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

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.

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Figure 4 Thermocouple Material Characteristics when Used with Platinum

Other materials may be used in addition to those shown in Figure 4. For example: ChromelConstantan is excellent for temperatures up to 2000F; Nickel/Nickel-Molybdenum sometimes replaces Chromel-Alumel; and Tungsten-Rhenium is used for temperatures up to 5000F. Some combinations used for specialized applications are Chromel-White Gold, Molybdenum-Tungsten, Tungsten-Iridium, and Iridium/Iridium-Rhodium.

Figure 5 shows the internal construction of a typical thermocouple. The leads of the thermocouple are encased in a rigid metal sheath. The measuring junction is normally formed at the bottom of the thermocouple housing. Magnesium oxide surrounds the thermocouple wires to prevent vibration that could damage the fine wires and to enhance heat transfer between the measuring junction and the medium surrounding the thermocouple.