TEMPERATURE MEASUREMENTS 17/09/2019 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 17/09/2019 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 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

Thermocouples Resistance thermometers Thermistors Solid state sensors 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

Thermoelectric effect 17/09/2019 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. 1 E 2 T1 T2 with T2 > T1 σ = SEEBECK coefficient (intrinsic property of any single conductor material) [=] mV/K E = direct voltage between points “1” and “2” [=] mV 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

Process Instrumentation and Control - Prof M. Miccio 17/09/2019 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) 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

SEEBECK EFFECT M1 Cold junction (2 measuring 17/09/2019 SEEBECK EFFECT E Cold junction (2 measuring terminals) at Tf Hot junction at Tc > Tf M1 M2 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. 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

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 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

MEASUREMENT SYSTEM WITH THERMOCOUPLE 17/09/2019 MEASUREMENT SYSTEM WITH THERMOCOUPLE  PROBLEM ! The electromotive force E depends on the cold junction temperature Tf, which is not fixed and is subject to variability, even unpredictable. E Cold junction (2 measuring terminals) at Tf Hot junction at Tc > Tf M1 M2 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

"IDEAL" MEASUREMENT SYSTEM WITH THERMOCOUPLE 17/09/2019 "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). 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

“REAL" MEASUREMENT SYSTEM WITH THERMOCOUPLE 17/09/2019 “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 Measuring device 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

LAW OF INTERMEDIATE TEMPERATURES 17/09/2019 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 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

“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 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

Process Instrumentation and Control - Prof M. Miccio 17/09/2019 “PRACTICAL" MEASUREMENT SYSTEM WITH THERMOCOUPLE software COLD JUNCTION COMPENSATION MEASURING DEVICE (Voltmeter) The measuring device “reads” VACQ = E(TC,TF) from the hot joint through copper (Cu) wires for the voltmeter TF is 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 TC from the static characteristic of the TC 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

“REAL” MEASUREMENT SYSTEM WITH THEMOCOUPLE 17/09/2019 “REAL” MEASUREMENT SYSTEM WITH 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 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

THERMOCOUPLE WIRES www.tc-srl.it 17/09/2019 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. Tc M1 Extension or compensation wires TERMINALS COPPER VOLTMETER M2 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

THERMOCOUPLE PROPERTIES 17/09/2019 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. 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

THERMOCOUPLE TYPES 17/09/2019 International standards define different TC types characterizing them with the relative E(Tc,0) type metals/alloy pairs normal range [°C] extended range [°C] J iron/costantan -100 750 -200 1100 K chromel/alumel 0 1100 -100 1370 E chromel/costantan -150 500 -200 1000 T copper/costantan -200 300 -230 400 B Pt80-Rh20/Pt94-Rh6 600 1650 100 1820 S Pt90-Rh10/Pt 550 1500 0 1700 R Pt87-Rh13/Pt 550 1500 N Nicrosil/nisil 0 1300 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. 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

TC STATIC CHARACTERISTIC 17/09/2019 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 J K T TC type K, J and T are the most used Sensitivity of about 58 di V/°C for J, 52 di V/°C for K, 50 di V/°C for T Accuracy between 0.5 and 2.5 °C 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

CONSTRUCTIVE FEATURES OF TCs 17/09/2019 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. 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

THERMOCOUPLES INTERNATIONAL COLOUR CODE 17/09/2019 THERMOCOUPLES INTERNATIONAL COLOUR CODE 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

LABORATORY THERMOCOUPLE 17/09/2019 LABORATORY THERMOCOUPLE Type J thermocouple (iron– constantan) 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

INSULATED HOT JUNCTION 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

INDUSTRIAL THERMOCOUPLE 17/09/2019 INDUSTRIAL THERMOCOUPLE TERMINALS BLOCK COUPLING ELEMENT CERAMIC INSULATOR EXTENSION PROTECTIVE TUBE terminals 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

Thermocouples Resistance thermometers Thermistors Solid state sensors 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

RESISTANCE TEMPERATURE DETECTOR (RTD) 17/09/2019 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 α1 temperature 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 transduction of the resistance value in voltage is performed by a Wheatstone bridge R 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

17/09/2019 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) 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

MORE COMMON RTD from Magnani, Ferretti e Rocco (2007) Cu: 10  17/09/2019 MORE COMMON RTD Cu: 10  from Magnani, Ferretti e Rocco (2007) 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

STATIC CHARACTERISTIC 17/09/2019 STATIC CHARACTERISTIC CALIBRATION CURVE OF A RESISTANCE TEMPERATURE DETECTOR 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

RTD Constructive features 17/09/2019 RTD Constructive features INSULATING DISK CERAMIC PLATINUM PLATINUM WIRES from Magnani, Ferretti e Rocco (2007) 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

RTD Constructive features 17/09/2019 RTD Constructive features SCHEMATIC DRAWING INDUSTRIAL RTD 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

Thermocouples Resistance thermometers Thermistors Solid state sensors 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

Process Instrumentation and Control - Prof M. Miccio 17/09/2019 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 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

STATIC CHARACTERISTIC 17/09/2019 STATIC CHARACTERISTIC NTC thermistor characteristic from Magnani, Ferretti e Rocco (2007) 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

Process Instrumentation and Control - Prof M. Miccio 17/09/2019 THERMISTORS Examples 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

Process Instrumentation and Control - Prof M. Miccio Thermocouples Resistance thermometers Thermistors non-contact temperature sensors 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

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 ⏎ 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

Process Instrumentation and Control - Prof M. Miccio CONCLUSIVE REMARKS 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

COMPARISON BETWEEN TCs AND OTHER SENSORS 17/09/2019 COMPARISON BETWEEN TCs AND OTHER SENSORS 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

COMPARISON BETWEEN SENSORS 17/09/2019 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. 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

COMPARISON BETWEEN SENSORS 17/09/2019 COMPARISON BETWEEN SENSORS From: Dr. Chi-fu Wu, ME 4903-Special Problem in ME, GTREP, Savannah (USA), Oct. 21, 2004 17.09.2019 Process Instrumentation and Control - Prof M. Miccio

Process Instrumentation and Control - Prof M. Miccio 17/09/2019 TYPICAL COSTS Type K thermocouple mineral insulated, inconel sheath, 3x3000mm        RS Code 455-3750 S X 1             1+ 6+ 12+ € 46,32 € 41,69 € 39,37 Resistance temperature PT100 mineral insulated, 1000mm length wire, 6.0mm diam.    RS code 397-1337 S Y 1   1+ 6+ € 91,98 € 78,18 17.09.2019 Process Instrumentation and Control - Prof M. Miccio