Temperature measurement thermoelectric

The basic ideas of thermoelectricity have been known for nearly two centuries, but until well after the Second World War the primary use was for temperature measurement (qv) using metallic wires. Then, upon improvements in semiconductor technology, thermoelectric power generation and refrigeration came under serious consideration. 

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... 

At relatively high temperatures thermocouple thermometers are most commonly used to measure temperature. The thermoelectric power of three frequently used thermocouples is compared in Figure 10.2. The choice of thermocouple depends on the temperature range, the chemistry of the problem in question, sensitivity requirements and resistance towards thermal cycling. The temperature range and typical uncertainty of some of the most commonly used thermocouple thermometers are given in Table 10.2. 

A second improvement in Calvet s calorimeter is that a differential set-up was adopted that aimed to suppress temperature drifts and fluctuations of the heat sink. This was achieved by coupling two calorimetric units in opposition to each other, so the measured thermoelectric force was the difference between the thermoelectric forces of the sample cell and the reference cell. The latter may remain at the temperature of the thermostat while the heat output or input related to the event under investigation occurs in the sample cell. 

Metal Oxide-Polymer Thermistors. The variation of electrical properties with temperature heretofore described can be used to tremendous advantage. These so-called thermoelectric effects are commonly used in the operation of electronic temperature measuring devices such as thermocouples, thermistors, and resistance-temperature detectors (RTDs). A thermocouple consists of two dissimilar metals joined at one end. As one end of the thermocouple is heated or cooled, electrons diffuse toward... 

In a nonisothermal system, an electric current (flow) may be coupled with a heat flow this effect is known as the thermoelectric effect. There are two reciprocal phenomena of thermoelectricity arising from the interference of heat and electric conductions the first is called the Peltier effect. This effect is known as the evolution or the absorption of heat at junctions of metals resulting from the flow of an electric current. The other is the thermoelectric force resulting from the maintenance of the junctions made of two different metals at different temperatures. This is called the Seebeck effect. Temperature measurements by thermocouples are based on the Seebeck effect. 

On the other hand, for slow reactions, adiabatic and isothermal calorimeters are used and in the case of very small heat effects, heat-flow micro-calorimeters are suitable. Heat effects of thermodynamic processes lower than 1J are advantageously measured by the micro-calorimeter proposed by Tian (1923) or its modifications. For temperature measurement of the calorimetric vessel and the cover, thermoelectric batteries of thermocouples are used. At exothermic processes, the electromotive force of one battery is proportional to the heat flow between the vessel and the cover. The second battery enables us to compensate the heat evolved in the calorimetric vessel using the Peltier s effect. The endothermic heat effect is compensated using Joule heat. Calvet and Prat (1955, 1958) then improved the Tian s calorimeter, introducing the differential method of measurement using two calorimetric cells, which enabled direct determination of the reaction heat. 

Since temperature measurements are required over such a wide range and diversity of situations, a large number of different types of thermometers with varying levels of accuracy and convenience have been developed over the years. Those most frequently used are based on the expansion of a gas, liquid or solid on changes in electrical resistance on the thermoelectric effect on changes in the thermal radiation of a system on changes in the thermal (Johnson) noise of electrical resistors on changes... 

Temperature measurement using thermocouples is based on the thermoelectric effect. Two dissimilar metals are joined together at a junction where an electromotive force (emf) is generated according to the Seebeck effect. The emf level depends on the junction temperature. The Peltier effect causes an emf to be generated when the dissimilar metals are connected to an electrical circuit. 

Experimental arrangement for measuring thermoelectric emfs of glass-forming melts by means of zirconia microelectrodes. (1) Zirconia electrodes (2) melt (3) temperature gradient furnace (4) leads from PtiOj reference electrodes (5) leads from thermocouples (6) reference gas inlet (7) outlet. 

The simplest and most inexpensive temperature monitors are thermocouples, such as copper-constantin (type T). Although the temperature coefficient of the thermoelectric effect is low (39 /nV/K at room temperature for type T), there exist excellent tables for common thermocouples which make them extremely reproducible (see, e.g., the helpful handbook on temperature measurements from Omega Engineering, Stamford, CT). Unfortunately, below about 30 K the thermoelectric coefficient decreases by at least a factor of 10 and thermocouples become not useful (unless one uses exotic and expensive couples like gold/iron). Further, because the voltages measured are so low, artifacts such as ground loops and stray return currents from the typical 1-5 A current being applied to the heater can lead to serious errors. 

The measurement and control of the temperature of experimental apparatus in cryogenic environments has been widely explored p]. Problems in such measurement and control by thermoelectric and thermal resistance effects are receiving constant attention. However, the application of Chromel-P vs. constantan thermocouples to cryogenic temperature measurement and control has not become widespread. The reason for this limited usage is not clear, especially since the sensitivity and potential 2. 3] fQj. his thermocouple system are higher than for the more popular copper vs. constantan thermocouple system. Furthermore, the use of low-thermal-conductivity Chromel-P P] wire, instead of copper wire, would reduce heat leaks into cryogenic systems. 

The additional, and most important feature of the Calvet type of calorimeter, is the existence of a second, identical microcalori-metric element situated elsewhere in the metal block. For temperature measurements the two cells ardF i iected in opposition, which means that it is the difference in thermoelectric e.m.f. be-... 

Thermoelectric ects occur whenever more than one type of metal is used in the measurement circuit, which includes the sensing element, the leads, and the readout instrumentatimi. The typical Seebeck coefficient between different metals is around 10 pV/K or more, and it is not unusual for temperature to vary by 5—10 °C between different parts of an apparatus even for room-temperature measurements. Thus, thermoelectric effects can be expected to contribute voltages of 100 pV or more if no precautions are taken, which may lead to errors of 1 °C when measuring the RTD temperature around 300 K, and potentially much worse away from 300 K. The first... 

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