Temperature Measurements Lecture Notes PDF

Temperature Measurements  Temperature Sensing Techniques Changes in Physical Dimensions Liquid-in-glass thermometers Bi-metallic thermometers Constant-volume gas thermometers https://circuitglobe.com/wp-content/uploads/2018/02/bimetallic-str...

Temperature Measurements  Temperature Sensing Techniques Changes in Physical Dimensions Liquid-in-glass thermometers Bi-metallic thermometers Constant-volume gas thermometers https://circuitglobe.com/wp-content/uploads/2018/02/bimetallic-strip-fixed-at-one- end.jpg Changes in Electrical Properties Thermocouples Resistance Temperature Detectors (RTDs) Thermistors https://upload.wikimedia.org/wikipedia/commons/thumb/4/41/ Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg/370px- Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg.png Integrated Circuitry (IC) Transistors and Diodes Changes in Emitted thermal radiation Infrared Pyrometers 3  Temperature Measurements The different sensing techniques used to measure temperature The advantages/limitations of each technique Properly select a temperature sensor for a given measurement situation Master the use of thermocouples, RTD’s, thermistors, etc…. 4  Liquid-in-Glass Thermometers The volumetric expansion of liquids and solids is used for temperature measurement Made of glass to see the expanding liquid clearly It has a bulb with a long capillary tube to hold the liquid Expansion observed is actually the difference between the expansion of the liquid and of the glass No need for additional indicator Inexpensive, simple and portable X Calibrated at a certain immersion depth X Not suitable for distant reading X Not suitable for surface temperature measurement 5  Liquid-in-Glass Thermometers This liquid is usually mercury or alcohol. Mercury: Clearly visible with light silvery colour Freezing point: - 37 C Boiling point: 356 C Accuracy ± 0.3 °C Mercury does not evaporate easily -> more durable than alcohol thermometer Does not wet the wall of the thermometer -> Better readability than alcohol X Toxic if the bulb breaks and mercury leaks out X Cannot measure cold temperatures 6  Liquid-in-Glass Thermometers Alcohol, ethanol, toluene, kerosene, etc…depending on the range of temperature required. Originally colourless -> dyed to give it a bright colour. Suitable to measure low temperatures Less toxic than mercury thermometer -> safer operation Larger value of temperature coefficient of expansion than a mercury -> larger sensitivity Less expensive than a mercury thermometer. X Less durable because alcohol evaporates faster than mercury X Cannot measure high temperature because of low boiling point 7  Bimetallic Thermometer Range of use: -65 to 430 °C Accuracy: ± 0.5 to 12°C Low cost Low maintenance Stable operation over time X Response time is mechanical in nature en.wikipedia.org/wiki/Bimetallic_strip#/ media/File:Bimetal_coil_reacts_to_lighter.gif 8  Bimetallic Thermometer Two different metals with different coefficients of thermal expansion are bonded together -> unequal expansion of the two metals -> strip will curl If one end is fixed the other end displaces in response to temperature changes. reviseomatic.org/help/e-components/Bimetal_Strip.gif Can be directly used in on/off temperature control (thermostats) www.electrical-forensics.com/BiMetal/Bimetal/BimetalStrip3-LG.jpg 9  Bimetallic Thermometer Bimetal strips can be fabricated into coils, spirals, and disks. www.polytechnichub.com/wp-content/uploads/2017/07/bimetal- temperature-gauge.jpg www.polytechnichub.com/wp-content/uploads/2017/07/ 10 bimetal-temperature-gauge.jpg  Bimetallic Thermometer r = Radius of curvature t=total thickness m = ratio of thicknesses Low/high expansion materials n=ratio of Young moduli of elasticity Low/high expansion materials 1=lower coefficient of thermal expansion, 1/°C 2=higher coefficient of thermal expansion, 1/°C T=Temperature, °C To=Initial bonding temperature, °C 11  Constant-volume gas thermometer A fluid filled bulb is connected to a pressure measuring device via a capillary tube. As fluid is heated it tries to expand -> pressure increases. Pressure is linked to temperature. Accuracy and range depends on fluid. Advantages/Disadvantages Low cost Stable in operation Widely used in industrial applications. Transient response is a function of bulb size and capillary tube length. 