Thermal EMF

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... 

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...

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

Measurement of a four-lead PRT with a three-terminal Wheatstone bridge. The resistance i , = Ri + R I2 see text for details. If the lead resistances and Rg are equal and there are no disturbing thermal emfs at the lead Junctions, R, can be determined from either of the single bridge balance conditions since Ri and R are then the same. 

What actually occurs during these final preparation procedures can be inferred from resistance measurements taken on pellets within the furnace. A DC technique with current reversal was used for these measurements. Four platinum leads were attached to the pressed pellet sample with silver epoxy. The epoxy was covered with a layer of protective ceramic paste. (8) To compensate for induced thermal emf s, an average of the forward and reversed current directions was... 

Some environmental limitations of the standard thermocouple materials compiled by ASTM [36] are reproduced in Table 16.11. The thermal EMF of standard thermoelements relative to platinum is shown in Fig. 16.20 [36]. Seebeck coefficients (first derivative of thermal EMF with respect to temperature) for each of the standard thermocouples as a function of temperature are tabulated in Table 16.12. 

Note 2—Stability under neutron radiation refers to chemical composition of thermoelement, not to stability of thermal EMF. 

In view of the correlations between a, II, and t, we need only to present data for one of these parameters, namely, thermal emf a... 

When two junctions of a material are maintained at different temperatures, then a potential difference or thermal emf develops between them. This phenomenon is referred to as the Seebeck effect 18), and the thermal emf is called the thermoelectric potential . The type of conductivity may be deduced by the sign of . 

The electrical properties of macromolecular semiconductors are generally characterized by the conductivity, the activation energy of the conductivity, the free radical concentration, and the thermal electromotive force (thermal emf). Since the polymers are usually in the form of amorphous powders, they are compressed into tablets. The contacts are either metal electrodes pressed into the surface or conductive pastes. The samples may not have any ionic conductivity (due to impurities) or surface conductivity and must be free of moisture, otherwise the conductivities will be too high. 

To determine the thermal emf, the sample is placed between two plates at different temperatures. The thermal voltage that occurs with a difference in temperature of 1°C is called the Seebeck coefficient. The Seebeck coefficient is positive when the hotter pole is positive. A positive Seebeck coefficient originates from an excess of electron defects (p-type conductivity) and a negative coefficient from an excess of conducting electrons ( -type conductivity). The concentration of free radicals, measured using electron-spin resonance, need not be identical, of course, with the concentration of conducting electrons. 

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