1. ELECTRICAL METHODS TO MEASURE TEMPERATURE

2. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
OVERVIEW
WHAT IS TEMPERATURE?
METHODS TO MEASURE TEMPERATURE
THERMISTOR
THERMOCOUPLE
RESISTANCE TEMPERATURE DETECTOR
ELECTRICAL METHODS OF TEMPERATURE MEASUREMENT
APPLICATIONS
REFERENCES
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

3. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
WHAT IS TEMPERATURE?
Temperature is a measure of the tendency of an object to spontaneously give up energy to its surroundings. When two objects are in thermal contact, the one that tends to spontaneously lose energy is at the higher temperature.
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

4. METHODS TO MEASURE TEMPERATURE
A few electrical methods to measure temperature are:
Thermistor
Thermocouple
RTD
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

5. Desirable Temperature Sensor Characteristics
ACCURATE
FAST RESPONSE
REPEATABLE
EASY CALIBRATION
WIDE TEMPERATURE RANGE
ELECTRICAL TEMPERATURE SENSOR
CHEAP
SIMPLE RELATIONSHIP
SENSOR OUTPUT  TEMPERATURE
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

6. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
Thermistor
Thermistor – a resistor whose resistance changes with temperature
Resistive element is generally a metal-oxide ceramic containing Mn, Co, Cu, or Ni
Packaged in a thermally conductive glass bead or disk with two metal leads
THERMISTOR = Thermal resistor
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

7. Negative Temperature Coefficient
Most materials exhibit a negative temperature coefficient (NTC)
Resistance drops with temperature
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

8. Converting Resistance to Temperature
The Steinhart-Hart Equation relates temperature to resistance
T is the temperature (in Kelvin)
R is the resistance at T and Rref is resistance at Tref
A1, B1, C1, and D1 are the Steinhart-Hart Coefficients
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

9. Thermistor Resistance (RT)
A thermistor produces a resistance (RT), which must be converted to a voltage signal
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

10. Power Dissipation in Thermistors
A current must pass through the thermistor to measure the voltage and calculate the resistance
The current flowing through the thermistor generates heat because the thermistor dissipates electrical power
P = I2RT
The heat generated causes a temperature rise in the thermistor
This is called Self-Heating
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

11. Thermistor Signal Conditioning Circuit
A voltage divider and a unity gain buffer are required to measure temperature in the lab
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

12. APPLICATIONS OF THERMISTOR
Temperature Control
Thermistors are used for high-accuracy temperature sensing, control, and compensation in medical, industrial, and automotive applications. They offer long-term stability and reliability over a broad temperature range.
NTC Thermistors used in Tesla Hyperloop Pod– Temperature Control
Learn how Tesla used NTC thermistor in the Hyper loop Pod project to create a heat-map to monitor and control temperatures throughout the Pods motor
Temperature Compensation Circuits– Temperature Compensation
Temperature compensation is a common problem among coils, or solenoids. These metals exhibit a positive temperature coefficient with rising temperature. Learn how a fixed parallel resistor is used to bring the temperature coefficient down to a usable limit.
Thermistor Fan Control in Gaming Applications– Temperature Control
NTC thermistors are expanding into new and unknown territories where they function very well. This year, EVGA; an American computer hardware company, redesigned its leading cooler fan to better monitor and cool power components with the added use of NTC thermistor
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

13. Resistive Temperature Detector (RTD)
RTD: Two potential barrier sandwiching a well region.
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

14. Resistive Temperature Detector (RTD)
Two terminal device
Usually made out of platinum
Positive temperature coefficient
Tends to be linear
R = R0(1+α)(T-T0) where T0 = 0oC
R0 = 100 Ω, α = Ω/ Ω oC
At 10oC, R = 100( )(10) = Ω
They are best operated using a small constant current source
Accuracy of 0.01 oC
They are expensive
SOURCE:
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

15. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
The RTD wire is a pure material, typically platinum, nickel, or copper.
The material has an accurate resistance/temperature relationship which is used to provide an indication of temperature.
As RTD elements are fragile, they are often housed in protective probes.
The three main categories of RTD sensors are thin-film, wire-wound, and coiled elements.
Other more exotic shapes are used; for example, carbon resistors are used at ultra-low temperatures (−173 °C to −273 °C).
Carbon resistor elements are cheap and widely used. They have very reproducible results at low temperatures.
Strain-free elements use a wire coil minimally supported within a sealed housing filled with an inert gas.
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

16. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
THIN FILM ELEMENTS
Have a sensing element that is formed by depositing a very thin layer of resistive material on a ceramic substrate.
This layer is usually 1 to 10 nanometers thick. 
This film is then coated with an epoxy or glass that helps protect the deposited film.
They are not very stable and can only be used over a limited temperature range.
These elements work with temperatures to 600 °C when suitably encapsulated in glass or ceramic.
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

17. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
WIRE WOUND ELEMENTS
Have greater accuracy.
The coil diameter provides a compromise between mechanical stability and allowing expansion of the wire to minimize strain.
The winding core must be electrical insulator.
The coefficient of thermal expansion of the winding core material is matched to the sensing wire.
This wire is selected to be compatible with the sensing wire, so that the combination does not generate an emf that would distort the thermal measurement.
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

18. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
COILED ELEMENTS
The design has a wire coil that can expand freely over temperature, held in place by some mechanical support.
The sensing wire expands and is contract free.
The basis of the sensing element is a small coil of platinum sensing wire.
This coil resembles a filament in an incandescent light bulb.
The coil is inserted in the housing of the mandrel and then packed with a very finely ground ceramic powder.
Work with temperatures to 850 °C.
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

19. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
Resistance thermometers are constructed in a number of forms and offer greater stability, accuracy and repeatability in some cases than thermocouples.
RTD use electrical resistance and require a power source to operate. The resistance ideally varies nearly linearly with temperature per the Callendar-Van Dusen equation.
The platinum detecting wire needs to be kept free of contamination to remain stable.
A platinum wire or film is supported on a former in such a way that it gets minimal differential expansion or other strains from its former, yet is reasonably resistant to vibration.
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

20. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
CONSTRUCTION
These elements nearly always require insulated leads attached.
Above this, glass fibre or ceramic are used.
The measuring point, and usually most of the leads, require a housing or protective sleeve, often made of a metal alloy that is chemically inert to the process being monitored.
Designing protection sheaths is important iy must withstand chemical or physical attack and provide convenient attachment points.
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

21. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
APPLICATION OF RTD
Air conditioning and refrigeration servicing
Food Processing
Stoves and grills
Textile production
Plastics processing
Petrochemical processing
Micro electronics
Air, gas and liquid temperature measurement
Exhaust gas temperature measurement
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

22. COMPARING RTD WITH THERMISTOR
Expensive
Slow
Needs I source
Self-heating
4-wire meas.
RTD
Most accurate
Most stable
Fairly linear
Thermistor
High output
Fast
2-wire meas.
Very nonlinear
Limited range
Needs I source
Self-heating
Fragile
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
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23. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
Thermocouple
Thermocouple – a two-terminal element consisting of two dissimilar metal wires joined at the end
SOURCE:
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

24. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
The Seebeck Effect
Seebeck Effect – A conductor generates a voltage when it is subjected to a temperature gradient
Measuring this voltage requires the use of a second conductor material
The relationship between temperature difference and voltage varies with materials
The voltage difference of the two dissimilar metals can be measured and related to the corresponding temperature gradient + -
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

25. VS = SΔT Nickel-Chromium Alloy Copper-Nickel Alloy 20-11-2017
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

26. Measuring Temperature
To measure temperature using a thermocouple, you can’t just connect the thermocouple to a measurement system (e.g. voltmeter)
The voltage measured by your system is proportional to the temperature difference between the primary junction (hot junction) and the junction where the voltage is being measured (Ref junction)
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

27. Ice Bath Method (Forcing a Temperature)
Thermocouples measure the voltage difference between two points
To know the absolute temperature at the hot junction, one must know the temperature at the Ref junction
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

28. Nonlinearity in the Seebeck Coefficient
Thermocouple output voltages are highly nonlinear
The Seebeck coefficient can vary by a factor of 3 or more over the operating temperature range of the thermocouples
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

29. Cold Junction Compensation
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

30. APPLICATION OF THERMOCOUPLE
K (-270 to +1370°C / -454 to +2498°F)
The Type K is a 'general purpose' thermocouple with a wide temperature range. With a variety of probe types available, it is suitable for use across many industries and processes.
Testing temperatures associated with process plants e.g. chemical production and petroleum refineries
Testing of heating appliance safety
Type J (-210 to +1200°C / -346 to +2192°F)
The Type J is a popular thermocouple that is commonly used to monitor temperatures of inert materials and in vacuum applications. This thermocouple is susceptible to oxidisation so is not recommended for damp conditions or low temperature monitoring. (Note the accuracy of this sensor may be permanently impaired if used above 760°C.)
Monitoring in a vacuum and for inert metals
Hot processes including plastics and resin manufacture
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31. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
Type T (-270 to +400°C / -454 to +752°F)
The Type T is used widely in the food industry, mainly due to the high level of accuracy it provides and because it performs well in the presence of moisture without oxidising. If in general a lower range temperature measurement is required, the Type T is popular choice.
Monitoring in food processing and production to identify potential food safety hazards and comply with HACCP regulations
Suitable for low temperature and cryogenic applications
Type N (-270 to +1300°C / -454 to +2372°F)
The Type N also has a wide temperature range, but is better suited to high temperature monitoring than the Type K because it is more stable and resists oxidisation.
Temperature profiling in ovens, furnaces and kilns
Temperature measurement of gas turbine and engine exhausts
Monitoring of temperatures throughout the production and smelting process in the steel, iron and aluminium industry
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

32. Temperature Measurement Devices
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THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

33. Tracking the Rate of Temperature Change
If a slow sensor is placed into a rocket that is launched to a high altitude, the sensor may not be able to track the rate of temperature change
A critical property of a temperature- measurement device is how quickly it responds to a change in external temperature
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

34. THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS
References
Previous E80 Lectures and Lecture Notes
Thermcouples White Paper
University of Cambridge Thermoelectric Materials for Thermocouples
National Instruments Temperature Measurements with Thermocouples: How-To Guide
ocouples.pdf
Vishay NTCLE100E3104JB0 Data Sheet
THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS