Calibration of a simple Iron-constantan thermocouple at freezing and boiling points of water and comparison of results-a Review

Transcription

1 INTERNATIONAL JOURNAL FOR RESEARCH & DEVELOPMENT IN TECHNOLOGY Volume-6, Issue-2 (Sep-16) ISSN (O) : Calibration of a simple Iron-constantan thermocouple at freezing and boiling points of water and comparison PROF SAJIDALI M HATTURKAR 1 1 Mechanical Engineering Department 2 BLDEA svp Dr P.G.Halkjatti College of Engineering and Technology Vijayapur (Karnataka) ABSTRACT - Thermocouples are widely used in science and industry. Its applications include temperature measurement for Kilns, gas turbine exhaust, diesel engines, and other industrial processes. Thermocouples are also used in homes, offices and businesses as the temperature sensors in thermostats, and also as flame sensors in safety engines for gas-powered major appliances. This paper is a literature review and it mainly focuses on three aspects of thermocouples:- i)theory of thermocouples. ii) Three principles of Thermocouple. Iii) Experimental work on calibration of iron constantan thermocouple at freezing and boiling points of water and comparison of results. Keywords:-Thermocouple, Calibration, freezing point, boiling point. INTRODUCTION When two dissimilar metal or alloy conductors are connected together to form a closed circuit and the two junctions are kept at different temperatures, thermal electromotive force (EMF) is generated at the temperature gradient zone along the conductors length in the circuit. Thus, when one end (cold or reference junction) is kept constant at a certain temperature, normally, and the other end (measuring junction) is exposed to unknown temperature, the temperature at the latter end can be determined by measuring EMF so generated. Such a combination of two dissimilar metal conductors is called "Thermocouple." As described, thermocouple is a "temperature difference sensor" to generate milli volt signal (EMF) only at the temperature gradient segment, which inevitably makes the thermocouple conductor heat treated in accordance with the temperature profile along the insertion depth. It is not correct, therefore, to use such a thermocouple as once heat treated and so stabilized, for measurement of the 87 other location that has different temperature gradient. Particularly, when measurement is made in shorter insertion depth than previous measurement, it will result in large reading error, since already heat treated segment is exposed to non-temperature gradient zone thus exhibiting spurious EMF, therefore, avoid re-using one thermocouple for measurements at the different locations. Generally, service life of the thermocouple cannot be predicted nor be guaranteed, as the environments of temperature measurement are so various involving handling, installation, corrosion, vibration, thermal cycles and steep change in temperatures [1]. A thermocouple, shown in Figure, consists of two wires of dissimilar metals joined together at one end, called the measurement (hot) junction. The other end, where the wires are not joined, is connected to the signal conditioning circuitry traces, typically made of copper. This junction between the thermocouple metals and the copper traces is called the reference (cold) junction [2]. Fig 1.1, Thermocouple theory The voltage produced at the reference junction depends on the temperatures at both the measurement junction and the reference junction. Since the thermocouple is a differential device rather than an absolute temperature measurement device, the reference junction temperature must be known to All rights reserved by

2 get an accurate absolute temperature reading. This process is known as reference junction compensation (cold junction caused by the Peltier effect alone. This behavior was discovered by Jean Charles Athanase Peltier ( ) compensation.) Thermocouples have become the industrystandard during experiments with Seebeck s thermocouple. He method for cost-effective measurement of a wide range of temperatures with reasonable accuracy. They are used in a variety of applications up to approximately C in boilers, water heaters, ovens, and aircraft engines to name observed that passing a current through a thermocouple circuit having two junctions, as in Figure, raised the temperature at one junction, while lowering the temperature at the other junction. just a few. The most popular thermocouple is the type K, consisting of Chromel and Alumel (trademarked nickel alloys containing chromium, and aluminum, manganese, and silicon, respectively), with a measurement range of 200 C to C Principles of emf generation in Thermocouple Thomas Johann Seebeck effect The Seebeck effect refers to the generation of a voltage potential, or emf, in an open thermocouple circuit caused by a difference in temperature between junctions in the circuit. There is a fixed, reproducible relationship between junctions in the circuit. There is a fixed, reproducible relationship between the emf and the junction temperatures T 1 and T 2. This relationship is expressed by the Seebeck coefficient,ὰ AB Thomson effect Consider the conductor shown in Figure, which is subjected to a longitudinal temperature gradient and also subject to a potential difference, such that there is a flow of current and heat in the conductor. defined as where σ is the Thomson coefficient Fig 1.2.1, Seebeck effect Peltier effect The Peltier heat is the quantity of heat in addition to the quantity I 2 R that must be removed from the junction to maintain the junction at a constant temperature. This amount of energy is proportional to the current flowing through the junction; the proportionality constant is the Peltier coefficient π AB, and the heat transfer required to maintain a constant temperature is 1.4 TYPES OF THERMOCOUPLES Type B Type B thermocouple has higher melting point and mechanical strength than other Pt/Rh thermocouples because of its higher content of Rhodium in both legs. Type B thermocouple can be used continuously in oxidizing and neutral atmospheres up to 1600 C and intermittently up to 1700 C. Even in reducing atmosphere, Type B may be used 88

3 for fairly longer period than other Pt/Rh thermocouples, but not generally recommended. Type B thermocouple is recommended especially for the applications requiring precision measurement and durability at high temperatures. This thermocouple has very small EMF up to 100C, thus for less critical applications, copper leads can be used as a compensating wire. Precious metal thermocouples are generally sensitive to contaminants and easily be corroded at However, it should not be used in sulphurous atmospheres above 538 C due to formation of the sulphides that leads conductors to embrittlement. The iron element is often rusted under high humidity environment; therefore, type J is less desirable than type T for low temperature measurements Type K Type K thermocouple was originally developed by Mr. A. L. Marsh of Hoskins Co., U.S.A. in 1906 and, since then, has elevated temperatures. It is essential to keep the thermocouple undergone many improvements. It has linear EMF wire clean and use dust free high purity (>99.5%) Alumina characteristics and most widely used as industrial insulators and protection tubes Type E Type E thermocouple has the highest EMF characteristics among industrial thermocouples which allows the best resolution to temperature change, type E thermocouple has met rapidly increasing demands and has been widely used even in large scale thermal and nuclear power stations. It can be used up to 750 C continuously. For practical use, precautions similar to those for type K are required. Careful attention is also needed in selection of the indicator to be connected because type E thermocouple has the highest resistivity among the base metal thermocouples Type J thermocouple with high reliability because of its versatile characteristics. It can be used in oxidizing or inert atmospheres at temperatures up to 1250 C. Type K thermocouple may be used in hydrogen or cracked ammonia atmospheres if the dew point is below -42 C. However, it should not be used in reducing, alternatively oxidizing and reducing, sulphurous or "green-rot" corrosive atmospheres unless properly protected."green-rot" can be minimized by increasing oxygen supply through the use of large diameter protection tube or ventilated protection tube. It can also be minimized by inserting a "getter" to absorb the oxygen in a sealed protection tube Type N Type J thermocouple has the second highest EMF This new thermocouple combination of 84Ni-14.2Cr-1.4Si vs. characteristics and is recommended for use in reducing, inert, oxidizing or vacuum atmospheres up to 750 C. Because of 95.5Ni.-4.4Si- 0.1Mg was first developed by Materials Research Laboratory of the Australian Department of comparatively less expensive price, type J has been easily Defence. Further research and evaluation have been accepted for use in various applications. extensively carried out by NIST (former NBS), Type N thermocouple exhibits superior long-term stability and oxidation resistance over type K when used at high temperatures ranging from 600 to 1250 C. By virtue of fine adjustment of chromium content with additions of Si and Mg, it has less EMF shift in the region of "short range ordering" and also resistant to "Green Rot" corrosion. In comparison with type K, rate of EMF drift is reported to be half or one third over the range of 1000 C and therefore recommended for use in oxidizing atmosphere of C continuous Type R Type R thermocouple has superior mechanical properties to Fig Iron-Constantan Thermocouple Type S and is recommended for continuous use in oxidizing 89

