gives an output
corresponding to some specified percentage.
e.g. if a mercury in glass thermometer is put in
hot liquid there can be an appreciable time
lapse, before the thermometer indicates the
actual temperature of the liquid.
16

17. 2.2.1 Static and Dynamic
Characteristics

Time Constant
The time constant is the measure
of the inertia of the sensor and so
how fast it will react to changes in
its input. The bigger the time
constant the slower will be the
reaction of a sensor to a changing
input signal.
17

18. 2.2.1 Static and Dynamic
Characteristics
Rise Time
This is the time taken for the output to rise to some
specified percentage of the steady state output.
Often the rise time refers to the time taken for the
output to rise from 10% of the steady-state value
to 90 or 95% of the steady-state value.
 Settling Time
This is the time taken for the output to settle to
with in some percentage, e.g. 2% of the steadystate value.

18

19. 2.3 Displacement, Position and
Proximity



Displacement sensors are concerned with
the measurement of the amount by which
some object has been moved.
Position sensors are concerned with the
determination of the position of some object
with reference to some reference point.
Proximity sensors are a form of position
sensor and are used to determine when an
object has moved to within some particular
critical distance of the sensor.
19

20. 2.3 Displacement, Position and
Proximity

Following points should be considered in
mind while selecting a displacement,
position or proximity sensor.
1-Size of Displacement.
2-Type of Displacement (linear/angular).
3-Required Resolution.
4-Accuracy Required.
5-Material of the measured object.
6-The Cost.
20

21. 2.3 Displacement, Position and
Proximity



Displacement and Position sensors are
divided into two basic types;
Contact sensors in which the measured
object comes into mechanical contact
with the sensor.
Non-Contacting sensor in which there is
no physical contact between the
measured object and the sensor.
21

22. 2.3.1 Potentiometer
Sensor



A Potentiometer consists of a resistance
element with a sliding contact which can be
moved over the length of the element.
Such element can be used for linear or rotary
displacements, the displacement being
converted into potential difference.
The rotary potentiometer consists of a circular
wire wound track or a film of conductive plastic
over which a rotatable sliding contact can be
rotated.
22

23. 2.3.1 Potentiometer
Sensor
23

24. 2.3.1 Potentiometer
Sensor


With the constant input voltage Vs, between
terminal 1 and 3, the output voltage Vo
between terminal 2 and 3 is a fraction of the
input voltage.
This fraction depends upon the ratio of the
resistance R23 between terminal 2 and 3
compared with the total resistance R13
between terminal 1 and 3. i.e.
Vo/Vs = R23/R13
24

25. 2.3.2 Strain-gauged
Element


The electrical resistance strain gauge is a metal
wire, metal foil strip, or a strip of semiconductor
material which is wafer like and can be struck in
to surfaces like a postage stamp.
When it is subjected to strain, its resistance R
changes, the fractional change in resistance
ΔR/R = Gε
where G, is the constant of proportionality and it
is termed as gauge factor.
25

26. 2.3.2 Strain-gauged
Element
26

27. 2.3.2 Strain-gauged
Element


Since strain is the ratio (change is length/
original length) then the resistance change
of the strain gauge is a measurement of
the change in length of the element to
which the strain gauge is attached.
A problem with all strain gauges is that
their resistance not only changes with
strain but also with temperature. So to get
an accurate result various ways of
temperature elimination are used.
27

28. 2.3.3 Capacitive Element

The capacitance C of a parallel plate
capacitor is given by;
C = (εr.εo. A)/d
where, εr is the relative permittivity of
the dielectric between the plates, εo is
the permittivity of free space, A the
area of overlap between the two plates
and d the plate separation. Capacitive
sensors used to measure linear
displacements are shown in next slide.
28

29. 2.3.3 Capacitive Element

Capacitor a) is used to measure
displacement by plate separation d.
 Capacitor b) is used to measure
displacement by overlap area A.
 Capacitor c) is used to measure
displacement by dielectric motion.
29

