Physics
Thermocouples
Thermocouples are devices used to measure temperature based on the principle of thermoelectric effect. They consist of two different metal wires joined at one end, and when the junction is exposed to a temperature gradient, it generates a voltage proportional to the temperature difference. This voltage can be measured and used to determine the temperature.
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10 Key excerpts on "Thermocouples"
- Dogan Ibrahim(Author)
- 2002(Publication Date)
- Newnes(Publisher)
Chapter 3 Thermocouple Temperature Sensors Thermocouples are simple temperature sensors consisting of two dissimilar metals joined together. In 1821 a German physicist named Thomas Seebeck discovered that thermoelectric voltage is produced and an electric current flows in a closed circuit of two dissimilar metals if the two junctions are held at different temper-atures. As shown in Fig. 3.1, one of the junctions is designated the hot junction and the other junction is designated as the cold or reference junction. The current developed in the closed loop is proportional to the types of metals used and the difference in temperature between the hot and the cold junctions. Hot junction Cold junction or Reference junction Metal A Metal B Fig. 3.1 A thermocouple circuit If the same temperature exists at both junctions, the voltages produced cancel each other out and no current flows in the circuit. A thermocouple therefore mea-sures the temperature difference between the two junctions, and not the absolute temperature. In order to measure the temperature we have to insert a voltage measuring device in the loop to measure the thermoelectric effect. Figure 3.2 shows such an arrangement where the measurement device is connected to the thermocouple with a pair of copper wires, using a terminal block. Thermocouple wires are usually different metals from the measuring device wires and as a result, an additional pair of Thermocouples are formed at the connection points. Figure 3.3 shows these additional undesirable Thermocouples as junc-tion 2 and junction 3. Although these additional Thermocouples seem to cause a problem, the application of the Law of Intermediate Metals show that these Thermocouples will have no effect if they are kept at the same temperature. The 63 64 Microcontroller-based Temperature Monitoring and Control Hot junction Metal A Metal B Copper Copper V Terminal block Fig.
eBook - PDF- Peter Childs, Peter R. N. Childs(Authors)
- 2001(Publication Date)
- Butterworth-Heinemann(Publisher)
Thermocouples 99 Thermocouples is their relatively weak signal, ≈ 4.1 mV at 100°C for a type K thermocouple. This makes their reading sensitive to corruption from electrical noise. In addition, their output is non-linear and requires amplification and their calibrations can vary with contamination of the thermocouple materials, cold-working and temperature gradients. The fundamental physical phenomenon exploited in Thermocouples is that heat flowing in a conductor produces a movement of electrons and thus an electromotive force (emf). This was originally demonstrated by Johann Seebeck who discovered that a small current flowed through the circuit shown in Figure 5.3 when the temperature of the two junctions was different (Seebeck, 1823). The emf produced is proportional to the temperature Figure 5.2 A commercial device providing a direct display of the temperature reading from a thermocouple V Voltmeter Thermoelement material B Thermoelement material A Thermoelemen t material A Junction at T 1 Junction at T 2 100 Practical Temperature Measurement difference and is called the Seebeck emf or thermoelectric potential. As well as being a function of the net temperature difference, the magnitude of the emf produced is also a function of the materials used. Thermocouples can be easy to use, but it is also possible to make errors in installation and interpreting a reading. Careful attention to the details of a thermocouple installation is therefore necessary. Some of the errors that occur in practice are due to a lack of appreciation of the behavior of Thermocouples, and in order to overcome this it is helpful to develop a basic understanding of how a thermocouple generates a signal and this is described in Section 5.2.
eBook - PDF- Alan S. Morris(Author)
- 2001(Publication Date)
- Butterworth-Heinemann(Publisher)
In the case of noble-metal Thermocouples, the use of large diameter wire incurs a substantial cost penalty. Some special applications have a requirement for a very fast response time in the measurement of temperature, and in such cases wire diameters as small as 0 . 1 μ m (0.1 microns) can be used. 282 Temperature measurement 14.2.6 The thermopile The thermopile is the name given to a temperature-measuring device that consists of several Thermocouples connected together in series, such that all the reference junc-tions are at the same cold temperature and all the hot junctions are exposed to the temperature being measured, as shown in Figure 14.7. The effect of connecting n ther-mocouples together in series is to increase the measurement sensitivity by a factor of n . A typical thermopile manufactured by connecting together 25 chromel–constantan Thermocouples gives a measurement resolution of 0 . 001 ° C. 14.2.7 Digital thermometer Thermocouples are also used in digital thermometers, of which both simple and intel-ligent versions exist (see section 14.13 for a description of the latter). A simple digital thermometer is the combination of a thermocouple, a battery-powered, dual slope digital voltmeter to measure the thermocouple output, and an electronic display. This provides a low noise, digital output that can resolve temperature differences as small as 0 . 1 ° C. The accuracy achieved is dependent on the accuracy of the thermocouple element, but reduction of measurement inaccuracy to š 0 . 5% is achievable. 14.2.8 The continuous thermocouple The continuous thermocouple is one of a class of devices that detect and respond to heat. Other devices in this class include the line-type heat detector and heat-sensitive cable . The basic construction of all these devices consists of two or more strands of wire separated by insulation within a long thin cable. Whilst they sense temperature, they do not in fact provide an output measurement of temperature.