12  Temperature Sensing Techniques Changes in Physical Dimensions Liquid in Glass Thermometers Bimetallic Thermometers Constant-volume gas thermometer https://circuitglobe.com/wp-content/uploads/2018/02/bimetallic-strip-fixed-at-one- end.jpg Changes in Electrical Properties Thermocouples Resistance Temperature Detectors (RTDS) Thermistors https://upload.wikimedia.org/wikipedia/commons/thumb/4/41/ Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg/370px- Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg.png Integrated Circuitry (IC) Transistors and Diodes Changes in Emitted thermal radiation Infrared Pyrometers 13  Thermocouples An electrical device consisting of two dissimilar metals forming an electrical junction. A thermocouple produces a voltage proportional to the difference between its hot and cold temperatures -> this voltage can be used to measure temperature. Thermocouples are widely used for temperature measurements https://upload.wikimedia.org/wikipedia/commons/thumb/4/41/ www.engineeringtoolbox.com/docs/documents/496/ Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg/370px- thermocouples.png Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg.png 14  Thermocouples: Grounded vs Ungrounded vs Exposed Junctions  Grounded junctions -> improved thermal conductivity -> quickest response time  Grounded junctions -> TC circuits more susceptible to electrical noise https://1igagd8yobp3o9y7m1f0yd11-wpengine.netdna-ssl.com/wp- content/uploads/2015/01/3.png  Ungrounded junctions -> Not susceptible to electrical noise but slower dynamic response than grounded junctions  Exposed junctions-> fastest response time but limited to noncorrosive and non- pressurized applications blog.wika.us/files/2017/12/ungrounded-thermocouple.jpg 15 Thermocouples: Law of intermediate temperatures If two junctions at temperatures T1 and T2 produce an output voltage V12, and temperatures T2 and T3 produce voltage V23, then temperatures T1 and T3 will produce a voltage V13 = V12 + V23 https://upload.wikimedia.org/wikipedia/commons/thumb/4/41/ Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg/370px- Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg.png Allows a thermocouple that is calibrated with a https://instrumentationtools.com/wp-content/uploads/2016/05/ instrumentationtools.com_law-of-intermediate-temperatures-principle.png reference temperature (usually 0 C) to be used with another reference temperature (usually the ambient temperature), without the need to use an ice-bath 16 Thermocouples: Law of intermediate metals  A third metal may be inserted into a thermocouple system with hot and cold temperatures T1 and T2 without affecting the eleceng.dit.ie/gavin/Instrument/Temperature/Laws%20of %20ThermoC_files/image002.gif output voltage as long as the two new junctions with the third metal are kept at the same temperature T3 According to this law, voltmeter leads can be added to the circuit without affecting the output voltage from the thermocouple https://cdn.instrumentationtools.com/wp-content/uploads/2016/05/ instrumentationtools.com_law-of-intermediate-metals-principle.png 17 The output voltage of a thermocouple system depends only on the temperatures of the hot and cold junctions (T1 and T2) and is independent of the temperatures of the wires connecting the junctions. If two thermocouple junctions are at T1 and T2, then the thermal emf generated is independent and unaffected by any temperature distribution along the wires. According to this law, the leads connecting the thermocouple to the voltmeter can be exposed to temperature fluctuations without affecting the measurement https://cdn.instrumentationtools.com/wp-content/uploads/2016/05/ instrumentationtools.com_law-of-homogeneous-circuits.png eleceng.dit.ie/gavin/Instrument/Temperature/Laws%20of%20ThermoC_files/ image001.gif 18  Signal conditioning