ECGR 6185 Advanced Embedded Systems

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ECGR 6185Advanced Embedded Systems

• TEMPERATURE SENSORS(Thermocouples, RTDs and
  Thermistors)
• University Of North Carolina Charlotte
• Karunakar Reddy Gujja

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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.

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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.

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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.

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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.

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Different types of thermocouples
Type Composition Range Good for Not recommended for Cost Sensitivity
Type K Chromel (Ni-Cr alloy) / Alumel (Ni-Al alloy) -200 C to 1200 C Oxidizing or neutral applications Use under 540ºC Low (11.65 to 48.63) 41 µV/C
Type E Chromel / Constantan (Cu-Ni alloy) -200 C to 900 C Oxidizing or inert applications Low 68 µV/C
Type J Iron / Constantan -40 C to 750 C Vacuum, reducing, or inert apps Oxidizing or humid environments Low 52 µV/C
Type N Nicrosil (Ni-Cr-Si alloy) / Nisil (Ni-Si alloy) -270 C to 1300 C Oxidizing or neutral applications Low 39 µV/C
Type T Copper / Constantan -200 C to 350 C Oxidizing, reducing or inert apps Wet or humid environments Low 43 µV/C
Type R Platinum /Platinum with 13 Rhodium 0 C to 1600 C High temperatures Shock or vibrating equipment High 10µV/C
Type S Platinum /Platinum with 10 Rhodium 0 C to 1600 C High temperatures Shock or vibrating equipment High 10µV/C
Type B Platinum-Rhodium/Pt-Rh 50 C to 1800 C High temperatures Shock or vibrating equipment High 10µV/C
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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).

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Thermocouples

• Calculations
• The voltage generated by the thermocouple is
  given by the equation
• V S ?T
• Where, V voltage measured (V)
• S Seebeck coefficient (V/C)
• ?T difference in temperature between
  two junctions
• Hence the unknown temperature can be calculated
  using the equation,
• T Tref V/S in C

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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.

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Characteristics of Thermocouples
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Characteristics of Thermocouples
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Precautions and considerations for using
thermocouples

• Connection problems
• Lead Resistance
• Decalibration
• Noise
• Common Mode Voltage
• Thermal Shunting

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Thermocouples

• Advantages
• Self-powered
• Simple in construction
• Rugged
• Wide temperature range
• Wide variety
• Inexpensive

• Disadvantages
• Non-linear
• Low voltage
• Less stable
• Reference required

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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

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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)

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RTDs

• Calculations
• R(T)R0(1aT bT2)
• 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 RTDs
  resistance is given by
• V (VrefR(T))/(R(0)R(T))

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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

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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

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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).

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Thermistors
Thermistor symbol

• Advantages
• High output
• Fast response
• Two wire ohms measurement

• Disadvantages
• Non-linear
• Limited temperature range
• Not self-powered
• Self heating

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Thermistors

• Applications
• Can be used as a liquid level indicator or as a
  liquid level controller
• To measure temperature in Medical Applications
• Temperature Control

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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.

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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

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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.

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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).

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Temperature controllers

• PID controller
• Proportional-Integral-Derivative controller.
• It is a closed loop control system.

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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.

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• END
• THANK YOU