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-59 to +93 ±0.8°C ±0.4°C
+93 to +371 ±3/4% ±3/8%
J (iron-Constantan) 0 to 277 ±2.2°C ±1.1°C
277 to 760 ±3/4% ±3/8%
E (Chromel-Constantan) 0 to 316 ±1.7°C
316 to 871 ±1/2%
K (Chromel-Alumel) 0 to 277 ±2.2°C ±1.1°C
277 to 1260 ±3/4% ±3/8%
R and S (platinum vs. 13% 0 to 538 ±1.4°C
(platinum vs. 10% rhodium-platinum) 538 to 1482 ±1/4%
B (platinum vs. 30% rhodium- 871 to 1705 ±1/2%
platinum vs. 6% rhodium)
temperature versus electromotive force as well as polynomial equations expressing the temperature-voltage relationship for different types of thermocouples are available in technical literature.
The iron-Constantan thermocouple is used most widely in industrial applications. The copper-Constantan thermocouple is used widely in industrial and laboratory thermometry.
The platinum -10 percent rhodium versus platinum (Type S) thermocouple serves as an instrument for defining the International Practical Temperature Scale from 630.74 to 1064.43°C. It is being used in industrial laboratories as a standard for base-metal thermocouples and other temperature-sensing devices.
Table M-4 lists the seven commonly used thermocouples and some of their characteristics.
REFERENCE JUNCTIONS I
FIG. M-3 Thermocouple thermometer systems. (Source: Demag Delaval.)
The electrical conductors connecting the thermocouple and the measuring instrument may use the actual thermocouple wires, extension wires, or connecting wires (see Fig. M-3). When it is not possible to run the thermocouple wires to the reference junction or to the measuring instrument, extension wires can be used. To assure a high degree of accuracy, extension wires should have the same thermoelectric properties as the thermocouple wires with which they are used. Significant uncertainties are introduced when extension wires are not matched properly. Calibration of the instrument with extension wires helps to minimize these uncertainties. Connecting wires are a pair of conductors that connect the reference junction to the switch or potentiometer. They are usually made of copper. They do not cause uncertainty in measurements when the reference junction is kept at constant temperature, for example, the ice point.
Indicating potentiometers are recommended by the ASME Power Test Codes for performance-test work, although recording potentiometers are used for industrial-process temperature measurement.
Thermocouples may be joined in series. The series connection, in which the output is the arithmetic sum of the emfs of the individual thermocouples, may be used to obtain greater measurement sensitivity and accuracy. A series-connected thermocouple assembly is generally referred to as a thermopile and is used primarily in measuring small temperature differences. A schematic diagram of a series-connected thermocouple is shown in Fig. M-4.
Thermocouples may also be joined in parallel. In the parallel-connected thermocouple circuit, a mean value of the individual thermocouples is indicated, and it will be the true arithmetic mean if all thermocouple circuits are of equal resistance. A schematic diagram of a parallel-connected thermocouple circuit is shown in Fig. M-5.
The installation of extensive thermocouple equipment requires the services of qualified instrument technicians, and special attention should be given to extension wires, reference junctions, switches, and terminal assemblies.
Opposed thermocouple circuits are sometimes used to obtain a direct reading of a temperature difference between two sets of thermocouples reading two levels of
FIG. M-4 Thermocouples connected in series. (Source: Demag Delaval.)
FIG. M-5 Thermocouples connected in parallel. (Source: Demag Delaval.)
temperature. The number of thermocouples in each set is the same. This method is considered to provide the highest degree of accuracy in the measurement of the critical temperature difference.