温度测量热电偶-外文文献及译文.doc

Page 1 University of Tasmania School of Engineering KNE222 Electronic Engineering Temperature Measurement – Thermocouples Introduction. A thermocouple is formed when two dissimilar metals are joined. Ifa second joint is made from the same materials, and one joint is heated with respect to the other, then a current will flow round the loop. Alternatively, if the circuit is opened, then a small voltage will appear at these terminals, proportional to the difference in temperature between the junctions. This voltage is known as the Seebeck voltage, after Thomas Seebeck who discovered the phenomenon in1821. Figure 1. Conceptual Application ofa Thermocouple (Ref: ” Operational Amplifier Circuits Theory and Applications ” Kennedy. HRW Inc.) In order to use a thermocouple to measure temperature with one junction, a reference temperature must be established for the other. This can bedone as illustrated in Figure 1, where a0oC ice bath is provided for the reference junction. However an ice bath isnot really practical for everyday measurements. Fortunately it is possible to introduce a correction voltage into the circuit so that the reference junction appears tobe at zero degrees C. An example of this will be presented later. Thermocouple Characteristics. Table 1 provides a list monly used thermocouple materials, together with the useful range of temperatures for each. While the Seebeck voltage increases with the junction temperature difference, the relationship is unfortunately not entirely linear. The proportionality constant is known as the Seebeck coefficient, α, and has the units μ V/o C. Table 1 provides typical α values at25o C; it also gives some idea of how α varies over the useable temperature range for each thermocouple. Table 1. Characteristics mon Thermocouples (Ref: “ Operational Amplifier Circuits Theory and Applications ” Kennedy. HRW Inc.) Page 2 Figure 2 shows the variation ofα asa function of temperature for K and T Type thermocouples. The K type dev