in Thermocouples Output signal from a thermocouple is: Low output voltage + Low sensitivity -> thermocouple amplifier may be used Output voltage  (Th – Tc) -> cold-junction compensation is required Non-linear output -> electronic circuits may be used for linearization https://upload.wikimedia.org/wikipedia/commons/thumb/4/41/ Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg/370px- Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg.png Analog signal -> Analog-to-digital conversion may be used upload.wikimedia.org/wikipedia/commons/thumb/c/c2/ RTD_3Wire.svg/730px-RTD_3Wire.svg.png 19  Signal Conditioning for Thermocouples Cold Junction Compensation Any error in reading the cold junction temperature will cause an error in the final temperature measurement. Many instruments have an internal temperature sensor (e.g., thermistor, LT1025A solid state temperature measurement device) that measures the actual cold junction temperature and implements cold junction compensation-> In this case, no ice- bath is required OR: Measure the cold junction temperature and then apply the law of intermediate temperatures www.engineeringtoolbox.com/docs/documents/496/ thermocouples.png Cold junction MUST be at 0 C (ice-bath) because calibration tables are provided at this reference 20 https://upload.wikimedia.org/wikipedia/commons/thumb/4/41/ -> Must use an ice bath unless cold Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg/370px- Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg.png junction compensation is used or a  Signal Conditioning for Thermocouples Cold Junction Compensation Proper use of thermocouple requires that the cold junction must be kept at 0C  This is inconvenient Another solution is to use electronic cold https://instrumentationtools.com/wp-content/uploads/2017/12/ Thermocouple-cold-junction-compensation.png junction compensation or to apply the law of intermediate temperatures https://upload.wikimedia.org/wikipedia/commons/thumb/4/41/ Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg/370px- https://instrumentationtools.com/wp-content/uploads/2017/12/ Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg.png Thermocouple-cold-junction-compensation.png 2121  Signal Conditioning for Thermocouples: AD954/AD595 Thermocouple Instrumentation Amplifier https://lh3.googleusercontent.com/proxy/ 8GhWgsJpCF2LWCaAS_bbjDnR8WbvFVI- nQesFnqPeWUS64bddJ8aHraHLof2UI968yqMntoWH42MI0iZm15B1XDw 22 ZrU3V1mUnPbJ46Lb 22  Thermocouple Amplification of Thermocouple Signals: Instrumentation Amplifiers  Instrumentation Amplifiers (in-amps) are very high gain differential amplifiers which have a high input impedance and a single ended output  Mainly used to amplify very small differential signals from strain gauges or thermocouples 23  Standards for Thermocouples  IEC 60584-1 : Thermocouples: basic values of the thermoelectric voltages  IEC 60584-2: Thermocouples: tolerance values of the thermoelectric voltages  IEC 60584-3: Thermocouples: Thermocouple cables and compensating cables  ASTM E230: Standard specification and temperature- electromotive force (EMF) tables for standardised thermocouples  ASTM E2846- 20: Standard Guide for Thermocouple https://www.astm.org/Standards/E2846.ht Verification 24  Thermocouple Classes https://tmseurope.co.uk/applications/thermocouple-rtd-colour-codes- 25  Thermocouples: Example #1 A type K thermocouple is used to measure the temperature in a freezer compartment. The reference junction is connected properly to an ice bath, and the voltage output from the thermocouple circuit is - 2.721 mV. Estimate the temperature in the freezer compartment. 