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AUTOMATION & ROBOTICS
LECTURE#04
TEMPERATURE SENSORS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor
TRANSDUCER
• Transducer is a device which transforms energy from
one type to another, even if both energy types are in
the same domain.
– microphone (converts sound into an electrical
signal).
• Typical energy domains are mechanical, electrical,
chemical, magnetic, optical and thermal.
• Transducer can be further divided into Sensors, which
monitors a system and Actuators, which impose an
action on the system.
TEMPERATURE SENSORS
Temperature is a scalar quantity that determines the direction of
heat flow between two bodies.
 Sensing methods: contact and non-contact
– Contact
• Sensor is in direct physical contact with the object to be
sensed
• To monitor solids, liquids, gases over wide range
– Non-contact
• Interprets the radiant energy of a heat source to energy in
electromagnetic spectrum
• Monitor non-reflective solids and liquids
 Temperature sensors generate output signals in one of two ways:
1. through a change in output voltage
2. through a change in resistance of the sensor‘s electrical circuit
TEMPERATURE SENSORS CLASSIFCATION:
Contact Sensing
• Thermocouple (Thermoelectric)
• Thermistor (Thermal Resistors)
– Negative temperature coefficient device (NTC)
– Positive temperature coefficient device (PTC)
• Resistive Temperature Detector (RTD)
• Semiconductor Temperature sensors
• Liquid-in-Glass Thermometers
• Bimetallic Thermometers
TEMPERATURE SENSORS CLASSIFCATION:
Non-Contact Sensing
• Radiation Thermometers
– Infrared thermal Imaging
– Scanners
– Spot Radiometers
• Thermal Imagers
• Ratio Thermometers
THERMOCOUPLES
• Most common temperature sensing device.
• Accurate temperature measurements can be made
with thermocouples sensors at low cost with shop-built
probes and ordinary low-level voltmeters.
• Thermocouples can measure temperature at a point in
a range of -250C to +3500C.
Typical Industrial Thermocouple
General Thermocouple
Thermocouple Concept
• Principle of operation is based on the Seebeck effect,
discovered by Thomas Seebeck in 1822,
– Electrons flow from one wire to other, due to different
energy potentials of alloys
– As temperature changes, current flows
– Voltage is measured between the two alloys (Small
voltage, less than 10 mV)
• Seebeck effect: when any conductor subjected to thermal
gradient, generates a voltage.
• A temperature gradient along a conductor creates an EMF.
Thermocouple Concept……
• If two conductors of different materials are joined at one
point, an EMF is created between the open ends which is
dependent upon the temperature of the junction.
• As T1 increases, so does Voltage (V).
• EMF also depends on the temperature of the open ends T2.
• The junction is placed in the process, the other end is in
iced water at 0C. This is called the reference junction.
Thermocouple Working Principle
• To measure this thermal
gradient– connect another
conductor.• voltage depends on type of
metal used.
• Difference –1 – 70 micro
volts per degree C
• Thermocouple
gives
difference in temperature
not the absolute value
• Current continues to flow
as long as T2>T1
• Emf e= a1 + a2 ()
2+…….+ a () n
n
• Finally reduced to e= a1
• Thermocouples
behaves
according to thermo electric
laws
Thermo electric laws
• Three laws govern operation of thermocouples:
• Law 1. A thermoelectric current cannot be established in a
homogeneous circuit by heat alone.
– This law establishes the need for junctions of dissimilar
materials since a single conductor is not sufficient.
Law 2. The algebraic sum of the thermoelectric forces in a
circuit composed of any number and combination of
dissimilar materials is zero if all junctions are at uniform
temperatures.
– Additional materials may be connected in the
thermoelectric circuit without affecting the output of the
circuit as long as any junctions added to the circuit are
kept at the same temperature.
– voltages are additive so that multiple junctions may be
connected in series to increase the output.
Thermo electric laws
• Law 3.If two junctions at temperatures T1 and T2 produce
Seebeck voltageV2 and temperatures T2 and T3 produce
voltage V1, then temperatures T1 and T3 produce V3=V1+V2.
– This law establishes methods of calibration of
thermocouples.
Law 4. Law of intermediate temp. Eac= Eab + Ebc
Law of intermediate metals:
• States that net emf in a circuit remains unaltered if a third
metal is introduced provided that the two junctions of third
metals are at same temperature.
