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Bachelor Degree in Chemical Engineering Course: Process Instrumentation and Control

Bachelor Degree in Chemical Engineering Course: Process Instrumentation and Control Measuring devices of the main process variables TEMPERATURE MEASUREMENTS Rev. 2.6 – March 13, 2019. MEASURE OF TEMPERATURE. The temperature control is fundamental in process engineer .

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Bachelor Degree in Chemical Engineering Course: Process Instrumentation and Control

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  1. Bachelor Degree in Chemical Engineering Course: Process Instrumentation and Control Measuring devices of the main process variables TEMPERATURE MEASUREMENTS Rev. 2.6 – March 13, 2019

  2. MEASURE OF TEMPERATURE The temperature control is fundamental in process engineer. For this reason it is indispensable to have instrumentation able to measure this important process variable with accuracy, repeatability and short response time. The common devices used in industrial plants operate based on thermomechanic and thermoelectric principles. Among the thermomechanic sensors we have: Liquid-in-glass column thermometer Bourdon tube gauges thermometer Bi-metal mechanic thermometer Among the thermoelectric sensors (usually transducer) we have: Thermocouples Resistance thermometers Thermistors Solid state sensors Thermal flux sensors Pyrometers Process Instrumentation and Control - Prof M. Miccio

  3. ThermocouplesResistance thermometersThermistorsSolid state sensors Process Instrumentation and Control - Prof M. Miccio

  4. Thermoelectric effect SEEBECK EFFECT (1822) When a homogeneous conductor is submitted to a temperature gradient and thus there are two different temperatures between the conductor terminals, a direct voltage E is produced. E 2 1 with T2 > T1 T1 T2 σ = SEEBECK coefficient (intrinsic property of any single conductor material) [=] mV/K E = directvoltage between points “1” and “2” [=] mV Process Instrumentation and Control - Prof M. Miccio

  5. SEEBECK EFFECT •  • PROBLEM ! • Usually, we cannot measure the voltage E from the point “2” because it is hot! • If a voltmeter is connected, a new metal-to-metal junction is introduced, which negatively affects the measure. •  • SOLUTION! • We use 2 conductors of different material connected in the “hot” point (hot junction) • We use the voltmeter and measure the voltage between the 2 “cold” metallic contacts (cold junction) Process Instrumentation and Control - Prof M. Miccio

  6. SEEBECK EFFECT M1 • M1 and M2 two different homogeneous conductors • In a non-closed circuit, with two junctions at different temperatures, the generated fem E is independent of the temperature distribution along the homogeneous wires • It is possible to use a voltmeter with each contact wire of the same metal of the terminal, without altering the measurement made by the thermocouple. Obviously, the wires must be homogeneous and without damage, which would lead to spurious voltages. Coldjunction(2 measuring terminals)atTf HotjunctionatTc> Tf E M2 Process Instrumentation and Control - Prof M. Miccio

  7. THERMOELECTRIC LAWS SUMMARY HOMOGENEOUS CIRCUIT LAW In a "closed" circuit of completely homogeneous material there is no difference in potential, whatever the temperature distribution in it, and even if the thickness of the wires is not constant.  V = 0 Process Instrumentation and Control - Prof M. Miccio

  8. MEASUREMENT SYSTEM WITH THERMOCOUPLE M1  PROBLEM ! The electromotive force E depends on the cold junction temperature Tf, which is not fixed and is subject to variability, even unpredictable. Coldjunction(2 measuring terminals)atTf HotjunctionatTc> Tf E M2 Process Instrumentation and Control - Prof M. Miccio

  9. "IDEAL" MEASUREMENT SYSTEM WITH THERMOCOUPLE  SOLUTION ! We transform the cold junction into a reference junction and keep it at a reference temperature, e.g. Tref = 0°C. The use of the two sections of copper (Cu) wire for the voltmeter, often present to limit the length of the TC wires, does not affect the VACQ reading, being subject to the same temperature difference (Homogeneous Circuit Law). Process Instrumentation and Control - Prof M. Miccio

  10. “REAL" MEASUREMENT SYSTEM WITH THERMOCOUPLE  PROBLEM ! In the measurement of temperature, both in the laboratory and in the industry, it is not practical to keep the "Cold junction” (measurement terminals) a TF = 0°C Measuringdevice Process Instrumentation and Control - Prof M. Miccio

  11. LAW OF INTERMEDIATE TEMPERATURES The electromotive force E generated in a pair with junctions respectively at temperatures T1 and T3 is equal to the sum of electromotive force of two pairs, of material identical to the first, with junctions respectively at T1 and T2 and at T2 and T3, whatever T2 EC = EB+ EA Process Instrumentation and Control - Prof M. Miccio

  12. “REAL" MEASUREMENT SYSTEM WITH THERMOCOUPLE  SOLUTION ! The problem is solved by observing that for TF ≠ 0°C : INTERMEDIATE TEMPERATURES LAW NOTE: Actually, TC measures the difference of temperature between 2 points, but NOT an absolute temperature Process Instrumentation and Control - Prof M. Miccio