eBook - PDF- Wolfgang Göpel, Joachim Hesse, J. N. Zemel, Wolfgang Göpel, Joachim Hesse, J. N. Zemel(Authors)
- 2008(Publication Date)
- Wiley-VCH(Publisher)
Near ambient temperature it can be within the range 0.1-0.2 K, where as at very high temperatures it can be of the order A thermocouple consists of two conductors (A and B) of different compositions, which are of +5-10 K. made up into an electrical circuit as shown in Figure 4-1. 0 Figure 41. Basic circuit of a thermocouple. In 1821 Johann Seebeck discovered that a small current flowed through a circuit, such as that in Figure 4-1, if junction 1 was brought to a different temperature (eg, higher) than junc- tion 2. The electrical potential created by the temperature difference is known as the Seebeck emf (electromotive force) or the thermolectric potential. The magnitude of the thermal emf depends on the composition of the conductors chosen (A and B) and the temperature dif- ference AT, and its polarity depends on the sign of A T In order to perform temperature deter- minations, one of the two junctions (eg, 2) is maintained at a known and constant temperature (eg, OOC). A thermocouple can be regarded as a device for the conversion of thermal energy into elec- trical energy. By ignoring the irreversible losses such as Joule heat and thermal conductivity losses, the thermoelectric potential produced can be determined from the algebraic sums of the Peltier potentials at junctions A and B together with the Thomson potentials in materials A and B (legs). In 1834, Peltier discovered that in a circuit according to Figure 4-2 temperature increase and decrease effects occurred at junctions 1 and 2 which depended on the intensity and direction of the current Z (Peltier effect). The transport of electrons across the junctions between the two materials requires work to be done by or on the system. This makes the thermal energy 0 Figure 42. Peltier effect. 4.1 Fundamentals 121 of the electrons either higher or lower, which leads to heating or cooling of the junction. If P is a constant of proportionality, the heat flow @ given by @ =PI.
- Richard S. Figliola, Donald E. Beasley(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
The output of a thermocouple circuit is a voltage, and there is a definite relationship between this voltage and the temperatures of the junctions that make up the thermocouple circuit. We will examine the causes of this voltage, and develop the basis for using Thermocouples to make engineering measurements of temperature. Table 8.3 Uncertainties in b Uncertainty T [ C] Random s b [K] Systematic b b [K] Total u b 95% [K] 100 13.3 37.4 79.4 125 10.6 53.9 109.9 150 8.8 78.4 157.8 175 150 125 100 75 Temperature [ C] 3000 Uncertainty intervals about mean 3200 3400 3600 3800 4000 Figure 8.12 Measured values of b and associated uncertainties for three temperatures. Each data point represents b u b . 330 Chapter 8 Temperature Measurements Consider the thermocouple circuit shown in Figure 8.13. The junction labeled 1 is at a temperature T 1 and the junction labeled 2 is at a temperature T 2 . This thermocouple circuit measures the difference between T 1 and T 2 . If T 1 and T 2 are not equal, a finite open-circuit electric potential, emf 1 , is measured. The magnitude of the potential depends on the difference in the temperatures and the particular metals used in the thermocouple circuit. A thermocouple junction is the source of an electromotive force (emf), which gives rise to the potential difference in a thermocouple circuit. It is the basis for temperature measurement using Thermocouples. The circuit shown in Figure 8.13 is the most common form of a thermocouple circuit used for measuring temperature. It is our goal to understand the origin of thermoelectric phenomena and the requirements for providing accurate temperature measurements using Thermocouples. In an electrical conductor that is subject to a temperature gradient, there will be both a flow of thermal energy and a flow of electricity.
eBook - PDFExperimental Combustion
An Introduction
- D. P. Mishra(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
Note that these two constants are dependent on the materials of thermocouple pairs and hence vary from one thermocouple to another. But the above expression may not predict the measured temperature correctly for a particular thermocouple pair over the entire range of temperature measurement and hence experimental calibration of each thermocou-ple pair must be carried about over a wide range of its operation. For this purpose, we are only interested in the total voltage generated during temperature measurement rather than the individual contribution of the Seebeck and Thompson effects. Hence the measurement of temperature by using a thermocouple is based on empirical calibration. In real situations, Thermocouples will be connected to microvoltmeters, potentiometers, or any other voltage measuring devices through extension wires. Hence the total voltage across the thermocouple during temperature measurement in practical systems will be dependent on the entire circuit and various junctions and their materials, for which the following thermoelectric laws are to be used. Law of homogeneous metals: If two different metal wires, A and B, form a thermocouple pair to produce emf, when other junctions are introduced in the thermocouple for measurement of voltage (e.g., microvoltmeter), then the tem-perature changes in the circuit does not affect the net emf provided all wires are made of the same material. This aspect is illustrated in Figure 5.22a. Law of intermediate metals: A and B form a thermocouple pair with two junctions to produce emf, but when the third metal C is included somewhere in between these two junctions, then the net emf remains unaffected with the addi-tion of metal C. For example, in the case of a Pt and Pt-Rh 13% thermocouple, if a copper metal is inserted as shown in Figure 5.22b with two new junctions, X and Y, which are held at the same temperature, the total emf in the circuit remains unchanged.
eBook - PDFIndustrial Heating
Principles, Techniques, Materials, Applications, and Design
- Yeshvant V. Deshmukh(Author)
- 2005(Publication Date)
- CRC Press(Publisher)
The change of color of a heated object with temperature is used to “judge” the temperature between 600–1300 ° C, but is not useful as an instrument. The pyrometers and thermometers that find wide use in an industrial environment are based on thermoelectrical and radiation phenomena. These are considered in detail in the next sections along with some other types of minor instruments. 634 Industrial Heating This introduction is an overall review of the science of temperature measurement. For reference and more details consult specialist literature 1,2 in the Bibliography . 14.2 THERMOCOUPLE PYROMETERS These pyrometers, invented long ago, still dominate the field of temperature measurement. The basic principle behind thermo-couple is known as the Seebeck effect which states that, “In a circuit made of two different metals, if one junction is kept at a temperature t 1 , and the other at a lower temperature t 2 then an e.m.f. E is created in the circuit (Figure 14.1(A)) which is pro-portional to the temperature difference ( t 1 − t 2 ), i.e., E = K s ( t 1 − t 2 ) (14.2) where K s is the proportionality constant known as the Seebeck coefficient. The junction at the higher temperature ( t 1 ) is called the hot junction and the other junction at the lower temperature is called the cold junction. A basic thermocouple pyrometer operating on the Seebeck effect is shown in Figure 14.1(B). Figure 14.1 A Seebeck effect . Temperature Measurement 635 Two other effects are associated with the Seebeck effect. 1. Peltier effect states that the Seebeck effect is revers-ible. If in a circuit of two dissimilar metals a direct e.m.f. is impressed (say with a cell or battery) then one of the junctions becomes hot and the other cool. The Peltier effect is the principle on which thermo-electric refrigeration is based. At least up to now, these cooling devices cannot economically compete with mechanical refrigeration. They have their advantages and limited applications.
eBook - PDF- Helcio R.B. Orlande, Olivier Fudym, Denis Maillet, Renato M. Cotta, Helcio R.B. Orlande, Olivier Fudym, Denis Maillet, Renato M. Cotta(Authors)
- 2011(Publication Date)
- CRC Press(Publisher)
consists in measuring the signal frequency as a function of the static pressure. Figure 3.43 gives the experimental and theoretical results of the oscillatory frequency versus pressure realized in the test section presented above. 3.4 Conclusion The thermocouple is one of the most widely used devices for temperature measurement. It presents advantages: inexpensive, rugged, simply constructed, fast in the response to changes in temperature (microThermocouples), and capable of being used to directly meas-ure temperatures from 200 8 C up to 2600 8 C. But, disadvantages exist too: temperature FIGURE 3.43 Frequency versus vacuum. 10 4 200 400 600 800 f (mHz) 10 0 10 1 10 2 10 3 10 5 Pressure (Pa) Experimental results Theoretical curve FIGURE 3.42 Frequency versus pressure in a test volume. Vacuum pump O 1 Pirani gauge O 2 Microthermocouple Th sens (diameter 25.4 μm) Measurement chamber (90 cm 3 ) Measurement chamber Temperature Measurements: Thermoelectricity and MicroThermocouples 137 measurement with a thermocouple requires in fact independent measurements of two temperatures, the junction at the hot junction and the junction where wires meet the instrumentation copper wires (cold junction). To avoid error, the cold junction temperature is in general compensated in the electronic instruments by measuring the temperature at the terminal block using with a semiconductor, thermistor, or RTD. Thermocouple operation is relatively complex with potential sources of error. The materials of thermocouple wires are not inert, and the thermoelectric voltage developed along the length of the thermocouple wire may be in fl uenced by corrosion, etc. The relationship between the process temperature and the thermocouple signal (millivolt) is not linear. The calibration of the thermocouple should be carried out while it is in use by comparing it to a nearby comparison thermo-couple. The size reduction of thermal sensors has been signi fi cant during the last 20 years.
