The material must be physically stable. It should not experience phase changes (especially in cryogenic applications). It should not be altered by mechanical handling (for example, it must not lose ductility).

The material should be insensitive to magnetic fields.

2. Homogeneity. It is important that the material be homogeneous not only along a given sample but also from sample to sample so that a single calibration be is valid for an entire batch.

3. Good thermoelectric power. The lower temperature limit for practical use of thermocouple thermometers (about 20 K) is the result of insufficient-values of a as absolute zero is approached.

4. Low thermal conductivity. This is important at cryogenic temperatures.

The choice of the type of thermocouple depends mainly on the temperature range desired, as indicated in Table 5.1.

To achieve the highest precision, it is essential to perform very careful calibration. The extremely meticulous procedures required are described in detail (with ample references) by Burns and Scroger (1969). As an example, a NIST (National Institute of Standards and Technology) publication reports the calibration of a thermocouple (submitted by a customer) as having uncertainties not exceeding 3 ¡V in the range of 0 C to 1450 C. Since the Seebeck voltage for this particular sample was 14,940 ¡V at the highest temperature mentioned, the uncertainty at that temperature was less than 0.02%.

Very complete data of thermoelectric voltages versus temperature, tabulated at intervals of 1 Celsius for various types of thermocouples are found in NIST ITS-90 Thermocouple Database at < its90/main/>. An example of such data appears in Figure 5.7. Observe that the voltages developed by a thermocouple thermometer are small compared to those developed by a thermoelectric generator. See Section 5.3.

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