  13. “PRACTICAL" MEASUREMENT SYSTEM WITH THERMOCOUPLEsoftware COLD JUNCTION COMPENSATION MEASURINGDEVICE (Voltmeter) The measuring device “reads” VACQ = E(TC,TF) from the hot joint through copper (Cu) wires for the voltmeter TFis measured separately by a different additional (small) sensor A special software (software compensation) or dedicated integrated circuit (hardware compensation) calculates E(TF,0) from the static characteristic of the TC,applies the intermediate temperature law and calculates E(TC,0). The software evaluates TCfrom the static characteristic of the TC Process Instrumentation and Control - Prof M. Miccio

  14. “REAL” MEASUREMENT SYSTEMWITH THEMOCOUPLE  PROBLEM! For the common laboratory and industrial application the cold junction is far from the voltage measurement device (voltmeter, etc.). The TC should be too “long”!  SOLUTION! In the practice, special Thermocouple Wires are used in laboratories and plants Process Instrumentation and Control - Prof M. Miccio

  15. THERMOCOUPLE WIRES www.tc-srl.it • The extension wires consist of wires of the same material or the same thermoelectrical behaviour of the thermocoulple materials. They come from waste of TC production. They are not cheap, but less expensive than thermocouple. They have a good accuracy. • The compensation wires are made of a material different but equivalent to the constructive metals of TC. They are cheaper, but have a worse accuracy. • These cables are commonly used as bipolar wires with an external insulator (PVC, silicon rubber, enameling) selected up to the temperature range. COPPER M1 Extension orcompensationwires Tc VOLTMETER M2 COPPER TERMINALS Process Instrumentation and Control - Prof M. Miccio

  16. THERMOCOUPLE PROPERTIES For TC construction, only those metals should be considered, the properties of which meet, at least partially, the following requirements: • High thermoelectric voltage, with quite linear variation in relation to the temperature; • Constancy of the thermoelectric characteristic over time; • Ability to preserve the thermoelectric characteristics unchanged due to mechanical deformations; • Resistance to aggressive, oxidizing or reducing environments; • Ductility, in order to obtain even thin threads; • Low cost; • Usability even at high temperatures; • Very large T range. Unable to combine all these features into a single thermocouple, different types of thermocouples were introduced. Process Instrumentation and Control - Prof M. Miccio

  17. International standards define different TC types characterizing them with the relative E(Tc,0) THERMOCOUPLE TYPES From Magnani, Ferretti e Rocco (2007) costantan = Cu-Ni alloy; chromel = Ni-Cr alloy; alumel = Ni-Al-Si alloy; nicrosil = 14.20 Cr, 1.40 Si, bal. Ni alloy; nisil = 4.40 Si, 0.10 Mg, bal. Ni alloy; materials are given in the order: positive leg/negative leg • E(Tc,0) are quite linear, con sensitivity of 10-50 V/°C • Measuring range max: -200 ÷ 2750 °C • Absolute accuracy 0.1 ÷ 4ºC depending on measuring range • Long response times (slow dynamic characteristic) except in the case of exposed hot joint. Process Instrumentation and Control - Prof M. Miccio

  18. J K T TC STATIC CHARACTERISTIC Each thermocouple is characterized by the static characteristic E(Tc,0)which is not difficult to measure in the laboratory, just placing the cold junction at temperature TF = 0 °C TC typeK, JandTare the mostused Sensitivity of about 58 di V/°C for J, 52 di V/°C for K, 50 di V/°C for T Accuracybetween 0.5 and 2.5 °C Process Instrumentation and Control - Prof M. Miccio

  19. CONSTRUCTIVE FEATURES OF TCs A- Bare hot junction It has a very short response time because of the hot junction is in direct contact with the environment in which the measurement is performed. However, it is not recommended in corrosive environment. B- Grounded hot junction In this type of realization the hot junction is an integral part of the protection sheath. The response time is reduced. This meets the standard ASTM-E-235. It is recommended for high pressure applications (up to 3500 kg/cm2). C- Insulated hot junction The hot junction is completely insulated form the protection sheath. It is particularly recommended when Eddy currents could compromise the measurement. This meets the standard ASTM-E-235. The hot junction selection depends on the operating condition of the thermocouple. It is important to bear in mind that type B and C joints are typical of thermocouples with compact ceramic insulation. For the thermocouples with the two wires isolated simply in PVC or fiber we are always in the case A. Process Instrumentation and Control - Prof M. Miccio

  20. THERMOCOUPLES INTERNATIONAL COLOUR CODE Process Instrumentation and Control - Prof M. Miccio

  21. LABORATORY THERMOCOUPLE Type J thermocouple (iron– constantan) Process Instrumentation and Control - Prof M. Miccio

  22. INSULATED HOT JUNCTION Process Instrumentation and Control - Prof M. Miccio

  23. INDUSTRIAL THERMOCOUPLE TERMINALSBLOCK COUPLING ELEMENT CERAMIC INSULATOR EXTENSION PROTECTIVETUBE terminals Process Instrumentation and Control - Prof M. Miccio

  24. ThermocouplesResistance thermometersThermistorsSolid state sensors Process Instrumentation and Control - Prof M. Miccio

  25. RESISTANCE TEMPERATURE DETECTOR (RTD) It is based on the temperature dependence of the electrical resistance of a conductor Materials: Platinum, Nickel, Copper A good approximation of the static characteristic is: R(T)= R0 (1+α1T+α2T2+ α3T3) [=] Ω where R0= resistance at T0 α1temperature coefficient [=] 1/K α1 > 0 <=> PTC = Positive Temperature Coefficient The sensitivity is about R01 Accuracy ass ± 0.02°C (in the range -50 ÷ +150°C) greater than TC The transductionof the resistance value in voltage is performed by a Wheatstone bridge R Process Instrumentation and Control - Prof M. Miccio

  26. WHEATSTONE BRIDGE The transduction of the resistance-voltage is realized with a Wheatstone bridge. Resistance-voltage transduction with Wheatstone bridge from Magnani, Ferretti e Rocco (2007) Process Instrumentation and Control - Prof M. Miccio

  27. MORE COMMON RTD Cu: 10  from Magnani, Ferretti e Rocco (2007) Process Instrumentation and Control - Prof M. Miccio

  28. STATIC CHARACTERISTIC CALIBRATION CURVE OF A RESISTANCE TEMPERATURE DETECTOR Process Instrumentation and Control - Prof M. Miccio

  29. RTDConstructive features INSULATINGDISK CERAMIC PLATINUM PLATINUMWIRES from Magnani, Ferretti e Rocco (2007) Process Instrumentation and Control - Prof M. Miccio

  30. RTDConstructive features SCHEMATIC DRAWING INDUSTRIAL RTD Process Instrumentation and Control - Prof M. Miccio

  31. ThermocouplesResistance thermometersThermistorsSolid state sensors Process Instrumentation and Control - Prof M. Miccio

  32. THERMISTORS Measurement principle Electrical resistance variation of a non-conductor with temperature: • semiconductor • metal oxides (e.g.: Mn oxides) In this case, the dependence with the temperature is non-linear α<0 <=> NTC (Negative Temperature Coefficient) but they can be also PTC type Max temperature range -50 ÷ 250 °C Absolute accuracy εass = ± 0,1 °C Disadvantage: non-linear static characteristic Process Instrumentation and Control - Prof M. Miccio

  33. STATIC CHARACTERISTIC NTC thermistor characteristic from Magnani, Ferretti e Rocco (2007) Process Instrumentation and Control - Prof M. Miccio

  34. THERMISTORS Examples Process Instrumentation and Control - Prof M. Miccio

  35. ThermocouplesResistance thermometersThermistorsnon-contact temperature sensors Process Instrumentation and Control - Prof M. Miccio

  36. Infrared Temperature Measurement Contact-based temperature sensors, such as thermocouples and resistance temperature detectors (RTDs), have demonstrated accurate and cost-effective operation throughout the chemical process industries (CPI). However, there are many applications and settings where they are simply not practical. In those cases, engineers can turn to a host of non-contact temperature measurement devices, many of which are based on measuring infrared (IR) radiation. Pyrometer ⏎ Process Instrumentation and Control - Prof M. Miccio

  37. CONCLUSIVE REMARKS Process Instrumentation and Control - Prof M. Miccio

  38. COMPARISON BETWEEN TCsAND OTHER SENSORS Process Instrumentation and Control - Prof M. Miccio

  39. COMPARISON BETWEEN SENSORS From: Hewlett-Packard Classroom Series 052 So we have already looked at the three types of absolute measuring devices: the RTD, the thermistor and the IC. Let's see how the thermocouple stacks up against these three. The thermocouple is useful in more types of atmospheres and over wider temperature ranges. Since you do not have to put current through the thermocouple to measure it, it has no self-heating and this can be extremely important. Because it has a very small output voltage, the thermocouple is hard to measure and is susceptible to noise, both magnetic and electrostatic. It only measures relative temperature, not absolute temperature. It is non-linear, and it requires either special connectors that are built with the same metal as the thermocouple itself, or a special "isothermal" junction. Process Instrumentation and Control - Prof M. Miccio

  40. COMPARISON BETWEEN SENSORS From: Dr. Chi-fu Wu, ME 4903-Special Problem in ME, GTREP, Savannah (USA), Oct. 21, 2004 Process Instrumentation and Control - Prof M. Miccio

  41. TYPICAL COSTS Process Instrumentation and Control - Prof M. Miccio

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