MIT Art Design and Technology University

MIT School of Computing, Pune

Department of Electrical and Electronics

Engineering

Subject - Electrical & Electronics

Engineering

NOTES: Unit V

Class – F.Y. (SEM-I)

2024-25
 Electrical and Electronics Engineering

UNIT V
TRANSDUCERS
Main Topic-1: Classification, Selection criteria, Sources of error for parameter under
measurement, Transducer specifications
Main Topic-2: Temperature transducer, Linear variable differential transducer, Strain gauge.
Main Topic-3: Various applications of transducers.

The transducer may be defined as any device that convert the energy from one form to another,
most of the transducers either convert electrical energy in to mechanical displacement and
convert some non-electrical physical quantities like temperature, Light, Pressure, Force, Sound
etc to an electrical signal. In an electronics instrument system, the function of transducers is of
two types.
1. To detect or sense the pressure, magnitude and change in physical quantity being
measured.
2. To produce a proportional electrical signal.

Classification of Transducers:
The Classification of Transducers is done in many ways. Some of the criteria for the classification
are based on their area of application, Method of energy conversion, Nature of output signal,
According to Electrical principles involved, Electrical parameter used, principle of operation, &
Typical applications.
The transducers can be classified broadly
• On the basis of transduction Principle used
• Primary and secondary transducers
• Active and passive transducers
• Transducers and inverse transducers.
• Analog and digital transducers

Classification on the Basis of Transduction Principle Used

This classification is done depending on the transduction principle i.e., how the input variable is
being converted into capacitance, resistance and inductance values. (These are named as
capacitive transducer, resistive transducer and inductive transducer respectively).
Examples of Capacitive Transducer:
1. Dielectric gauge: It is used to measure, (i) Thickness and (ii) Liquid level

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2. Capacitor Microphone: It is used to measure, (i) Noise (ii) Speech and Music

Examples of Resistive Transducer:

1. Resistance thermometer: Used in the measurement or, (i) Temperature and (ii) Radiant
heat.
2. Potentiometer device: (i) Used in displacement measurement and (ii) Used in pressure
measurement.

Examples of Inductive Transducer:

1. Reluctance pick up: It is used to measure, (i) Pressure (ii) Vibrations (iii) Position and (iv)
Displacement.
2. Magnetostriction gauge.: It is used to measure, (i) Sound (ii) Force (iii) Pressure.

Primary and Secondary Transducers:

• Transducers, on the basis of methods of applications, may be classified into primary and
secondary transducers.
• When the input signal is directly sensed by the transducer and physical phenomenon is
converted into the electrical form directly then such a transducer is called the primary
transducer. For example, consider a thermistor used for the measurement of temperature
fall in this category. The thermistor senses the temperature directly and causes the
change in resistance with the change in temperature.
• When the input signal is sensed first by some detector or sensor and then its output being
of some form other than input signals is given as input to a transducer for conversion into
electrical form, then such a transducer falls in the category of secondary transducers.
• For example, in case of pressure measurement, bourdon tube is a primary sensor which
converts pressure first into displacement, and then the displacement is converted into an
output voltage by an LVDT. In this case LVDT is secondary transducer.

Examples of Primary Transducer:

1. Bourdon tube: Used in pressure
2. Strain gauge: Used in measurements

Examples of Primary Transducer:

1. LVDT: Used to measure, (i) Displacement J (ii) Force (iii) Pressure and (iv) Position

Active and Passive Transducers

• Transducers, on the basis of methods of energy conversion used, may be classified into
active and passive transducers.

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• Self-generating type transducers i.e. the transducers, which develop their output in the
form of electrical voltage or current without any auxiliary source, are called the active
transducers. Such transducers draw energy from the system under measurement.
Normally such transducers give very small output and, therefore, use of amplifier becomes
essential.
• Transducers, in which electrical parameters i.e. resistance, inductance or capacitance
changes with the change in input signal, are called the passive transducers. These
transducers require external power source for energy conversion. In such transducer
electrical parameters i.e. resistance, inductance or capacitance causes a change in
voltages current or frequency of the external power source. These transducers may draw
source energy from the system under measurement. Resistive, inductive and capacitive
transducer falls in this category.

Examples of Active Transducer:

1. Photo voltaic cell: Used in light meters and in solar cells.
2. Thermocouple: Used to measure, (i) Temperature (ii) Radiation and (iii) Heat flow.

Examples of Passive Transducer:

1. Capacitive transducers.
2. Resistive transducers.
3. Inductive transducers

Analog and digital transducers

• Analog transducer is the transducer which provides output signal in analog form (of
voltage or current) i.e., a continuous function of time in response of input quantity to be
measured.
• Digital transducer is the transducer which provides output electrical signal in digital form
i.e., discrete signal in response of input quantity to be measured. Here Output is in the
form of square pulses and having two states (high and low); hence it is called a digital
transducer.

Examples of Analog Transducer:

1. Strain gauge: Used to measure, (i) Displacement (ii) Force and (iii) Torque.
2. Thermistor: Used to measure, (i) Temperature and (ii) Flow.

Examples of Digital Transducer:

1. Turbine meter: Used in flow measurement.

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Transducers and Inverse Transducers

• Transducer, as already defined, is a device that converts a non-electrical quantity into an

electrical quantity. Normally a transducer and associated circuit have a non-electrical input
and an electrical output, for example a thermo- couple, photoconductive cell, pressure
gauge, strain gauge etc.
• An inverse transducer is a device that converts an electrical quantity into a non-electrical
quantity. It is a precision actuator having an electrical input and a low-power non-electrical
output. For examples a piezoelectric crystal and transnational and angular moving-coil
elements can be employed as inverse transducers. Many data- indicating and recording
devices are basically inverse transducers. An ammeter or voltmeter converts electric
current into mechanical movement and the characteristics of such an instrument placed
at the output of a measuring system are important. A most useful application of inverse
transducers is in feedback measuring systems.

Selection criteria of a Transducer

In a measurement system the transducer is the input element with the critical function of
transforming some physical quantity to a proportional electrical signal. The following is the
summary of the factors influencing the choice of a transducer for measurement of a physical
quantity:
1. Operating principle: The transducer are many times selected based on the operating
principle used by them. The operating principles used may be resistive, inductive, capacitive,
optoelectronic, piezoelectric, etc.
2. Sensitivity: It describes the smallest absolute amount of change that can be detected by a
measurement.
3. Operating Range: The transducer should maintain the range requirements and have a good
resolution over its entire range.
4. Accuracy: The degree of closeness of the measured value to a standard or true value. The
ability of an instrument to measure the accurate value is known as accuracy.
5. Errors: The transducer should maintain the expected input-out relationship as described by
its transfer function so as to avoid errors.
6. Transient and Frequency Response: The transducer should meet desired time domain
specifications like peak overshoot, rise time, settling time and small dynamic error. It should
ideally have a flat frequency response curve. In practice, however, there will be cutoff
frequencies and higher cut off frequency should he high in order to have a wide bandwidth.
7. Environmental Compatibility: It should be assured that the transducer selected to work

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under specified environmental conditions maintains its input/ output relationship and does not
break down. For example, the transducer should remain operable under its temperature
range. It should be able to work in corrosive environments, should be able to withstand
pressures and shocks and other interactions to which it is subjected to.
8. Insensitivity to Unwanted Signals: The transducer should be minimally sensitive to
unwanted signals and highly sensitive to desired signals.
9. Stability and Reliability: The transducers should exhibit a high degree of stability during its
operation and storage life. Reliability should be assured in case of failure of transducer in
order that the functioning of the instrumentation system continues unaffected.

Types of Measurement Errors

We can classify the measurement errors into the following three types.
• Gross Errors
• Random Errors
• Systematic Errors
Now, let us discuss about these three types of measurement errors one by one.

Gross Errors
The errors, which occur due to the lack of experience of the observer while taking the
measurement values are known as gross errors. The values of gross errors will vary from
observer to observer. Sometimes, the gross errors may also occur due to improper selection of
the instrument. We can minimize the gross errors by following these two steps.

