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TEMPERATURE MEASUREMENT

TEMPERATURE MEASUREMENT. CONTENTS. INTRODUCTION RESISTANCE TEMPERATURE DETECTOR (RTD) THERMISTOR THERMOCOUPLE CALIBRATION METHOD. MATERIAL BEHAVIOUR. INTRODUCTION. TEMPERATURE UNLIKE OTHER QUANTITIES (LENGTH, TIME, MASS) IS AN ABSTRACT QUANTITY THAT MUST BE DEFINED IN TERMS

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TEMPERATURE MEASUREMENT

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  1. TEMPERATURE MEASUREMENT

  2. CONTENTS • INTRODUCTION • RESISTANCE TEMPERATURE DETECTOR (RTD) • THERMISTOR • THERMOCOUPLE • CALIBRATION METHOD

  3. MATERIAL BEHAVIOUR INTRODUCTION TEMPERATURE UNLIKE OTHER QUANTITIES (LENGTH, TIME, MASS) IS AN ABSTRACT QUANTITY THAT MUST BE DEFINED IN TERMS OF THE BEHAVIOUR OF MATERIALS AS THE TEMPERATURE CHANGES CHANGE IN VOLUME OF LIQUID CHANGE IN LENGTH OF BAR CHANGE IN ELECTRICAL RESISTANCE OF A WIRE CHANGE IN PRESSURE OF GAS AT CONSTANT VOLUME ETC TEMPERATURE CHANGE

  4. CONTENTS • INTRODUCTION • RESISTANCE TEMPERATURE DETECTOR (RTD) • THERMISTOR • THERMOCOUPLE • CALIBRATION METHOD

  5. RESISTANCE THERMOMETERS CONSISTS OF A SENSOR ELEMENT THAT EXHIBITS A CHANGE IN RESISTANCE WITH A CHANGE IN TEMPERATURE, A SIGNAL CONDITIONING THAT CONVERTS THE RESISTANCE CHANGE TO AN OUTPUT VOLTAGE AND INSTRUMENTATION TO RECORD AND DISPLAY THE OUTPUT VOLTAGE • TYPES OF SENSOR USED: • RESISTANCE TEMPERATURE DETECTOR (RTD) • SIMPLE RESISTIVE ELEMENTS FORMED OF SUCH MATERIALS • AS PLATINUM, NICKEL, OR NICKEL-COPPER ALLOY BECAUSE • THEY EXHIBIT A POSITIVE COEFFICIENT OF RESISTIVITY, STABLE • GOOD REPRODUCIBILITY • THERMISTOR • FABRICATED FROM SEMICONDUCTION MATERIALS, SUCH AS • OXIDES OF MANGANESE, NICKEL OR COBALT • EXHIBIT A HIGH NEGATIVE COEFFICIENT OF RESISTIVITY

  6. RTD Resistance-Temperature relationship RT=Ro{1+a[T-d(0.01T-1)(0.01T)-b(0.01T-1)(0.01T)3]} (Calendar-Van Dusen equation) where a, b and d are constants determined by calibration T is temperature in degrees Celcius Ro is a resistance at reference temperature (usually 00C) a=0.003921/0C , d=1.49 and b=0 for T>0, b =0.11 for T<0 (US calibration) • Advantages using RTD • Platinum RTDs are more accurate than thermocouples • Output response is more linear

  7. US Calibration of Platinum RTD: R versus T

  8. RTD instrumentation Wheatstone bridge for RTD Three wire Two wire Four wire Assume resistance of two wires low, R1=R4 R1=R4, if lead wire resistance is not neglected, the change in lead resistance cancelled each other

  9. Example: An RTD probe has a resistance of 100 W at 00C. The Calendar-Van Dusen constants are a=0.00392, d=1.49 and b=0 for T>00C. What will be the resistance at 3500C? Solution: RT=Ro{1+a[T-d(0.01T-1)(0.01T)-b(0.01T-1)(0.01T)3]} if all variables are put into this equation RT=232.08 W or from calibration table we have 231.89 W

  10. CONTENTS • INTRODUCTION • RESISTANCE TEMPERATURE DETECTOR (RTD) • THERMISTOR • THERMOCOUPLE • CALIBRATION METHOD

  11. THERMISTOR Resistance Temperature relationship where: R=resistance at any temperature T, in K R0= resistance at reference temperature T0, in K b=a constant, in K Generally respond to an increase in temperature with a decrease in resistance Using semiconductor b depends on thermistor material, typically between 1000-5000 K for metal oxide thermistor

  12. Thermistor Instrumentation Since the change in resistance for thermistor is so large (DR/R=80W/K) a common multimeter (4 digits) can be employed to measure R within ± 1 W, no bridge required If using DAQ with computing microprocessor, temperature can be approximated very closely using Steinhart-Hart relation Where A, B and C are coefficients determined from calibration curve

  13. CONTENTS • INTRODUCTION • RESISTANCE TEMPERATURE DETECTOR (RTD) • THERMISTOR • THERMOCOUPLE • CALIBRATION METHOD

  14. THERMOCOUPLE Seebeck Effect Temperature gradient between two conductors induces a voltage potential where (sometimes a0 is used) is average Seebeck coefficient at T1≤T≤T2

  15. Thermocouple is a simple temperature sensor consists of two dissimilar materials in thermal contact Thermocouple are only capable of measuring temperature differences. To measure the temperature of an object, we need a known reference temperature. The thermocouple is used to measure the temperature difference between the object and the known temperature

  16. Law of intermediate metals

  17. EXAMPLE A type R thermocouple system with an ice reference has an output of 9.1 mV. What is the temperature of the sensing junction? Solution: In table, for R type thermocouple, 7.949 mV corresponds to 8000C and 9.203 corresponds to 9000C. Linear interpolation gives a temperature of 891.80C for 9.1 mV.

  18. Example: An iron constantan thermocouple is connected to a potentiometer whose terminals are 250C. The potentiometer reading is 3.59 mV. What is the temperature of the thermocouple junction? Solution: The thermoelectric potential corresponding to 250C from table is E25oC=1.277 mV The emf of the thermocouple based on a 00C reference temperature is ET=1.277+3.59=4.867 mV Which corresponds to 92.50C temperature

  19. For laboratory work reference temperature usually forced to 00C using ice bath

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