30. 2.3.3 Capacitive Element

For the displacement changing the
plate separation, if the separation d is
increased by displacement x then the
capacitance becomes;
C- ΔC = (εr.εo. A)/(d+x)
Change in capacitance as a fraction of
the initial capacitance is given by;
ΔC/C = - (x/d)/[1+(x/d)]
30

31. 2.3.4 Differential
Transformers



The Linear Variable Differential Transformer
(LVDT) consists of three coils symmetrically
spaced along an insulated tube.
The central coil is the primary coil and the other
two are identical secondary coils which are
connected in series in such away that their
outputs oppose each other.
A magnetic core is moved through the central
tube as a result of the displacement being
monitored.
31

32. 2.3.4 Differential
Transformers
When there is an
alternating voltage input
to the primary coil, alternating
e.m.fs are induced in the secondary coil.
 With the magnetic core central, the amount of
magnetic material in each of the secondary
coils is the same.
 But when the core is displaced from the
central position there is a greater amount of
magnetic core in one coil than the other, e.g.
more in secondary coil2 than coil 1.

32

33. 2.3.4 Differential
Transformers


The result is that a greater e.m.f is induced
in one coil than the other. There is then a
net output from the two coils.
Since a greater displacement means even
more core in one coil than the other, the
output, the difference between the two
e.m.fs increases the greater the
displacement being monitored.
33

34. 2.3.4 Differential
Transformers



LVDTs have operating ranges from about ±2mm to
±400mm with non-linearity errors of about ±0.25%.
LVDTs are very widely used as primary transducers
for monitoring displacements. The free end of the
core may be spring loaded for contact with the
surface being monitored, or threaded for mechanical
connection.
They are also used as secondary transducers in the
measurement of force, weight and pressure; these
variables are transformed in to displacements
which can be monitored by LVDT’s.
34

35. 2.3.5 Eddy Current Proximity
Sensor


If a coil is supplied with an alternating
current, an alternating magnetic field is
produced. If there is a metal object in close
proximity to this alternating magnetic field,
then eddy currents are induced in it.
The eddy currents themselves produce a
magnetic field. This distorts the magnetic
field responsible for their production.
35

36. 2.3.5 Eddy Current Proximity
Sensors


As a result, the impedance of the coil
changes and so the amplitude of the
alternating current. At some preset level,
this change can be used to trigger a switch.
This type of sensor is used for detection of
non-magnetic but conductive materials.
They are inexpensive, small in size, highly
reliable and are very sensitive to small
displacements.
36

37. 2.3.6 Inductive proximity
Switch


This consists of a coil wound round
a core. When the end of the coil is
close to a metal object its
inductance changes. This change
can be used to trigger a switch.
It is used for detection of metal
objects and is best with ferrous
metals.
37

38. 2.3.8 Pneumatic Sensors


Pneumatic sensors involve the use of
compressed air, displacement or the
proximity of an object being transformed in
to a change in air pressure.
Low pressure air is allowed to escape
through a port in the front of the sensor.
This escaping air in the absence of any
close by object, escapes and in doing so
also reduces the pressure in the nearby
sensor output port.
38

39. 2.3.8 Pneumatic Sensors
 But if there is a close by object, the air cannot so readily
escape and the result is that the pressure increases in the
sensor output port. The output pressure from the sensor thus
depends on the proximity of objects.
 Typically 3-12mm displacements can be measured by this
sensor.
39

40. 2.3.9 proximity Switches




There are many forms of switches which are
activated by the presence of an object, to give
an output to sensor which is either on or off.
Microswitch is a small electrical switch which
requires physical contact and a small
operating force to close the contacts.
On a conveyor belt presence of an item is
determined by the weight on the belt.
Lever operated, Roller Operated and Cam
Operated switches are examples of Proximity
Microswitches.
40

41. 2.3.9 Proximity Switches


Reed Switch consists of two magnetic switch
contacts sealed in a glass tube.
When a magnet is brought close to the switch,
the magnetic reeds are attracted to each other
and close the switch contacts.
41

42. 2.3.9 Proximity Switches

Photosensitive devices can be used to detect
the presence of an opaque object by it
breaking a beam of light, or infrared radiation,
falling on
such a device or by detecting
the light reflected back by
the object.
42

43. 2.3.10 Hall Effect Sensors
Home Work for Students
43

44. 2.4 Velocity and Motion
These sensors are used to monitor
linear and angular velocities and
detect motion.
 The following are the main types of
these sensors;
1- Incremental Encoders
2- Tachogenerator
3- Pyroelectric Sensors.

44

45. 2.4.1 Incremental
Encoders

The incremental encoders are used
to determine angular velocity by
measuring number of pulses
produced per second.
45

46. 2.4.1 Incremental
Encoders



A beam of light passes through slots in a disc
and is detected by a suitable light sensor.
When the disc is rotated, a pulsed output is
produced by the sensor with the number of
pulses being proportional to the angle
through which the disc rotates.
Hence rotation of disc can be obtained by
number of pulses produced.
46

47. 2.4.2 Tachogenerator



A Tachogenerator is used to measure
angular velocity. Variable Reluctance
Tachogenerator is most commonly used
form of tachogenerator.
It consists of a toothed wheel of
ferromagnetic material which is
attached to the rotating shaft.
A pick-up coil is wound on a permanent
magnet.
47

48. 2.4.2 Tachogenerator
Variable Reluctance Tachogenerator:
48

49. 2.4.2 Tachogenerator



As the wheel rotates, so the teeth move
past the coil and the air gap between
the coil and ferromagnetic material
changes.
Thus we have a magnetic circuit with an
air gap which periodically changes.
As a result flux linked by a pick-up coil
changes which in turn produces an
alternating e.m.f. in the coil.
49

50. 2.4.2 Tachogenerator

If wheel contains ‘n’ teeth and rotates
with an angular velocity ω, then the flux
change with time for the coil is given as;
Ф= Ф̥ + Фa cos nωt
where Ф̥ is the mean value of flux and
Фa the amplitude of the flux variation.
The induced e.m.f. e in the N turns of
the pick-up coil is thus:
50

51. 2.4.2 Tachogenerator
e= -N d(Ф)/dt
=-N d/dt (Ф̥ + Фa cos nωt)
= N Фa n ω sin nωt
We can write;
e= Emax sin ωt
where the maximum value of the
induced e.m.f. Emax is NФanω and so it
is the measure of the angular velocity.
51

52. 2.4.3 Pyroelectric Sensor
Pyroelectric Materials are crystalline
materials which produce charge in response
to heat flow.
 When such a material is heated to a
temperature just below the Curie
temperature in an electric field and material
cooled while remaining in the field, electric
dipoles with in the material line up and it
becomes polarised.

52

53. 2.4.3 Pyroelectric Sensor
Curie Temperature:
53

54. 2.4.3 Pyroelectric Sensor



When the field is removed the
material retains its polarisation.
When the pyroelectric material is
exposed to infrared radiation, its
temperature rises and this reduces
the amount of polarisation in the
material.
Hence the dipoles being shaken up
more and losing their alignment.
54

55. 2.4.3 Pyroelectric Sensor

Pyroelectric sensor consists of a
polarised pyroelectric crystal with
thin metal film electrodes on
opposite faces.
55

56. 2.4.3 Pyroelectric Sensor


Because the crystal is polarised with
charged surfaces, ions are drawn from the
surrounding air and electrons from any
measurement circuit connected to the
sensor to balance the surface charge.
If than infrared radiation is incident on the
crystal and changes its temperature, the
polarisation in the crystal is reduced thus
results in charge reduction on the crystal
surface.
56

57. 2.4.3 Pyroelectric Sensor


The excess charge leaks away through
measurement circuit until the charge on
the crystal once again is balanced by
the charge on the electrodes.
The pyroelectric sensor thus behaves as
a charge generator which generates
charge when there is a change in its
temperature. The relationship between
change in charge Δq is proportional to
the change in temperature Δt;
Δq = kp Δt (Kp = sensitivity constant)
57

58. 2.5 Force


A spring balance is an example of
force sensor in which a force, a
weight, is applied to the scale pan.
This causes a displacement, i.e.
the spring stretches. This
displacement is then a measure of
the force.
58

59. 2.5.1 Strain Gauge Load
Cell



The most widely used form of forcemeasuring transducer is based on the use of
electrical resistance strain gauges.
These are used to monitor the strain
produced in some member when stretched,
compressed or bent by the application of the
force.
This arrangement is generally known as
Load Cell.
59

60. 2.5.1 Strain Gauge Load
Cell
Load Cell:
60

61. 2.5.1 Strain Gauge Load
Cell




This is a cylindrical tube to which strain
gauges have been attached.
When forces are applied to the cylinder to
compress it, then strain gauges give a
resistance change.
This resistance is the measure of the strain
and hence applied forces can be
determined from it.
A signal conditioning circuit is used to
eliminate the effect of temperature because
temperature has an effect on resistance.
61

62. 2.6 Fluid Pressure


Fluid pressure in industrial applications
can be measured by monitoring the
elastic deformation of diaphragms,
capsules, bellows and tubes.
The type of pressure measurements that
can be required are, Absolute Pressure
(where pressure is measured relative to
zero-pressure), Differential Pressure
(where a pressure difference is
measured) and Gauge Pressure (where
the pressure is measured relative to
barometric pressure).
62

63. 2.6 Fluid Pressure



In a diaphragm, when there is a
difference in pressure than the centre of
diaphragm becomes displaced/bends.
This form of movement can be
monitored by some form of
displacement sensors e.g. strain gauge.
Corrugation in diaphragm results in
greater sensitivity.
63

64. 2.6 Fluid Pressure

Diaphragms a) Flat b) Corrugated
64

65. 2.6 Fluid Pressure




Capsules can be considered to be just two
corrugated diaphragms combined and give
greater sensitivity.
Bellow is a stake of capsules and even more
sensitive.
A bellow can be combined with a LVDT to
give a pressure sensor with an electrical
output.
Capsules and Bellows are made of materials
such as Stainless Steel, Phosphor Bronze,
Nickel, with rubber and nylon.
65

66. 2.6 Fluid Pressure
Capsules, Bellows and LVDT with Bellows:
66

67. 2.6 Fluid Pressure




A different form of deformation is obtained
using a tube with an elliptical cross-section.
Increasing the pressure in the tube causes
it to tend to more circular cross-section.
When such a tube is in the form of Cshaped tube it is known as Bourdon Tube.
The C opens to some extent when the
pressure in the tube increases.
These are made up of materials as stainless
steel and phosphor bronze.
67

68. 2.6 Fluid Pressure

Tube pressure sensors:
68

69. 2.6.1 Piezoelectrical
Sensors



Piezoelectric materials are those which when
stretched or compressed generate electric
charges with one face of the material becoming
positively charged and the opposite face
negatively charged.
As a result voltage is produced.
During stretching or compressing charge
distribution in the crystal takes place so that
there is a net displacement of charge.
69

70. 2.6.1 Piezoelectrical
Sensors

The net charge q on a surface is proportional
to the amount x by which the charges have
been displaced, and since the displacement
is proportional to the applied force F;
q = kx = SF
Where k is a constant and S a constant
termed the charge sensitivity and it depends
upon the material and orientation of its
crystals.
70

71. 2.6.1 Piezoelectrical
Sensors

Piezoelectric Sensors
71

72. 2.6.1 Piezoelectrical
Sensors

Metal electrodes are deposited on
opposite faces of the piezoelectric
crystal. The capacitance C of the
piezoelectric material between the
plates is;
C = (εo εr A)/t
where εr is the relative permittivity of
the material, A is area and t its
thickness.
72

