ELECTRICAL METHODS TO MEASURE TEMPERATURE

THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS OVERVIEW WHAT IS TEMPERATURE? METHODS TO MEASURE TEMPERATURE THERMISTOR THERMOCOUPLE RESISTANCE TEMPERATURE DETECTOR ELECTRICAL METHODS OF TEMPERATURE MEASUREMENT APPLICATIONS REFERENCES 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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. 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

METHODS TO MEASURE TEMPERATURE A few electrical methods to measure temperature are: Thermistor Thermocouple RTD 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

Desirable Temperature Sensor Characteristics ACCURATE FAST RESPONSE REPEATABLE EASY CALIBRATION WIDE TEMPERATURE RANGE ELECTRICAL TEMPERATURE SENSOR CHEAP SIMPLE RELATIONSHIP SENSOR OUTPUT  TEMPERATURE 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

Negative Temperature Coefficient Most materials exhibit a negative temperature coefficient (NTC) Resistance drops with temperature 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

Thermistor Resistance (RT) A thermistor produces a resistance (RT), which must be converted to a voltage signal 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

Thermistor Signal Conditioning Circuit A voltage divider and a unity gain buffer are required to measure temperature in the lab 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

Resistive Temperature Detector (RTD) RTD: Two potential barrier sandwiching a well region. 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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 = 0.03385 Ω/ Ω oC At 10oC, R = 100(1+0.385)(10) = 103.85 Ω They are best operated using a small constant current source Accuracy of 0.01 oC They are expensive SOURCE: http://www.omega.com/prodinfo/images/RTD_diag1.gif 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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. 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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. 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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. 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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. 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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. 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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. 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS 025

THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS Thermocouple Thermocouple – a two-terminal element consisting of two dissimilar metal wires joined at the end SOURCE: http://upload.wikimedia.org/wikipedia/en/e/ed/Thermocouple_(work_diagram)_LMB.png 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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 + - 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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

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) 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

Cold Junction Compensation 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

Temperature Measurement Devices > 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

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 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS

THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS References Previous E80 Lectures and Lecture Notes http://www.eng.hmc.edu/NewE80/TemperatureLec.html Thermcouples White Paper http://www.ohio.edu/people/bayless/seniorlab/thermocouple.pdf University of Cambridge Thermoelectric Materials for Thermocouples http://www.msm.cam.ac.uk/utc/thermocouple/pages/ThermocouplesOperatingPrinciples.html National Instruments Temperature Measurements with Thermocouples: How-To Guide http://www.technologyreview.com/sites/default/files/legacy/temperature_measurements_with_therm ocouples.pdf Vishay NTCLE100E3104JB0 Data Sheet http://www.eng.hmc.edu/NewE80/PDFs/VIshayThermDataSheet.pdf 20-11-2017 THERMO - ELECTRICAL TEMPERATURE MEASUREMENTS