1. ECGR 6185 Advanced Embedded Systems TEMPERATURE SENSORS (Thermocouples, RTDs and Thermistors) University Of North Carolina Charlotte Karunakar Reddy Gujja

2. 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.

3.  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

4.  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. Temperature sensors

5. 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.

6. Different types of thermocouples: TypeCompositionRangeGood forNot recommended for CostSensitivity Type KChromel (Ni-Cr alloy) / Alumel (Ni- Al alloy) −200 °C to 1200 °COxidizing or neutral applications Use under 540ºC Low (11.65$ to 48.63$) 41 µV/°C Type EChromel / Constantan (Cu-Ni alloy)−200 °C to 900 °COxidizing or inert applications Low 68 µV/°C Type JIron / Constantan−40 °C to 750 °CVacuum, reducing, or inert apps Oxidizing or humid environments Low52 µV/°C Type NNicrosil (Ni-Cr-Si alloy) / Nisil (Ni-Si alloy) −270 °C to 1300 °COxidizing or neutral applications Low 39 µV/°C Type TCopper / Constantan−200 °C to 350 °COxidizing, reducing or inert apps Wet or humid environments Low43 µV/°C Type RPlatinum /Platinum with 13% Rhodium0 °C to 1600 °CHigh temperaturesShock or vibrating equipment High10µV/°C Type SPlatinum /Platinum with 10% Rhodium0 °C to 1600 °CHigh temperaturesShock or vibrating equipment High10µV/°C Type BPlatinum-Rhodium/Pt-Rh50 °C to 1800 °CHigh temperaturesShock or vibrating equipment High10µV/°C

7. 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).

8. 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

9. 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.

10. Characteristics of Thermocouples:

12. Precautions and considerations for using thermocouples: –Connection problems –Lead Resistance –Decalibration –Noise –Common Mode Voltage –Thermal Shunting

13. Thermocouples Advantages: –Self-powered –Simple in construction –Rugged –Wide temperature range –Wide variety –Inexpensive Disadvantages: –Non-linear –Low voltage –Less stable –Reference required

14. 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

15. 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)

16. 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) )

17. 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

18. Applications: They are used for precision process temperature control. Widely used in industrial applications. Directly used in recorders, temperature controllers, transmitters and digital ohmmeters RTDs

19. 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).

20. Advantages: High output Fast response Two wire ohms measurement Disadvantages: Non-linear Limited temperature range Not self-powered Self heating Thermistors Thermistor symbol

21. Applications: Can be used as a liquid level indicator or as a liquid level controller To measure temperature in Medical Applications Temperature Control Thermistors

22. 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.

23. 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

24. 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.

25. 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).

26. Temperature controllers PID controller: Proportional-Integral-Derivative controller. It is a closed loop control system.

27. 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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