Thermo electric laws
Practical Thermocouple Construction
• A thermocouple construction consist of two conductors,
welded together at the measuring point and insulated
from each other long the length.
• It will usually have an outer protection sheath.
Thermocouple Materials Classification
thermocouple materials are classified as
•
BASE METAL : Types E, J, K, N, and T
• NOBLE METAL: Types R, S, and B
Comparison of Thermocouples & Applications
Type J:
• Type J is useable up to 720°C.
• It is not very susceptible to aging up to about 540°C.
• It is very cost effective and is the thermocouple of choice in the
plastics processing industry where temperatures rarely exceed
400 °C.
• The iron conductor is subject to oxidation (due to the iron wire) at
higher temperatures and when unprotected.
• generates about 50 µV/°C (28 µV/°F)
Type K:
• Type K is useable up to 1150°C in an oxidizing atmosphere.
• Metallurgical changes can cause a calibration drift of 1 to 2°C in a
few hours, increasing to 5 °C over time.
• generates about 40 µV/°C (22 µV/°F)
Comparison of Thermocouples & Applications
Type E:
• Type E is useable up to 820 °C.
• It has the highest mV output of all the thermocouples and has similar
calibration drift to that of Type K so the same precautions are
recommended.
Type T:
• Type T oxidization of the copper limits the useable temperature to
about 370 °C.
• Type T is thermocouple of choice for applications down to –200 °C.
• generates about 40 µV/°C (22 µV/°F).
Type N :
• Type N is useable up to1260°C.
• It was developed to overcome several problems inherent in Type K
TCs.
• Aging in the 300 to 600 °C range is considerably less.
• Also Type N has also been found to be more stable than Type K in
nuclear environments, where Type K has been the sensor of choice.
Comparison of Thermocouple & Applications
Types R and S:
• Types R and S are usable up to 1480°C.
• They are extremely stable but reducing atmospheres are
particularly damaging.
• This type should be protected with a gas-tight ceramic tube
and a secondary tube of porcelain, silicon carbide or metal
outer tube, as conditions require.
• Type R delivers some 15% more mV than type S.
Type B
• Type B is usable up to 1700 °C. Also easily contaminated,
and damaged by reducing atmospheres.
• The same protective measures for R and S shown above
apply to type B thermocouples.
Thermocouples used in industrial process
• Type K thermocouples are
the most linear of the three
Types J and E have better
relative output than type K
Thermocouples Capabilities
• Wide Range
• Fast Response
• Passive
• Inexpensive
Thermocouples Limitations
• Non-Linear
• Accuracy
– Often between 0.5 and 2.2ºC, depending on TC type
• Noise
– Long leads can attract electrical signals
– Already low signal from thermocouple
• Thermal shunting
– Heating of wire mass can affect measurements by
absorbing energy
• Corrosion
– High alkali or water environments can modify calibration
Thermistors: THERMal resISTORS
• Thermistor is a combination of the words thermal and resistor.
• Thermistor was invented by Samuel Ruben in 1930
• A thermistor is a type of resistor used to measure temperature
changes, relying on the change in its resistance with changing
temperature.
– Typically have a negative temperature coefficient (NTC),
– Resistance decreases with increasing temperature
• Thermistor can measures across the range of -40~150 ±0.35 °C
Leads, coated
Glass encased
Surface mount
Relationship b/w resistance & temperature
• Assume a simple linear relationship between resistance
and temperature for the following discussion:
ΔR = k ΔT
Where
•
ΔR = change in resistance
•
ΔT = change in temperature
•
k = first-order temperature coefficient of resistance
Thermistors Classification
• Thermistors can be classified into two types depending on
the sign of k.
• If k is positive, the resistance increases with increasing
temperature, and device is called a positive temperature
coefficient (PTC) thermistor, some time also known as
Resistance Temperature Detector (RTD)
– If Platinum and nickel are used as metal in RTD.
• If k is negative, the resistance decreases with increasing
temperature, and the device is called a negative
temperature coefficient (NTC) thermistor.
• Resistors that are not thermistors are designed to have
the smallest possible k, so that their resistance
remains almost constant over a wide temperature
range.
Thermistor construction
NTC Thermistors
• NTC Thermistors are made from the oxides of metals
such as manganese, cobalt, nickel and copper.
• The metals are oxidized through a chemical reaction,
ground to a fine powder, then compressed and
subject to very high heat.
