Thermocouple emfs

To avoid or minimize unwanted thermal EMFs, thermocouple junctions are used in pairs as shown in the bottom portion of Figure 12.10. One of the junctions is maintained at a known temperature - often an ice bath. The temperature of the other junction can then be inferred from the resulting total EMF using standard thermocouple tables or special calibration tables prepared for your specific thermocouple. 

In industrial appHcations it is not uncommon that the thermocouple must be coupled to the readout instmment or controUer by a long length of wire, perhaps hundreds of feet. It is obvious from the differential nature of the thermocouple that, to avoid unwanted junctions, extension wine be of the same type, eg, for a J thermocouple the extension must be type J. Where the thermocouple is of a noble or exotic material, the cost of identical lead wine may be prohibitive manufacturers of extension wine may suggest compromises which are less costiy. Junctions between the thermocouple leads and the extension wine should be made in an isothermal environment. The wine and junctions must have the same electrical integrity as the thermocouple junction. Because the emf is low, enclosure in a shield or grounded conduit should be considered. 

The thermal emf of the thermocouple is a function of the difference between the hot end and the cold end, the latter usually located at the readout instmment thus the measurement can be no mote accurate than the isothermality of the leads at the cold end and the accuracy with which this temperature is known. Three common methods for addressing these problems foUow. 

Thermocouples Temperature measurements using thermocouples are based on the discovery by Seebeck in 1821 that an electric current flows in a continuous circuit of two different metalhc wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammaticaUy as shown in Fig. 8-60. A and B are the two metals, and T and To are the temperatures of the junctions. Let T and To be the reference junction (cold junction) and the measuring junc tion, respectively. If the thermoelectric current i flows in the direc tion indicated in Fig. 8-60, metal A is customarily referred to as thermoelectricaUy positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), cmromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between To and T. In control systems the reference junction is usually located at... 

ASTM E230-96el. Standard Specification for Temperature-Electromotive Force (EMF) Tables for Standardized Thermocouples. American Society for Testing and Materials, 1996. 

Seebeck used antimony and copper wires and found the current to be affected by the measuring instrument (ammeter). But, he also found that the voltage generated (EMF) was directly proportional to the difference in temperature of the two junctions. Peltier, in 1834, then demonstrated that if a current was induced in the circuit of 7.1.3., it generated heat at the junctions. In other words, the SEEBECK EFFECT was found to be reversible. Further work led to the development of the thermocouple, which today remains the primary method for measurement of temperature. Nowadays, we know that the SEEBECK EFFECT arises because of a difference in the electronic band structure of the two metals at the junction. This is illustrated as follows ... 

In differential thermal aned rsls, l.e.- DTA, we use one thermocouple Taucked" against the voltage output of another of the same composition to produce a "net" EMF. What this means Is that either the positive (or negative) legs of both thermocouples are electrically connected so that the net EMF at any given temperature of the two Is zero. Only If one thermocouple temperature differs from that of the other does one obtain an EMF response. 

If we put a sample next to one thermocouple and a "standard" or reference" next to the other, we can follow any thermal changes that may take place as both are heated since each TC generates Its own EMF as the temperature changes. Thus, If we put a reference material, R, directly in contact with the "TC(1)" thermocouple junction (hereinafter, we will refer to this thermocouple junction as "R") and a sample, S, at TC(2), l.e.- S , then we can detect any thermal change that may occur if either R or S undergoes a transformation as it is heated. 

The relation between the emf of the thermoelectric pile and the heat flux from the calorimeter cell will be first established. Let us suppose (Fig. 8) that the process under investigation takes place in a calorimeter vessel (A), which is completely surrounded by n identical thermoelectric junctions, each separated from one another by equal intervals. The thermocouples are attached to the external surface of the calorimeter cell (A), which constitutes the internal boundary (Eint) of the pile and to the inside wall of the heat sink (B), constituting the external boundary (Eext) of the thermoelectric pile. The heat sink (B) is maintained at a constant temperature (6e). 

Fig. 6. Rule 3 of thermocouples, where (a) represents a four-junction and (b) a two-junction thermocouple. A and B, the two legs of the thermocouple, are wires of dissimilar materials. Junctions are at temperatures Tp T2, and Ty Ep E2> and E3 are thermal emfs. The thermal emf at T3 is the same for both...