4 and inert atmospheres around temperatures up to 1400 C and Grounded Junction intermittently up to 1600 C. However, it should not be used in vacuum, reducing or metallic vapor atmospheres unless properly protected with clean high purity (>99.5%) Alumina insulators and protection tubes. Among precious metal thermocouples, Type R is most widely used. Fig 1.5.3, Grounded junction Type S The sheath and conductors are welded together, forming a Type S thermocouple is the first historic thermocouple completely sealed, integral junction. The grounded junction is originally developed by Le Chatelier in It had been recommended in the presence of liquids, moisture, gas or high widely used as a standard thermometer as an interpolation pressure. The wire is protected from corrosive or erosive means to determine the temperature scale between the fixed conditions. Response time with this style approaches that of (freezing) points ranging from C of Antimony to the exposed junction C of Gold as defined by the International Practical Ungrounded Junction Temperature Scale (IPTS). Applications are similar to Type R, but it has less mechanical strength Type T Type T thermocouple has good resistance to corrosion in moist atmospheres and is suitable for sub-zero temperature Fig 1.5.4, Ungrounded junction measurements. It can be used in vacuum and in oxidizing, The thermocouple junction is fully insulated from the welded reducing or inert atmospheres up to 400 C. At higher sheath end. The ungrounded junction is excellent for temperatures, it is susceptible to rapid oxidation by water applications where stray EMFs would affect the reading and vapor. Because of its stable and precise EMF characteristics, for frequent or rapid temperature cycling. Response time is type T is widely used in laboratories. Type T is the first longer than with the grounded junction. thermocouple for which tolerance in the sub-zero temperature Ungrounded Dual Isolated Junction range has been established. Due to high thermal conductivity of the conductors, care must be exercised to eliminate heat conduction error that often occur on short stem length type T thermocouple unit JUNCTION TYPES Fig 1.5.5, ungrounded dual isolated junction Exposed Junction Two separate thermocouples are encased in a single sheath. The isolation prevents ground loop errors if wired to separate instruments. Only available as ungrounded junctions. 2. OBJECTIVES: 1. Develop a simple iron constantan of Thermocouple. 2. Calibrate the Thermocouple at freezing point and boiling Fig 1.5.2, exposed junction point of water. Thermocouple wires are butt welded, insulated and sealed 3. Draw the calibration curves. against liquid or gas penetration. This junction style provides Compare the results of Iron-Constantan with the Base metal. the fastest possible response time but leaves the thermocouple EXPERIMENTAL WORK wires unprotected against corrosive or mechanical damage. 90

5 3.1. EXPERIMENTAL SET UP Set up comprises of RTD sensor as a reference and Ironindicator. 5. Record the room temperature from the RTD constantan Thermocouple to be calibrated.all four sensors temperature indicator. can be placed in a hot bath where the water can be heated up 6. Adjust the zero setting knob of the thermocouple to boiling temperature through heating coil. Heater of capacity 500watts is provided which will be connected to the temperature indicator until the display shows the room temperature. 230v/50Hz power supply through three-pin mains cord. 7. Connect the power supply to heating coil and heat ELECTRIC RESISTANCE THERMOMETER the water in the water bath. (Resistance Temperature Detector, RTD) 8. Set the temperature of thermocouple to the One of accurate methods o temperature measurement is the electrical resistance thermometer. It consists of some type of temperature of RTD indicator when the water is boiling, using CAL knob. resistive element which is exposed to the temperature to be 9. Now the given thermocouple is calibrated with measured.since the resistance of an element depends on the reference to RTD. temperature, the temperature is indicated through a 10. Record the RTD and thermocouple temperature measurement of the change in resistance of the element. indicator reading simultaneously at regular intervals. 3.3 TABULAR COLUMNS RTD type: Resistance temperature detector Materials for thermocouple wires: Iron- Constantan Table Boiling of water set-1 Fig 4.1. Experimental setup DIGITAL TEMPERATURE INDICATOR Temperature indicator for both thermocouples and RTD are provided in a single unit with two separate displays. For thermocouples, the output of the sensor is amplified through electronic circuits. Calibration provision is provided outside to calibrate any sensor required. RTD sensor is calibrated and the output in terms temperature in degree centigrade is displayed. 3.2 PROCEDURE 1. Turn the type of selector to the desired position according to the designed T.C. probe. 1. Connect the RTD probe to the resistance temperature detector display. 2. Connect the given thermocouple to the thermocouple temperature display. 3. Place the thermocouple hot junction and the RTD probe into a beaker containing water at room temperature. 4. Connect the power supply to the temperature Temperature of Thermocouple C RTD C Error Table Freezing of water set-1 Temperatur Temperatur e of e of RTD Thermocou C ple C Error

6 Table Boiling of water set-2 Thermocouple C RTD C Error Table Freezing of water set-3 Thermocouple C RTD C Error Table Emf of Thermocouple vs. RTD Table Freezing of water set-2 emf of RTD C Thermocouple mv RESULTS AND DISCUSSIONS Calibration of Iron-Constantan thermocouple with respect to base metal, the following results are obtained. The results are discussed as follows, Table Boiling of water set-3 Fig 4.1. Boiling of water set-1 92

7 reads same temperature with less error from 0 C to 100 C. And at freezing point of water error increases with decrease in the temperature Fig4.2. Freezing of water set-1 From fig (4.1) and fig (4.2), it shows that with increase in boiling water temperature, the temperature difference in thermocouple and RTD i.e the error also increases gradually. And at freezing point of water the error decreases with decreasing in temperature of water and at 20 C the error is zero, at 0 C the thermocouple reads 10 C. Fig 4.5. Boiling of water set-3 Fig 4.3. Boiling of water set-2 Fig 4.6. Freezing of water set-3 From fig(4.5) and fig(4.6) of set-3,it is observed that error at boiling and freezing point of water is less compared to previous sets and it attains zero error at higher temperature Fig 4.4. Freezing of water set-2 From fig (4.3) and fig (4.4) set-2, the increase in the temperature of boiling water the Thermocouple and RTD Fig 4.7. Error vs. Temp of TC (boiling-2) 93

8 From fig (4.10) the emf generation from thermocouple is zero up to 30 C and increases linearly with increase in the temperature. CONCLUSION Calibration of thermocouple with reference of Resistance Temperature Detector (RTD) at boiling point and freezing point of water shows that, the initial level of calibration the thermocouple during boiling of water is more sensitive at low temperature i.e below 30 C and less error is observed, but Fig 4.8. Error vs. Temp of TC (boiling-3) with increase in the temperature the error also increases. The From fig (4.7) and fig (4.8), it may be conclude that with fig(5.8) shows that at second time calibration the error is increase in temperature of thermocouple with respect to RTD decreased by increasing the temperature of boiling water. And the error decreases, and attain zero value above 90oC and it in fig (5.9) the freezing of water shows that by decreasing the shows that sensitive of Iron-Constantan increases with temperature, the error also increases. It is concluded that temperature sensitive of thermocouple is increased gradually by increase in temperature, at low temperature more error is occurred, and by increase in temperature the error decreases, at 100 C the error is zero. Comparison between Iron-Constantan and Chromel- Alumel (type K),it shows that Chromel-Alumel thermocouple have less temperature difference between thermocouple and RTD(error) from 0 C to 100 C. And by repeated calibration of type-k thermocouple attain nearly zero error at temperature from 0 C to 100 C,and it shows that Chromel-Alumel(type K) is sensitive at all temperature, but Iron-Constantan is more sensitive at higher temperature(>90 C). REFERENCES Fig 4.9. Error vs. Temp of TC (freezing-3) [1]. Measure Temperature using Thermocouples by Matthew Duff and Joseph Towey, Analog Dialogue 44-10, October (2010). [2]. F. R. Caldwell Thermocouple Materials text book. [3]. Calibration of Thermocouples EURAMET cg-8 (10/2011). [4]. P. A. Zajtsev, S. V. Prijmak Models of the drift of the calibration Curves of thermocouples and resistance Thermometers under reactor conditions Atomic Energy, Vol. 113, No. 3, January, 2013 (Russian Original Vol. 113, No. 3, September, 2012). [5]. K. M. Garrity D. C. Ripple M. Araya A Regional Fig 4.10.Temp of RTD vs. emf of TC Comparison of Calibration Results for Type K Thermocouple Wire from (100 to 1,100 C), Int J 94

9 Thermophys (2008) 29: DOI /s ). [6]. R. Morice F. Edler J. Pearce G. Machin, High- Temperature Fixed-Point Facilities for Improved Thermocouple Calibration Euromet Project 857, Int J Thermophys (2008) 29: [7]. H. Lehmann, Fixed-Point Thermocouples in Power Plants: Long-Term Operational Experiences. Int J Thermophys (2010) 31: [8]. J. V. Pearce. V. Montag. D. Lowe. W.Dongm, Melting High-Temperature Fixed Points for Thermocouple Calibrations, Int J Thermophys (2011) 32: [9]. F. Edler,Material Problems in Using High-Temperature Thermocouples, Int J Thermophys (2011) 32: