THERMOCOUPLES - h1013v1

Instrumentation and Control Volume 1 of 2
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Temperature Detectors THERMOCOUPLES THERMOCOUPLES The thermocouple is a device that converts thermal energy into electrical energy. EO 1.4 DESCRIBE the basic construction of a thermocouple including: a. Major component arrangement b. Materials used EO 1.5 EXPLAIN how a thermocouple provides an output representative of the measured temperature. Thermocouple Construction Figure 4 Thermocouple Material Characteristics When Used with Platinum A thermocouple is constructed of two dissimilar metal wires joined at one end. When one end of each wire is connected to a measuring instrument, the thermocouple becomes a sensitive and highly accurate measuring device. Thermocouples may be constructed of several different combinations of materials. The performance of a thermocouple material is generally determined by using that material with platinum. The most important factor to be considered when selecting a pair of materials is the "thermoelectric difference" between the two materials. A significant difference between the two materials will result in better thermocouple performance. Figure 4 illustrates the characteristics of the more commonly used materials when used with platinum. Other materials may be used in addition to those shown in Figure 4. For example: Chromel- Constantan is excellent for temperatures up to 2000°F; Nickel/Nickel-Molybdenum sometimes replaces Chromel-Alumel; and Tungsten-Rhenium is used for temperatures up to 5000°F. Some combinations used for specialized applications are Chromel-White Gold, Molybdenum-Tungsten, Tungsten-Iridium, and Iridium/Iridium-Rhodium. Rev. 0 Page 5 IC-01

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