Iron-constantan thermocouples, temperature range

BL TEMPKRATURF- Thermocouples are the most commonly used temperature sensing devices. The two dissimilar wires produce a millivolt signal that varies with the hot-junction temperature. Iron-constantan thermocouples are commonly used over the 0 to 1300°F temperature range. 

When certain materials are bonded together, electrons tend to transfer from one to the other. This is called the Volta effect. If two such materials are joined together with two junctions at the same temperature, the plus Volta emf at one junction will be balanced by a minus emf at the other and no current will flow. However, if the two junctions have different temperatures, a current will flow from one junction to the other. This is called the Seebeck effect, and is the basis of the thermocouple. Figure 11.1 (a) shows a thermocouple with a small voltmeter in series with two thermocouple wires (iron and 60 Cu-40 Ni constantan). The emf will be proportional to (T2 - Tj). Figure 11.1 (6) gives the calibration curve for an iron-constantan couple. Thermocouples are used to measure very high (furnace) temperatures, and when the upper range of an iron-constantan thermocouple is reached, a platinum-rhodiiun couple [also shown in Fig. 11.1(6)] may be employed. 

Temperature measurements ranging from 760 to 1760°C are made usiag iron—constantan or chromel—alumel thermocouples and optical or surface pyrometers. Temperature measuriag devices are placed ia multiple locations and protected to allow replacement without iaciaerator shutdown (see... 

Thermocouples are based on the thermoelectric Seebeck effect, which generates a voltage at the junction between two metallic conductors, which depends on temperature [13]. Thus, in the measuring circuit, two junctions are created, namely, a sensitive (or hot) junction at the point where temperature has to be measured and a nonsensitive (cold) junction, kept at a constant known temperature, where the voltage established between the conductors can be easily measured [19]. Different typologies of thermocouples exist for application in a wide range of conditions they essentially differ by the materials, the most common being J (iron/constantan), K (chromel/alumel), T (copper/constantan), and E (chromel/constantan). 

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