26  Thermocouples: Example #2 Given: A type S thermocouple is placed into an oven to measure the temperature. An ice bath is used as a reference junction. The output voltage is 4.005 mV. (a) Estimate the temperature of the oven (b) If ambient air (20°C) was used as the reference junction instead of the ice bath, what voltage would the thermocouple read? 27  Thermocouples: Example #2 Given: A type S thermocouple is placed into an oven to measure the temperature. An ice bath is used as a reference junction. The output voltage is 4.005 mV. (a) Estimate the temperature of the oven (b) If ambient air (20°C) was used as the reference junction instead of the ice bath, what voltage would the thermocouple read? 28  Thermocouples: Advantages  Wide temperature range: Depending on the metal wires used, a thermocouple is capable of measuring temperature in the range -200°C to +2500°C -> wide temperature ranges, from cryogenics to jet-engine exhaust  Rugged devices -> immune to shock and vibration and suitable for use in hazardous environments.  Small with low thermal capacity -> fast dynamic response, especially if the sensing junction is exposed.  Active devices -> No self heating -> not prone to self heating and are intrinsically safe  Active devices -> intrinsically safe www.engineeringtoolbox.com/docs/documents/496/ thermocouples.png 29  Thermocouples: Limitations X Complex signal conditioning: Cold junction compensation, amplification and linearization X Accuracy: inherent inaccuracies due to their metallurgical properties + measurement is only as upload.wikimedia.org/wikipedia/commons/thumb/c/cb/UTP_cable.jpg/ accurate as the reference junction temperature can be 220px-UTP_cable.jpg measured, traditionally within 1°C to 2°C. X Susceptibility to corrosion: Because thermocouples consist of two dissimilar metals, in some environments corrosion over time may result in deteriorating accuracy -> apply protection X Small signal range -> susceptibility to noise. Action plan: avoid noise from stray electrical and magnetic fields; Twist the thermocouple wire pair to cdn.shopify.com/s/files/1/0869/5892/files/shielded-cable-blog.jpg? reduce magnetic field pickup; Use a shielded cable or v=1499255103 run the wires in metal conduit to reduce electric field pickup; apply filtering (either in hardware or by software) with strong rejection of the line frequency (50 Hz/60 Hz) and its harmonics. 30  Resistance Temperature Detectors RTDs Nick R(250C) Each metal has a specific resistivity, , which varies R(T) el with temperature and is determined Platinu experimentally. m The change in resistance can be used to estimate the temperature Uses metal conductors (typically a fine platinum T(0C wire winding or thin metallic layer applied to a Sensitivity = dR/dT ) substrate) RTD: Metal resistance increases almost linearly with L temperature R  Positive coefficient of resistance A  = o [1 + (T-To)+ …] 31  Resistance Temperature Detectors RTDs  = o [1 + (T-To)] Nick R(250C) R(T) Wide operating range (-200 °C to 850 °C) el Platinu L m High sensitivity (compared to thermocouples) R  A High accuracy (±0.0006 °C to 0.1°C) T(0C High repeatability and stability (drift of 0.0025 Sensitivity = dR/dT ) °C/year in lab models) Industrial models drift < 0.1 °C/year Slower response times than thermocouples Sensitive to shock and vibration upload.wikimedia.org/wikipedia/commons/thumb/c/c2/ RTD_3Wire.svg/730px-RTD_3Wire.svg.png 32 Resistance Temperature Detectors RTDs To measure resistance must pass current through sensor -> self-heating = I 2R Low resistance: 100 (most common) to 1000 > Hence lead wire issues upload.wikimedia.org/wikipedia/commons/thumb/c/c2/ RTD_3Wire.svg/730px-RTD_3Wire.svg.png 33  Signal conditioning in RTDs Output signal from a RTD is: A change in resistance -> Wheatstone bridge and bridge excitation are required Low resistance (100  typical) -> resistances of lead wires are an issue -> Four-wire or three-wire resistance measurements may be used Low sensitivity -> amplifier may be used after the Wheatstone bridge 50/60 Hz may be present -> filter may be used after the amplifier Analog signal -> Analog-to-digital conversion may be used after the filter upload.wikimedia.org/wikipedia/commons/thumb/c/c2/ RTD_3Wire.svg/730px-RTD_3Wire.svg.png Non-linear output -> Linearization may be employed 34  Thermistors  Made from semiconductor materials  Resistance is highly nonlinear  Mostly negative temperature coefficient (NTC) https://sc02.alicdn.com/kf/ HTB1jJHYogLD8KJjSsze761GRpXaG.png 35  Thermistors  Operating range is between -200 °C and 1000°C (using different units)  High