MIDDLE EAST TECHNICAL UNIVERSITY DEPARTMENT OF METALLURGICAL AND MATERIALS ENGINEERING

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1 MIDDLE EAST TECHNICAL UNIVERSITY DEPARTMENT OF METALLURGICAL AND MATERIALS ENGINEERING Met E 206 MATERIALS LABORATORY EXPERIMENT 5 Prof.Dr. Yavuz TOPKAYA Prof.Dr. İshak KARAKAYA TEMPERATURE MEASUREMENT THEORY Modern experimental studies are unthinkable without accurate measurement of temperatures. In many cases the accuracy of temperature measurement determines the total error of an experiment. In industry the usual objective is to find a way to measure temperature that yields: -accuracy and speed of response required by the application -acceptable initial investment -low maintenance cost, which implies long life and stability of calibration. All methods of temperature measurement can be divided into direct ( contact methods affected by means of thermometers, thermocouples and resistance thermometers ) and indirect ( contactless, for instance, optical methods ). In the former, the sensor of measuring instrument is placed in close vicinity of a heated object whose temperature is to be measured. Optical methods measure either the intensity of radiation, or else a change in the spectral distribution of radiation as a function of temperature. Thermocouples A thermocouple is a system composed of two homogeneous metal wires dissimilar in chemical composition that are soldered, fused or welded together at one end, forming a closed circuit when the free-ends are connected to an indicating instrument which is mostly a potentiometer. ( See Figure 1 ) Figure 1. Schematic Drawing of a Thermocouple System.

2 If the junction of these two dissimilar metal conductors is heated, there appears an electromotive force ( emf ). This emf is a function of temperature for a given pair of metals. If one of the junctions of a thermocouple is held at constant temperature T o, the thermo-emf of the thermocouple will be a function of the temperature T 1 at which the other junction is kept. A thermocouple can be used for temperature measurement after calibration, i.e. after determining how its thermo-emf varies with the temperature of one of the junctions ( the other junction being kept at a constant temperature ). The working junction of a thermocouple is usually called the hot junction and that kept under thermostatic conditions, cold ( reference ) junction. Thermo-emf of a thermocouple is measured by a millivoltmeter or a potentiometer connected by copper wires ( leads ) to the free-ends of the thermocouple. Types of Thermocouples Any dissimilar metal junction could generate a thermo-emf when it is heated, but those pairs used for temperature measurements are limited. The thermocouples and the symbols used for them are listed in Table 1. Several types of thermocouples may have similar accuracy and initial cost. The objective is to select the type with greatest stability and longest life. Selection may depend on whether the atmosphere will corrode or poison the thermocouple. Table 1. Symbols for Thermocouples and Temperature Ranges. ANSI SYMBOL Wire Combination Temperature Range( o C ) * NBS Limits ( o C ) ** J Iron-Constantan -190 to to 760 K Chromel-Alumel -190 to to 1370 T Copper-Constantan -200 to to 400 E Chromel-Constantan -190 to to 1000 S Platinum - ( 10% Rhodium+Platinum ) 0 to to 1760 R Platinum - ( 13% Rhodium+Platinum ) 0 to to 1760 B ( Platinum+30%Rh ) - ( Platinum+6% Rh) 0 to to 1820 * Handbook of Physics and Chemistry ** National Bureau of Standards Constantan (55-60% Cu+ rest Ni ), Chromel ( 90% Ni+10% Cr), Alumel (94%Ni+2%Al+4%Mn) Standardized calibration tables are available for these pairs. (Consult instructor to see a calibration table ). These tables are based on a reference ( cold ) junction temperature of 0 o C. Correction operation is required if the cold junction is different from 0 o C ( see Procedure Section for corrections ). Type J : Not used below 0 o C because condensation may cause rusting of iron. They are best used in reducing atmospheres. Type T: May be used in slightly oxidizing or reducing atmospheres and are frequently used for low temperature work. 2

3 Type K: Used only in an oxidizing atmosphere. A reducing atmosphere may cause green rot, which shifts calibration. OBJECTIVE To conduct temperature measurement tests in order to demonstrate the use of thermocouples for high temperature measurements. APPARATUS 1. Vertical tube furnace 2. Thermocouple calibration cell 3. Thermocouple 4. Potentiometer 5. Mercury in glass thermometer PROCEDURE PART A : 1. Assemble the thermocouple calibration cell ( See Figure 2 ). 2. Power up the vertical tube furnace, by setting up the controller to a desired temperature ( a temperature above the melting point of calibration material ). 3. Measure the temperature of the furnace intermittently until it reaches the desired temperature and stabilizes. 4. Connect thermocouple from the calibration cell to a potentiometer and turn the furnace off. 5. Measure the thermo-emf from the calibration cell every 30 seconds and record them until the temperature given by the instructor. 6. Apply corrections to the measured thermo-emf values if the cold junction temperature is not 0 o C. Use the following procedure for correction : i. Measure the reference ( cold ) junction temperature of your thermocouple in o C using a mercury in glass thermometer. ii. Find out the corresponding voltage to the temperature measured in step (i) from the calibration tables. iii. Add voltage measured from the thermocouple and the voltage found in step (ii) to obtain the corrected voltage. iv. Find out the corresponding temperature to the corrected voltage in step (iii) from the calibration table. 3

4 Figure 2. Schematic Cross-section of Thermocouple Calibration Cell. PART B: ( OPTIONAL ) Use ice-water mixture to construct a cold junction and assemble the thermocouple calibration cell as given in Figure 3. Then, repeat the procedure given in Part A, except the correction part. Figure 3. Schematic Cross-section of Thermocouple Calibration Cell with Cold Junction Application. 4

5 QUESTIONS 1. Write a short assay on the optical methods of temperature measurement. 2. Explain why an emf is developed (i.e., Seebeck effect) when two dissimilar metal wires are welded at one end. SUGGESTED READINGS 1. O. Kubaschewski, E.LL. Evans and C.B. Allock, "Metallurgical Thermochemistry", Pergamon Press, 81-85, S. Cankut, "Ekstraktif Metalurji", İTÜ Yayınları, No: 884, , J. Newton, "Extractive Metallurgy", John Wiley and Sons Inc., New York, , P.A. Kinzie, "Thermocouple Temperature Measurement", Wiley, New York, H.E. Bennet, "Noble Metal Thermocouples", Johnson, Matthey and Co., London, B.F. Billing, "Thermocouples: Their Instrumentation, Selection and Use", Techniprint, London, T.J. Quinn, "Temperature", Academic Press, London, J.D. Lenk, "ABC's of Thermocouples", H.W. Sams, Indianapolis, W.F. Coxon, "Temperature Measurement and Control", McMillan, New York, "Temperature, its Measurement and Control in Science and Industry", Vol.1-3, Reinhold Pub. Corporation, New York, G.V. Somsonov and P.S. Kislyi, "High Temperature Non-metallic Thermocouples and Sheats", Consultants Bureau, New York, D. Pollock, "Theory and Properties of Thermocouple Elements", American Society for Testing of Materials (ASTM), B. Linchevsky, Methods of Metallurgical Experiment, MIR Publishers, Temperature Measurement Handbook, Omega Engineering, Inc.,