• Some NTC thermistors are crystallized from
semiconducting material such as silicon and
germanium.
PTC Thermistors
• PTC Thermistors are made by introducing small
quantities of semiconducting material into a
polycrystalline ceramic.
• When temperature reaches a critical point, the
semiconducting material forms a barrier to the flow of
electricity and resistance climbs very quickly.
PTC working principle
• Resistance of these types of
thermistors increases with the
rise in temperature.
• Due to the special ResistanceTemperature-characteristic,
there
is
no
additional
temperature
regulation
or
safety device necessary while
reaching high heat-power level
when using the low resistance
area .
NTC working principle
• Resistance of NTC thermistors decreases proportionally
with increases in temperature.
• Thermistor resistance-temperature relationship can be
approximated by,
where:
• T is temperature (in Kelvin),
• TRef is the reference temperature,
usually at room temp (25°C;77°F;
298.5 K),
• R is the resistance of the thermistor
(W),
• RRef is the resistance at TRef,
• b is a calibration constant depending
on the thermistor material, usually
between 3,000 and 5,000 K.
PTC Thermistor :Application configuration
• There are very few commercial
applications
involving
PTC
thermistors that are based upon
the
resistance
-temperature
characteristic.
• Most PTC thermistor applications
are based upon either the steady
state
self-heated
condition
(voltage-current characteristic)
or upon the dynamic self-heated
condition
(current
-time
characteristic) or a combination
of both.
• Dramatic rise in resistance of a
PTC at and above the transition
temperature makes it ideal for
over current protection.
PTC Thermistor: Industrial Applications
Time Delay, Motor Starting, Degaussing
• These three applications are somewhat similar in that they
all rely on the dynamic operation (Current-Time
Characteristic) of a self-heated PTC thermistor.
• In each case, current is allowed to pass through a series
circuit for a prescribed amount of time before the thermistor
self-heats into a high resistance condition.
Time Delay Circuit:
In the time delay circuit, as the
thermistor
self-heats
and
increases its resistance, the
voltage drop on the fixed resistor
decreases to a minimum.
PTC Thermistor: Industrial Applications
Motor Starting
In the motor start circuit, PTC has a
low resistance at turn-on so that a
significant amount of current is
permitted to flow in the starting
winding of the motor.
After the thermistor has self-heated
into a high resistance state, the
current through the starting winding
becomes negligible.
degaussing circuit
In the degaussing circuit, current
through the demagnetizing coil is
initially large and decreases to a
negligible value as the thermistor selfheats.
In all of these cases, the length of time
required for the switching action to
occur depends upon the amount of
power applied to the PTC and its
thermal characteristics.
NTC Thermistor :Application configuration
• NTC thermistor is a versatile component that can be used
in a wide variety of applications where the measured is
temperature dependent.
• Thermistor applications are grouped according to one of
the three fundamental electrical characteristics;
 The current-time characteristics
 The voltage-current characteristic
 The resistance-temperature characteristic
NTC Thermistor :Application configuration
• Application based on Current-Time characteristic
 Time delay, surge suppression, inrush current limiting and
sequential switching represent some of the earliest, high volume
uses of thermistors.
• Application based on Resistance -temperature characteristic
 Temperature measurement, control, and compensation.
NTC Thermistor :Application configuration
Linear Voltage Divider
• Simplest thermistor network used in
many applications is the voltage
divider circuit
 The output voltage is taken across
the fixed resistor.
 This has the advantages of
providing an increasing output
voltage for increasing temperatures
and allows the loading effect of any
external measurement circuitry to be
included into the computations for
the resistor, R .
 The loading will not affect the output
voltage as temperature varies
Thermistors Specifications
• Thermistor-choice is based
on the nominal resistance
you want at the operating
temperature range, on the
size, and on the time
constant.
• Time constants are about 510 seconds. (Check this out
with your thermistor).
Thermistor Linearizing Circuit
Application of PTC Thermistors
•
•
•
•
•
They are used as resettable fuses.
They are used in time delay circuits.
PTC Thermistors are used in motor starting circuits.
They are also used in Degaussing circuitry.
The PTC Thermistor can provide a combination of
heater and thermostat in one device
• They are used as ‘liquid level’ and ‘flow sensors’.
