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ECGR 6185
Advanced Embedded Systems
TEMPERATURE SENSORS
(Thermocouples, RTDs and
Thermistors)
University Of North Carolina Charlotte
Karunakar Reddy Gujja
Temperature sensors
 Temperature Sensors are the devices which are used to measure the
temperature of an object.
 These sensors sense the temperature and generate output signals in one of the
two forms: change in voltage or change in resistance.
 In order to select a sensor for a particular application - accuracy, range of
temperature, response time and environment are considered.
Temperature sensors
 Temperature sensors are categorized into two types:
– Contact type sensors
– Non-Contact type sensors
 Contact type sensors:
These measure their own temperature i.e., they are in contact with the metal and
will be in thermal equilibrium.
 Non-Contact type:
These infer temperature by measuring the thermal radiations emitted by the
material.
Temperature sensors
 Contact type sensors:
• Thermocouples
• Resistive temperature devices
 Non-Contact type sensors:
• IR thermometers
-These measure the temperature by detecting the infrared energy emitted by
the material.
-This consists of a lens which senses the IR signal and converts it into
electrical signal which is displayed in temperature units.
-These are applied when the object is moving, surrounded by EM field or
when a fast response is required.
Thermocouple Temperature Measurement Sensors
 Principle of operation:
Thermocouples work on the principle of Seebeck effect.
 They are available in bead type or probe type construction.
 They consist of two junctions: cold junction and hot junction.
 The voltage developed between two junctions is called Seebeck voltage.
 Voltage is in the order of millivolts.
 They generate energy in the order of microwatts-milliwatts.
Different types of thermocouples:
Type
Composition
Range
Good for
Type K
Chromel (Ni-Cr alloy) / Alumel (NiAl alloy)
−200 °C to 1200 °C
Oxidizing or neutral
applications
Type E
Chromel / Constantan (Cu-Ni alloy)
−200 °C to 900 °C
Oxidizing or inert
applications
Type J
Iron / Constantan
−40 °C to 750 °C
Vacuum, reducing,
or inert apps
Type N
Nicrosil (Ni-Cr-Si alloy) / Nisil (Ni-Si
alloy)
−270 °C to 1300 °C
Oxidizing or neutral
applications
Type T
Copper / Constantan
−200 °C to 350 °C
Oxidizing, reducing
or inert apps
Type R
Platinum /Platinum with 13% Rhodium
0 °C to 1600 °C
Type S
Platinum /Platinum with 10% Rhodium
Type B
Platinum-Rhodium/Pt-Rh
Not recommended
for
Cost
Sensitivity
Low
(11.65$ to
48.63$)
41 µV/°C
Low
68 µV/°C
Low
52 µV/°C
Low
39 µV/°C
Wet or humid
environments
Low
43 µV/°C
High temperatures
Shock or vibrating
equipment
High
10µV/°C
0 °C to 1600 °C
High temperatures
Shock or vibrating
equipment
High
10µV/°C
50 °C to 1800 °C
High temperatures
Shock or vibrating
equipment
High
10µV/°C
Use under 540ºC
Oxidizing or humid
environments
Thermocouples
Theory of operation:
– Figure 1 shows the typical
Type-J thermocouple.
– The emf shown in the figure is
the Seebeck voltage which is
developed because of the
temperature difference.
– Figure 2 shows the cold
junction compensation (CJC).
Thermocouples
Calculations:
The voltage generated by the thermocouple is given by the equation:
V= S* ΔT
Where, V= voltage measured (V)
S= Seebeck coefficient (V/°C)
ΔT= difference in temperature between two junctions
Hence the unknown temperature can be calculated using the equation,
T= Tref + V/S in °C
Thermocouples
• Thermocouples are available in wire bead type or probe type.
• Bead type are used for low temperature applications and probe type for high
temperature applications.
• In selecting a thermocouple for particular application type, insulation and
probe construction is considered.
• Location of the thermocouple plays a major role for accurate measurement.
As a ‘rule of thumb’ it is located at 1/3rd distance from the heat source and
2/3rd distance from workload.
Characteristics of Thermocouples:
Characteristics of Thermocouples:
Precautions and considerations for using thermocouples:
– Connection problems
– Lead Resistance
– Decalibration
– Noise
– Common Mode Voltage
– Thermal Shunting
Thermocouples
Advantages:
Disadvantages:
– Self-powered
– Non-linear
– Simple in construction
– Low voltage
– Rugged
– Less stable
– Wide temperature range
– Reference required
– Wide variety
– Inexpensive
Resistance Temperature Devices
 They work by undergoing change in electrical resistance, with change in
temperature.
 These are low cost and low temperature range sensors.
 These are of two types:
• RTDs
• Thermistors
Resistance Temperature Detectors (RTDs)
 They work on the principle of positive temperature coefficient.
 RTDs are used to measure the temperatures ranging from -196 to 482 deg C
or (-320 to 900 deg Fahrenheit)
 Common Resistance Materials for RTDs:
• Platinum (most popular and accurate)
• Nickel
• Copper
• Balco (rare)
• Tungsten (rare)
RTDs
Calculations:
 R(T)=R0*(1+a*T+ b*T^2)
–
–
–
R (T) = Resistance at temperature T
R0 = Resistance at Nominal Temperature
a and b are calibration constants, where
a= 3.9692 * 10^-3 /°C
b= -5.8495 * 10^-7 /°C

The relationship between voltage and RTD’s resistance is given by:
V= (Vref*R(T))/(R(0)+R(T))
RTDs
Advantages:
•
•
•
•
Stable output for a long period of time
Ease of recalibration
Accurate readings over narrow temperature range
Linear output
Disadvantages:
• Smaller temperature range when compared to thermocouples
• High initial cost and less rugged to environmental vibrations
• Not self-powered
• Self heating
RTDs
Applications:
•
They are used for precision process temperature control.
•
Widely used in industrial applications.
•
Directly used in recorders, temperature controllers, transmitters and digital
ohmmeters
Thermistors
 These are similar to RTDs.
 These work on negative temperature coefficient.
 These are made up of semiconductor devices.
 Variation is non-linear.
 Thermistors are used to measure the temperatures ranging from -45 to 260
deg C or (-50 to 500 deg Fahrenheit).
Thermistors
Thermistor symbol
Advantages:
Disadvantages:
•High output
•Fast response
•Two wire ohms measurement
•Non-linear
•Limited temperature range
•Not self-powered
•Self heating
Thermistors
Applications:
•
Can be used as a liquid level indicator or as a liquid level controller
•
To measure temperature in Medical Applications
•
Temperature Control
Software aspect: (Thermistor and RTD application)
• Application of RTD for detecting the environment temperature.
• This uses the microcontroller board which has an inbuilt Thermistor which is
used to compare the readings of both sensors.
• The environmental temperature is measured and displayed on the LCD screen
of the microcontroller and updated every 1 second.
• RTD is connected to one of the ADCs of the microcontroller and this value is
also displayed on the LCD and updated for every 1 second.
Temperature Controllers
What are temperature controllers?
How to select a controller?
The following items should be considered when selecting a controller:
–Type of input sensor (thermocouple, RTD) and temperature range
–Type of output required (electromechanical relay or analog output)
–Control algorithm needed (on/off, proportional, PID)
–Number and type of outputs (heat, cool, alarm, limit)
Different types of controllers:
•On/Off controller
•Proportional controller
•PID controller
Temperature controllers
On-Off controller:
• This is a simple mechanism for temperature control device, whenever
temperature crosses the set point, controller switches the output.
• It is a cyclic process.
• In order to prevent the continual operation, a differential or hysteresis is used.
• It is used in slow temperature change applications.
Eg: Temperature alarm system.
Temperature controllers
Proportional controller:
• It eliminates the cyclic problem of on-off controller.
• This slows down the time at which heater approaches the set point by
decreasing the average power supplied.
• This time proportioning phenomenon controls the ON time and OFF time of
the controller.
• Proportioning action occurs within a proportional band.
• Output is ON within the band (below set point) and OFF outside the band
(above the set point).
Temperature controllers
PID controller:
• Proportional-Integral-Derivative controller.
• It is a closed loop control system.
Conclusion
 Thermocouples,
– Produce a difference voltage in response to a temperature gradient
developed along its length.
– Must be referenced to a known temperature reference, a ‘cold junction’
for accurate measurement.
– Requires linearization for best over-temperature linearity response.
 Resistance temperature devices,
 RTD produce fast response than thermocouples at low temperatures and is
accurate and stable when compared to other sensors.
 Thermistors are sensitive and less expensive compared to RTDs.
END
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