73. 2.6.2 Tactile Sensor




Tactile sensor is a particular form of pressure sensor and
used on the finger tips of robots to determine contact of
hand with object.
They are also used for touch display screens where a
physical contact has to be sensed.
One form of tactile sensor uses piezoelectric
polyvinylidene fluoride (PVDF) film.
Two layers of film are used and are separated by a soft
film which transmit vibrations.
73

74. 2.6.2 Tactile Sensors




The lower PVDF film has an alternating
voltage applied to it and this results in
mechanical oscillations of the film.
The intermediate film transmit these
vibrations to the upper PVDF film.
These vibrations cause an alternating
voltage to be produced across the upper
film.
When pressure is applied to the upper PVDF
film its vibrations are effected and the
output alternating voltage is changed.
74

75. 2.6.2 Tactile Sensors
Tactile Sensor
75

76. 2.7 Liquid Flow



The traditional methods of measuring the flow
rate of liquids involves devices based on the
measurement of pressure drop occurring
when a liquid flows through a constriction.
For a horizontal tube, where v1 is the fluid
velocity, P1the pressure and A1 the crosssectional area of the tube prior to the
constriction.
v2 the velocity, P2 the pressure and A2 the
cross-section area at the constriction, ρ the
fluid density. Then Bernoulli’s equation gives;
76

77. 2.7 Liquid Flow
77

78. 2.7 Liquid Flow


Since mass of liquid passing per second
through the tube prior to the
constriction, we have A1v1ρ = A2v2ρ.
The quantity Q of liquid passing through
the tube per second is A1v1=A2v2,
hence
78

79. 2.7 Liquid Flow


Thus it is seen that quantity of fluid
flowing through the pipe per
second is proportional to
√(pressure difference).
Measurements of pressure
difference can thus be used to give
a measure of the rate of flow.
79

80. 2.7.1 Orifice Plate



The Orifice plate is simply a disc, with a
central hole, which is placed in the tube
through which the fluid is flowing.
The pressure difference is measured
between a point equal to the diameter
of the tube upstream and a point equal
to half the diameter downstream.
It is cheap, simple with no moving parts
but does not work well with slurries.
80

81. 2.7.1 Orifice Plate
81

82. 2.7.2 Turbine Meter



The turbine flow meter consists of a multibladed rotor that is supported centrally in
the pipe along which the flow occurs.
The fluid flow results in rotation of the rotor,
the angular velocity being proportional to
the flow rate.
The rate of revolution of the motor can be
determined by using a magnetic pickup.
82

83. 2.7.2 Turbine Meter

The pulses are counted and so the
number of revolutions of the rotor
can be determined.
83

84. 2.8 Liquid Level


The level of liquid in a vessel can be
measured directly by monitoring the
position of the liquid surface or indirectly by
measuring some variable related to the
height.
Direct methods involve floats while indirect
methods include the monitoring of the
weight of the vessel by load cells.
84

85. 2.8.1 Floats



A direct method of monitoring the level
of liquid in a vessel is by monitoring the
movement of a float.
The displacement of the float causes a
level arm to rotate and so move a slider
across a potentiometer.
The result is an output of a voltage
related to the height of liquid.
85

86. 2.8.1 Floats

Other forms of this involve the
lever causing the core in a LVDT to
displace, or stretch or compress a
strain gauged element.
86

87. 2.8.2 Differential Pressure



Two basic types of instruments are used for
measurement of differential pressure.
In figure 2.48 (a), the differential pressure cell
determines the pressure difference between the
liquid at the base of the vessel and atmospheric
pressure. The vessel is being open to the
atmospheric pressure.
In figure 2.48 (b) the differential pressure cell
monitors the difference in pressure between the
base of the vessel and the air or gas above the
surface of the liquid.
87

88. 2.8.2 Differential Pressure
88

89. 2.9 Temperature


Temperature can be measured by
changes it causes in the form of
expansion or contraction of gases,
liquids or solids.
The change in electrical resistance
of conductors, semiconductors and
thermoelectric e.m.f.s.
89

90. 2.9.1 Bimetallic Strips



Bimetallic Strips consists of two different
metal strips bonded together.
The metals have different coefficients of
expansion and when temperature changes the
composite strip bends into a curved strip, with
the higher coefficient metal on the outside of
the curve.
This deformation may be used as a
temperature-controlled switch, meaning that
the switch contacts close at a different
temperature from that at which they open.
90

91. 2.9.1 Bimetallic Strips
91

92. 2.9.2 Resistance
Temperature Detectors
(RTDs)

The resistance of most metals
increases, over a limited temperature
range, in a reasonably linear way with
temperature. The relationship is as;
Rt = R0 (1 + at)
Where Rt is the resistance at
temperature t °C, R0 is resistance at
temperature 0 °C and a is temperature
coefficient of resistance.
92

93. 2.9.2 Resistance
Temperature Detectors
(RTDs)

Resistance Temperature Detectors are simple
resistive elements in the form of coils of wire
of metals as platinum, nickel or nickel-copper
alloys.
93

94. 2.9.3 Thermistors





These are small pieces of material made from
mixtures of metal oxides, such as those of
chromium, cobalt, iron, manganese and nickel.
These metal oxides are semiconductors.
The material is formed into various forms of
elements such as beads, discs and rods.
The resistance of conventional metal oxide
thermistors decreases in a very non-linear way
with an increase in temperature.
These thermistors have negative temperature
coefficients (NTC).
94

95. 2.9.3 Thermistors
95

96. 2.9.3 Thermistors
The change in resistance per degree change in
temperature is considerably larger than that
which occurs with metals. The equation for
resistance-temperature for a thermistor can be
written as;
β/t
Rt = K (e)
Where Rt is resistance at temperature t, with K
and β being constants.
 They are small in size and hence respond very
rapidly to changes in temperature. They give
very large changes in resistance per degree
change in temperature.

96

97. 2.9.5 Thermocouples



If two different metals are joined
together, a potential difference occurs
across the junction.
The potential difference depends upon
the metals used and the temperature of
the junction.
A thermocouple is a complete circuit
involving two such junctions. If both the
junctions are at same temperature than
there is no net e.m.f.
97

98. 2.9.5 Thermocouples
98

99. 2.9.5 Thermocouples


The value of this e.m.f E depends on the two metals
concerned and the temperature t of both junctions.
Usually one junction is held at 0°C and then to a
reasonable extent, the following relationship holds;
E = at + b (t)²
Where a and b are constants for metals concerned. Table
2.1 shows commonly used thermocouples with
temperature ranges and sensitivities. Figure 2.55 shows
thermoelectric e.m.f – temperature graphs of these
thermocouples.
99

100. 2.9.5 Thermocouples
100

101. 2.10 Light Sensors



Photodiodes are semiconductor junction
diodes which are connected in a circuit in
reverse bias, giving a very high resistance.
So when light falls on the junction the diode
resistance drops and current in the circuit
rises appreciably.
A photodiode can thus be used as a variable
resistance device controlled by the light
incident on it. These have a very fast
response to light.
101

102. 2.10 Light Sensors




Phototransistors have a light-sensitive
collector-base p-n junction.
When there is no incident light there is
a very small collector-to-emitter
current.
When light is incident, a base current is
produced that is directly proportional to
the light intensity.
This leads to the production of a
collector current which is then a
measure of the light intensity.
102

103. 2.10 Light Sensors


Photoresistor has a resistance
which depends on the intensity of
the light falling on it, decreasing
linearly as the intensity increases.
The cadmium sulphide
photoresistor is most responsive to
light.
103

104. 2.11 Selection of sensors
Following factors needs to be considered
while selecting a sensor.
1- The nature of the measurement
required.
2- The nature of the output required.
3- Then possible sensors can be identified
taking into account such factors as range,
accuracy, speed of response, reliability,
availability, cost.

104

105. 2.12 Inputting data by
switches
Home Work for Students
105