resistance 1 k to 100 k ->no lead resistance 1 1  issues       T To  R R o e  Small physical size -> Fast response time  Lower cost than RTD’s where R o = reference resistance measured at T o  Very high sensitivity and resolution -> Up to 1000 T = measured temperature times more sensitive than RTD’s  = material constant  Not sensitive to shock and vibration X Highly non-linear resistance-to-temperature relationships X Narrow operating range for a single unit X More susceptible to internal/self heating issues than RTD’s 36 Thermistors 37  Signal Conditioning in Thermistors Output signal from a thermistor is: A change in resistance -> Wheatstone bridge and 1 1       bridge excitation are required to induce a change in  T To  voltage R R o e Very non-linear output -> Linearization may be where required R o = reference resistance measured at T o  High sensitivity T = measured temperature  High resistance -> resistances of the lead wires are  = material constant not an issue 50/60 Hz may be present -> filter may be required upload.wikimedia.org/wikipedia/commons/thumb/c/c2/ Analog signal -> Analog-to-digital conversion may be RTD_3Wire.svg/730px-RTD_3Wire.svg.png required 38  Temperature Integrated Circuits Semiconductor temperature sensors are produced in the form of ICs. Made of semiconductors with temperature-sensitive voltage vs. current characteristics The output is linearly proportional to the absolute temperature. upload.wikimedia.org/wikipedia/ An input voltage must be applied to the commons/5/5c/Microchips.jpg sensor -> hence a power supply is required -> passive sensor The use of IC temperature sensors is limited to applications where the temperature is within a –55° to 39  Temperature Integrated Circuits  Can be used in cold junction compensating temperature circuits.  Accuracy is about 0.5 °C https://upload.wikimedia.org/wikipedia/commons/thumb/4/41/  Lead-wire errors are minimal Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg/370px- Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg.png  Small, accurate, and inexpensive  Easy to interface with other devices such as amplifiers, regulators, DSPs, and microcontrollers X Small measurement range as compared to that of thermocouples 40 Electric Temperature Sensing Techniques: Summary 41  Temperature Sensing Techniques Changes in Physical Dimensions Liquid-in-glass thermometers Bi-metallic thermometers Constant-volume gas thermometers https://circuitglobe.com/wp-content/uploads/2018/02/bimetallic-strip-fixed-at-one- end.jpg Changes in Electrical Properties Thermocouples Resistance Temperature Detectors (RTDS) Thermistors https://upload.wikimedia.org/wikipedia/commons/thumb/4/41/ Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg/370px- Thermocouple_circuit_Ktype_including_voltmeter_temperature.svg.png Integrated Circuitry (IC) Transistors and Diodes Changes in Emitted thermal radiation Infrared Pyrometers 42  Radiation and Infrared Pyrometers All objects above zero Kelvin emits radiation Infra red pyrometers measures the radiant heat and infers the temperature Emissivity must be known https://upload.wikimedia.org/ wikipedia/commons/2/2a/ 1024_Pyrometer-8445.jpg 43  Radiation and Infrared Pyrometers  Non-contact measurement  Moving objects can be studied  Pattern observation  Collection of large amounts of thermal data in a 2-D plan  Measurement through hazardous atmospheres upload.wikimedia.org/wikipedia/ commons/c/cf/Ir_girl.png 44 Temperature Sensors: Primary Calibration 45  Temperature Sensors: Selection Guides Availability? Can provide a control action? Stability and drift? Cost? Remote reading? Hysteresis? Range? Analog/Digital display? Dynamic response Accuracy? Interface with other devices: Precision? time? Computers, Amplifiers, Regulators, Sensitivity? Heating issues? Resolution? Digital signal processors, and Rugged? Configurations? microcontrollers? Moving objects? Signal-conditioning needed? Large amounts of measured data? Linear output? Hazardous atmospheres? Self-powered? Sensitivity to shock and vibration? Is excitation needed? Is a reference required? Lead-wire issues? 46