eBook - ePub- Cecil L. Smith(Author)
- 2011(Publication Date)
- Wiley-AIChE(Publisher)
To be completely independent of intermediate temperatures, the thermocouple wires must be homogeneous. This affects the manufacturing process for the thermocouple wires. The purity must be exact and uniform; the wires must be drawn in such a manner that no imperfections are created.JunctionsIn reality, the thermocouple responds to a temperature difference. It cannot be used to directly measure the value of a temperature. But if we know the temperature at one of the junctions, the temperature at the other junction can be computed from the temperature difference. This is the basis of the following terminology:Process junction or measuring junction. This is the junction whose temperature is to be determined.Reference junction. This is the junction whose temperature is known.The process, or measuring, junction is commonly referred to as the hot junction , and the reference junction as the cold junction. But there is one problem with this terminology—in an occasional application the process junction is at a temperature below that of the reference junction.Law of Intermediate MetalsLet’s change the resistor in our circuit to copper, as illustrated by the upper circuit in Figure 2.13 . What effect does this have on the current or voltage? None, but with one proviso. Let the temperatures at the terminals of the resistor be T 3 and T 4 . As long as these temperatures are the same, using a copper resistor has no effect on the current or voltage.Figure 2.13 Law of intermediate metals.Inserting a voltage-sensing device into the circuit is equivalent to inserting a copper resistor. Normally, this is inserted at the reference junction, giving the lower circuit in Figure 2.13 . We no longer have an iron-constantan junction at the reference junction. Instead, we have an iron-copper junction and a copper-constantan junction. But provided the two terminals at the reference junction are at the same temperature, this is equivalent to an iron-constantan junction.The two junctions involving copper must be at the same temperature. In practice, this receives special attention, resulting in the so-called isothermal terminal block that contains the termination screws for the thermocouple at the reference junction.
eBook - PDF- Walt Boyes(Author)
- 2002(Publication Date)
- Butterworth-Heinemann(Publisher)
Figure 14.33(b) shows a block diagram of such an arrangement. Thermocouple input cir- cuits are available as encapsulated electronic modules. These modules contain input amplifier and cold junction compensation. Siiice the cold junction consists of the input connections of the module, the coiinections and the cold junction sensor can be accurately maintained at the same temperatnre by encapsulation, giving very accu- rate compensation. These modules can be very versatile. Many are available for use with any of the normal Thermocouples. The cold junction compensation is set to the thermocouple in use by connecting a specified value resistor across two terminals of the module. Where the thermocouple instrument is based on a microcomputer the cold junction compensation can be done by software, the microcomputer being programd to add the compensation value to the thermocouple output. In all electronic equipment for thermocouple signal processing the location of the sensor for cold junction temperature sensing is critical. It must be very close to the cold junction terminals and pre- ferably in physical contact with them. 14.5.2 Thermocouple materials Broadly, thermocouple materials divide into two arbitrary groups based upon cost of the mater- ials, namely, base metal Thermocouples and pre- cious metal Thermocouples. 14.5.2.1 Bme metal tlzermocozyles The most coniionly used industrial thermo- couples are identified for convenience by type letters. The main types, together with the relevant British Standard specification and permitted tolerance on accuracy, are shown in Table 14.13. Also shown are their output e.m.f.s with the cold junction at 0 C. These figures are given to indicate the relative sensitivities of the var- ious couples. Full tables of voltages against hot junction temperatures are published in BS 4937. The standard also supplies the equations gov- erning the thermocouple e.m.f.s for convenience for computer programming purposes.
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