Application of NTC Thermistors
•





•
General industrial applications
Industrial process controls
Plastic laminating equipment
Fiber processing & manufacturing
Hot mold equipment (thermoplastics)
Solar energy equipment
Automotive
&
Transportation
Application
 Emission controls
 Engine temperatures
 Aircraft Temperatures.
Medical Applications
Fever Thermometers
Fluid temperature
Dialysis Equipment
Consumer/Household
Applications
Burglar alarm
Refrigeration & air conditioning
Fire detection
Oven temperature control
Thermistor Advantages
• High resistance (1 kΩ to 100 kΩ)
– Eliminates problems with resistance in lead wires
• Highly non-linear RT vs. T relationships
– Mostly negative temperature coefficients (NTC) from metal
oxides, but positive temperature coefficient (PTC) models are
available from barium and strontium titanate mixtures
– Can be linearized
• Small physical size
• Fast response time
• Not as small as thermocouples
• Lower cost than RTDs
• Easy to manufacture in bulk
• Wide temperature range
• Very high sensitivity and resolution
• Up to 1000 times more sensitive than RTDs
• Can withstand shock and vibration
• Accurate
Thermistor Disadvantages
• Limited temperature range, typically -100 ~ 150 °C (-148 ~
302 °F).
• Nonlinear resistance-temperature relationship, unlike RTDs
which have a very linear relationship.
• Errors can result from self excitation currents being
dissipated by the thermistors.
• They get de-calibrated on exposure to higher temperatures
• The nonlinear response requires extra circuitry
• Limited range of linear response with this additional circuitry
• Narrow “linear” operating range for a single sensor
• The glass can break if mishandled
• Requires an excitation current
• High resistance leads to self-heating errors (more so than
RTDs)
• Less stable than RTDs
Resistance Temperature Detector- RTD
• RTD is a temperature sensitive resistor, It is a
positive temperature coefficient device, which
means
that
the
resistance
increases
with
temperature.
• This type of sensors is based on the observation
that different materials can have different resistive
profiles at different temperatures.
– Properties are mainly electrical in nature.
• Industrial RTDs are very accurate: the accuracy
can be as high as ±0.1°C.
• The ultra high accurate version of RTD is known
as Standard Platinum Resistance Thermometers
(SPRTs) having accuracy at ±0.0001°C.
Platinum Wire RTDs (PRTs)
PRTs have established themselves as the de-facto industry
standard for temperature measurement, and for many
reasons:
 linear temperature sensors
 linear positive temperature coefficient (-200 to 800 °C)
 Resistance vs. temperature characteristics are stable and
reproducible
 very accurate and suitable for use as a secondary standard
Platinum Wire RTDs (PRTs)
Platinum Scale ( 0 to 100 °C )
Thin-Film RTDs
• Thin-film RTD design is a newer technology and is gaining
favor due to lower cost.
• It is designed to minimize strain on the platinum due to
thermal expansion since strain also cause changes in
resistance, R =(L/A).
RTD: small resistance change measurement
• Bridge circuit:
– Can detect small resistance changes
– If R1=R4, RRTD= R2(Vs-2Vo)/(Vs+2Vo)
“Supply”
Voltage
R1
R2
Vo
Vs
R4
RRTD
Circuits Used to Determine the Resistance
of an RTD
 Four Wire Design
• Two-wire: Non-linear relationship between the
measured voltage and the RTD resistance.
• Three-wire: Better results.
• Four-wire: Resistance is a linear function of the
measured voltage.
RTDs: Characteristics and Applications
• Characteristics:
–
–
–
–
–
n
Resistive device, active, linear
Large range: -200 to +850oC for Platinum
High accuracy: 0.001oC
Low sensitivity: 0.39 % per oC
Don’t need reference temperature
Applications:
u
Industries and laboratories where high accuracy of
temperature measurements are required.
Selecting A Temperature Sensor
Three things that we need to keep in mind when selecting
temperature sensor:
1) What is the desired temperature range, the tolerable limit to
the error in measurement and the conditions under which the
measurement is to be performed?
2) Is it possible to touch the object and if so would the sensor or
the temperature of the object be likely to be seriously affected by
the contact?
If the answer is yes, then a non-contact temperature sensor is
needed. If no, then the answer probably lies with one of the other
sensor types.
3) If a contact sensor appears satisfactory, then questions
revolve more around temperature measuring range, satisfying
the conditions of use and meeting the acceptable error
allowance.