THERMOCOUPLE. In 1821, Seebeck discovered that an electric current flows in a continuous circuit of two metals if the two junctions are at different temperatures, as shown in Fig. 1. A and B are two metals, T and T are the temperatures of the junctions. I is the thermoelectric current. A is thermoelectncally positive to B if 7i is the colder junction. In 1834, Peltier found that current flowing across a junction of dissimilar metals causes heat to be absorbed or liberated. The direction of heat flow reverses if current flow is reversed. Rate of heat flow is proportional to current but depends upon bodi temperature and the materials at die junction. Heat transfer rate is given by PI, where P is the Peltier coefficient in watts per ampere, or die Peltier emf in volts. Many studies of the characteristics of thermocouples have led to the formulation of three fundamental laws ... 

Common thermocouple wire combinations used in industry are listed in Table 1. A choice of different metals is needed to fulfill a broad range of temperatures as well as for oxidizing or reducing conditions in use. The temperature-thermal emf curves for common types of thermocouples are... 

Fig. 2. Law nf Intermediate Metals makes it possible to use foreign wires to connect thermocouple to measuring instrument. Thermocouple materials A and B can be connected to the instrument by use of connecting materials C and D. If the temperatures at Xj and X2 are both at T and if temperatures at Y and Y2 are both at Ti, the emf of the circuit will be independent of materials C and D...

Fig. 3. Temperature-thermal emf curves for common types of thermocouples. (Honeywell)...

The emf developed by a thermocouple depends upon the temperature of both the measuring and reference junctions. Thus, to determine temperature, the following data musi be known (1) the calibration data for the particular thermocouple (2) the measured emf and (3) the temperature of the reference junction. In laboratoiy cases, the reference junction can be maintained at the freezing temperature of water. However, in most modem instruments, the ambient temperature of the reference junction is sensed, and the correction is incorporated in the measurement circuitry. 

The temperature of a gas oil product flowing through a pipe is monitored using a chromel/alumel thermocouple. The measurement junction is inserted into the pipe and the reference junction is placed in the plant control room where the temperature is 20°C. The emf at the thermocouple junction is found to be 6.2 mV by means of a potentiometer connected into the thermocouple circuit adjacent to the reference junction. Find the measured temperature of the gas oil. 

Thermojunctions may be formed by welding, soldering or pressing the materials together. Such junctions give identical emfs (by law (iii)), but may well produce different currents as the contact resistance will differ depending on the joining process utilised. Whilst many materials exhibit thermoelectric effects, only a small number are employed in practice. The characteristics of the more common thermocouple materials are listed in Table 6.4. 

A typical thermocouple installation for an industrial application is shown in Fig. 6.23. Instead of placing the reference junction in a temperature controlled environment (which is often inconvenient), an automatic reference junction compensation circuit is fitted. This provides a second source of emf Sj,° in series with the thermocouple emf E. The meter thus measures 0 = E 0 where E%-0... 

Thermocouples consist of two dissimilar electrical conductors which are joined to form a measuring junction, with the free ends of the wires constituting the reference junction. When a temperature difference exists between the measuring and reference junctions, an emf is produced between the free ends of the device. This emf, which is a function of the temperature difference, can be used to determine the temperature at the measuring junction if the reference junction temperature is known. A schematic of a typical thermocouple circuit is shown in Fig. 9.12. 

Temperature control is normally carried out using thermocouples in a stainless steel pocket. The type of thermocouple used is either a platinum resistance detector (RTD) or a thermocouple using two dissimilar metals that produce a voltage (EMF). The indicators for these thermocouples must match the probe type and grade. The positioning of the probes is very important as well as any lag (delay) in the system. The output from the probe is connected to the indicator and/or controllers. Most indicators have at least a set point with an on/off output. The more advanced units will allow anticipated switching, more than one set point, temperature ramping between temperatures, time, and hold facilities. Thermocouple break and over-temperature alarm outputs are also commonly provided features. 

The most often used temperature detectors in renewable energy and most other processes are the thermocouples (TCs). Their operation is based on the principle known as the Seebeck effect. T. J. Seebeck discovered that heating the junction of dissimilar metals generates a small, continuous electromotive force (EMF). The name is a combination of thermo and couple denoting heat and two junctions, respectively. The dissimilar TC wires are joined at the hot (or measurement) end and also at the cold junction (reference end),... 

It is important to understand that the tables and polynomials are based on the assumption that the cold junction of the thermocouple pair is at zero degrees Celsius. In the laboratory, the cold junction is generally at room temperature or slightly above (the temperature at the screw terminals where the thermocouple wires and lead-wires join), hence a correction factor is needed. The law of successive potentials (Figure 2.6) may be stated as The sum of the EMF s from the two thermocouples is equal to the EMF of a single thermocouple spanning the entire temperature range ... 

E Chromel - Con-stantan (Cu-Ni) -200 to 900 0 to 200 Recommended for vacuum use or for inert, mildly oxidizing or reducing atmospheres. These thermocouples produce the greatest amount of emf and therefore can detect small temperature changes. 

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