• Choose the best suitable instrument, based on the range of values to be measured.
• Note down the readings carefully

Systematic Errors
If the instrument produces an error, which is of a constant uniform deviation during its operation
is known as systematic error. The systematic errors occur due to the characteristics of the
materials used in the instrument. Types of Systematic Errors The systematic errors can be
classified into the following three types.
• Instrumental Errors − This type of errors occur due to shortcomings of instruments
and loading effects.
• Environmental Errors − This type of errors occur due to the changes in environment
such as change in temperature, pressure & etc.
• Observational Errors − This type of errors occur due to observer while taking the
meter readings. Parallax errors belong to this type of errors.

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Random Errors
The errors, which occur due to unknown sources during measurement time are known as random
errors. Hence, it is not possible to eliminate or minimize these errors

Strain Gauges
• Strain gauges are devices whose resistance changes under the application of force or strain.
They can be used for measurement of force, strain, stress, pressure, displacement,
acceleration etc. A strain gauge is a resistor used to measure strain on an object. When an
external force is applied on an object, due to which there is a deformation occurs in the shape
of the object. This deformation in the shape is both compressive or tensile is called strain,
and it is measured by the strain gauge. When an object deforms within the limit of elasticity,
either it becomes narrower and longer or it become shorter and broadens. As a result of it,
there is a change in resistance end-to-end.
• The strain gauge is sensitive to that small changes occur in the geometry of an object. By
measuring the change in resistance of an object, the amount of induced stress can be
calculated.
• The change in resistance normally has very small value, and to sense that small change,
strain gauge has a long thin metallic strip arrange in a zigzag pattern on a non-conducting
material called the carrier, as shown below, so that it can enlarge the small amount of stress
in the group of parallel lines and could be measured with high accuracy. The gauge is literally
glued onto the device by an adhesive.
• When an object shows physical deformation, its electrical resistance gets change and that
change is then measured by gauge.

Principle of Working of Strain Gauges

When force is applied to any metallic wire its length increases due to the strain. The more is the
applied force; more is the strain and more is the increase in length of the wire. If L 1 is the initial
length of the wire and L2 is the final length after application of the force, the strain is given as:

ε =(L2-L1)/L1

Consider a wire strain gage, as illustrated above. The wire is composed of a uniform conductor
of electric resistivity 𝜌 with length l and cross-section area A. Its resistance R is a function of the
geometry given by

𝐥
𝐑=𝛒
𝐀

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Fig Construction of Strain gauge

Strain gauge has a long thin metallic strip arrange in a zigzag pattern on a non-conducting
material. So that it can enlarge the small amount of stress in the group of parallel lines and could
be measured with high accuracy. When an object shows physical deformation, its electrical
resistance gets change and that change is then measured by gauge.
Working:
Strain gauge bridge circuit shows the measured stress by the degree of discrepancy, and uses a
voltmeter in the centre of the bridge to provide an accurate measurement of that imbalance:

Fig Working of strain gauge

In this circuit, R1 and R3 are the ratio arms equal to each other, and R2 is the rheostat arm has a
value equal to the strain gage resistance. When the gauge is unstrained, the bridge is balanced,
and voltmeter shows zero value. As there is a change in resistance of strain gauge, the bridge
gets unbalanced and producing an indication at the voltmeter. The output voltage from the bridge
can be amplified further by a differential amplifier.
Applications of the Strain Gauges
The strain gauges are used for two main purposes:
1. Measurement of strain: Whenever any material is subjected to high loads, they come under
strain, which can be measured easily with the strain gauges. The strain can also be used to carry
out stress analysis of the member.
2. Measurement of other quantities: The principle of change in resistance due to applied force
can also be calibrated to measure a number of other quantities like force, pressure, displacement,

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acceleration etc since all these parameters are related to each other. The strain gauges can
sense the displacements as small as 5 µm. They are usually connected to the mechanical
transducers like bellows for measuring pressure and displacement and other quantities.

Linear Variable Differential Transformer – LVDT Transducer

• LVDT is a positive or magnetic displacement transducer; it is commonly used to measure
force, weight, pressure and acceleration which depend on force in terms of amount and
direction of displacement.
• It is an inductive transformer which converts the linear displacement into an electrical signal.
• It works on the principle of mutual induction, i.e., the flux induced in the primary winding is
linked to the secondary winding.
• The output of the transformer is obtained because of the difference of the secondary voltages,
and hence it is called a differential transformer.

Fig Working of LVDT

Principle:
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, eout=e1−e2
CASE-I:
When the core is at null position when the core is at null position then the flux linking with both
the secondary windings is equal so the induced emf is equal in both the windings. So for no
displacement the value of output eout is zero as e1 and e2 both are equal. (eout=𝟎)
CASE II:
When the core is moved to upward of null position In this case the flux linking with secondary
winding S1 is more as compared to flux linking with S2. Due to this e1 will be more as that of
e2. Due to this output voltage eout is positive. (eout=+𝒗𝒆)
CASE III:

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When the core is moved to downward of Null position (for displacement to the downward of
the reference point). In this case magnitude of e2 will be more as that of e1. (eout=-𝒗𝒆)

LVDT Characteristics
Advantages of LVDT
• High Range: the LVDTs has a very high range for measurement of displacement This can be
used for measurement of displacement ranging from 1.25 mmto 2.50 mm
• Friction and Electrical Isolation
• Immunity from External Effects
• High input and high sensitivity
• Ruggedness: The transducer can usually tolerate high degree of shock and vibration
• Low Hysteresis
• Low Power consumption
Disadvantage of LVDT
• Relatively large displacement is required for appreciable differential output
• They are sensitivity to stray magnetic fields but shielding is possible
• Many times, the transducer performance is affected by vibrations
• The receiving instrument must be selected to operate on ac signal
• The dynamic response is limited mechanically by the mass of the core and electrically by
frequency of applied voltage. The frequency of the carrier of the carrier should be at least ten
times the highest frequency component to be measured
• Temperature affects the performance
Applications of LVDT
Acting as a secondary transducer it can be used as a device to measure force, weight and
pressure etc. The force measurement can be done by using a load cell as the primary transducer
while fluid pressure can be measured.by using Bourdon tube which acts as primary transducer.
The force or the pressure is converted into a voltage. In these applications the high sensitivity of
LVDTs is a major attraction. Used in reducing atmosphere but at low temperatures.

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Resistance temperature detector (RTD)

When a metal wire is heated the resistance increases. So, a temperature can be measured using
the resistance of a wire. RTD incorporates pure metals or certain alloys that increases resistance
as temperature increases and it conversely decreases resistance as temperature decreases.
RTDs act similar to an electrical transducer. And it converts changes the temperature to voltage
signals by the measurement of resistance.

Working principle of RTD

Resistance thermometers or resistance temperature detector works on the principle of positive
temperature coefficient of resistance i.e. as temperature increases, resistance offered by
thermometer also increases. The resistance element of platinum and iron metal wire is wrapped
around an electrically insulating support of glass, ceramic or mic and from the outside, the
protective sheath of metallic tube can be provided. The lead wires are taken out from the
resistance elements which are joined to the circuitry.

Fig Constriction of RTD

RTD formula:
The resistance thermometers which are alternatively known as RTD works on the principle that
“the resistance of a metal varies with a change in temperature” according to the relation as,
RT =R0 [1+α(T-T0)] where,
RT : Resistance at temperature ( T)
R0: Resistance at temperature ( 0˚C)
α: Temp. coeff.
T: temp (˚C)
T0: Initial temp.
To measure the change in the resistance bridge network is used.

Fig 5.6 Working of RTD

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Figure shows the construction of an RTD. It has a resistor element connected to a Wheatstone
bridge. The element and the connection leads are insulated and protected by a sheath. A small
amount of current is continuously passing through the coil. As the temperature changes the
resistance of the coil changes which is detected at the Wheatstone bridge.
RTDs are used in the form of thin films, wire wound, or coil. They are generally made of metals
such as platinum, nickel, or nickel-copper alloys. Platinum wire held by a high temperature glass
adhesive in a ceramic tube is used to measure the temperature in a metal furnace.

Working of RTD
Steel protective sheath detects the temperature and transfer it to the platinum filament.
The change in the resistance value of the Platinum coil is very small with respect to the
temperature.
So, the RTD value is measured by using a bridge circuit.
Temperature is determined by converting the RTD resistance value using a calibration
expression.
Dummy wire reduces impedance effect and so the error.

Applications of RTD
• Air conditioning and refrigeration servicing
• Food Processing
• Stoves and grills
• Textile production
• Plastics processing
• Petrochemical processing
• Microelectronics
• Air, gas and liquid temperature measurement in pipes and tanks
• Exhaust gas temperature measurement
Advantages of RTD
• It is suitable for measuring high temperatures
• It has a high degree of accuracy
• It ensures good stability and repeatability
• It does not need a reference temperature junction
Disadvantages of RTD
• Size is more than the thermocouple
• Power supply is required
• It needs an auxiliary apparatus to get the required form of output

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• Resistance element is more expensive than a thermocouple

Thermocouple
• The thermocouple is a temperature measuring device. Thermocouple is a transducer
which converts the thermal energy into electrical energy.
• It is used to measure the temperature at one specific point in the form of the EMF or an
electric current.
• A thermocouple consists of two different metal wires joined together to form two junctions.
One junction is connected to the body whose temperature is to be measured; this is the
hot or measuring junction
• The other junction is connected to a body of known temperature; this is the cold or
reference junction.
• Therefore, the thermocouple measures unknown temperature of the body with reference
to the known temperature of the other body.
• The amount of EMF generated in the thermocouple is very minute (millivolts), so very
sensitive devices must be utilized for calculating the e.m.f. produced in the circuit. • The
common devices used to calculate the e.m.fs are voltage balancing potentiometer and
the ordinary galvanometer.

Fig Constriction of thermocouple

Working Principal
• The working principle of the thermocouple depends on the Seeback effect. • Seeback
Effect: When the temperature difference exists between junctions of two dissimilar metals
then emf is generated at the two junctions.
• See back Effect – The See back effect occurs between two different metals. When the
heat provides to any one of the metal, the electrons start flowing from hot metal to cold
metal. Thus, direct current induces in the circuit.

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Fig. See back effect

• In short, it is a phenomenon in which the temperature difference between the two different
metals induces the potential differences between them. The See beck effect produces
small voltages for per Kelvin of temperature.
• If the temperature of both the junctions is same, equal and opposite emf will be generated
at both junctions and the net current flowing through the junction is zero.
• If the junctions are maintained at different temperatures, the emf’s will not become zero
and there will be a net current flowing through the circuit.
• The total emf or the current flowing through the circuit can be measured easily by the
suitable device.
• The device for measuring the current or emf is connected within the circuit of the
thermocouple. It measures the amount of current flowing through the circuit. • Now, the
temperature of the reference junctions is already known, while the temperature of
measuring junction is unknown. The output obtained from the thermocouple circuit is
calibrated directly against the unknown temperature.
• Thus, the voltage or current output obtained from thermocouple circuit gives the value of
unknown temperature directly.

Advantages of Thermocouple
• The following are the advantages of the thermocouples.
• The thermocouple is cheaper than the other temperature measuring devices. •
The thermocouple has the fast response time.
• It has a wide temperature range.
Disadvantages of the Thermocouples
• The thermocouple has low accuracy.
• The recalibration of the thermocouple is difficult.

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Thermistors
Principle
A resistor is an electrical component that limits the amount of current flows through a circuit.
Thermistor is special type of resistor, whose resistance varies more significantly with temperature
than in standard resistors. Generally, the resistance increases with the temperature for most of
the metals but the thermistors respond negatively i.e., the resistance of the thermistors decrease
with the increase in temperature. This is the main principle behind thermistor. As the resistance
of thermistors depends on the temperature, they can be connected in the electrical circuit to
measure the temperature of the body.

Types of Thermistors
There are mainly 2 types of thermistors namely
• Positive-temperature coefficient (PTC) and
• Negative-temperature coefficient (NTC).
Positive Temperature Coefficient (PTC)
PTC thermistors increase their resistance as the temperature rises. The relationship between
resistance and temperature is linear, as expressed in the following equation: deltaR = k(deltaT)
where deltaR is the change in resistance, deltaT is the change in temperature and k is the
temperature coefficient. When k is positive, it causes a linear increase in resistance as the
temperature rises.
Negative Temperature Coefficient (NTC)
Many NTC thermistors are made from a pressed disc or cast chip of a semiconductor such as a
sintered metal oxide. They work because raising the temperature of a semiconductor increases
the number of electrons able to move about and carry charge – it promotes them into the
conduction band. Definition: The thermistor is a kind of resistor whose resistivity depends on
surrounding temperature. It is a temperature sensitive device. The word thermistor is derived from
the word, thermally sensitive resistor. The thermistor is made of the semiconductor material that
means their resistance lies between the conductor and the insulator.
The variation in the thermistor resistance shows that either conduction or power dissipation
occurs in the thermistor. The circuit diagram of thermistor uses the rectangular block which has
a diagonal line on it.

Fig Symbol of thermistor

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Construction of Thermistor
The thermistor is made with the sintered mixture of metallic oxides like manganese, cobalt, nickel,
cobalt, copper, iron, uranium, etc. It is available in the form of the bead, rod and disc. The different
types of the thermistor are shown in the figure below.

Fig Physical view of Thermistor

The bead form of the thermistor is smallest in shape, and it is enclosed inside the solid glass rod
to form probes.

Fig Physical view of Thermistor

Resistance Temperature Characteristic of Thermistor

The relation between the absolute temperature and the resistance of the thermistor is
mathematically expressed by the equation shown below

Where
RT1 – Resistance of the thermistor at absolute temperature T1 in Kelvin.
RT2 – Resistance of the thermistor at absolute temperature T2 in Kelvin.
Β – a temperature depending on the material of thermistor.
The resistance temperature coefficient of the thermistor is shown in the figure below. The graph
below shows that the thermistor has a negative temperature coefficient, i.e., the temperature is
inversely proportional to the resistance. The resistance of the thermistor changes from 105 to 10-
2
at the temperature between -100C to 400C.

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Advantages of Thermistor
The following are the advantages of the thermistor.
1. The thermistor is compact, long durable and less expensive.
2. The properly aged thermistor has good stability.
3. The response time of the thermistor changes from seconds to minutes. Their response time
depends on the detecting mass and the thermal capacity of the thermistor.
4. The upper thermistor limit of the temperature depends on the physical variation of the
material, and the lower temperature depends on the resistance reaching a large value.
5. The self-heating of the thermistor is avoided by minimising the current passes through it.
6. The thermistor is installed at the distance of the measuring circuit. Thus the reading is free
from the error caused by the resistance of the lead.
7. The thermistor has more advantages as compared to the conventional thermocouple and
resistance thermometer. Along with the temperature sensing the thermistor are also used in
various other application.

Applications of Transducer
The following are the application of the transducers.
1. It is used for detecting the movement of muscles which is called acceleromyograph.
2. The transducer measures the load on the engines.
3. It is used as a sensor for knowing the engine knock.
4. The transducers measure the pressure of the gas and liquid by converting it into an electrical
signal.
5. It converts the temperature of the devices into an electrical signal or mechanical work.
6. The transducer is used in the ultrasound machine. It receives the sound waves of the patient
by emitting their sound waves and pass the signal to the CPU.

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7. The transducer is used in the speaker for converting the electrical signal into acoustic sound.
8. It is used in the antenna for converting the electromagnetic waves into an electrical signal.

Comparison of RTD Thermocouple and Thermistor

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