Question

Answer all questions. Explain what each of the following is in relation to thermocouples: (a) extension leads, (b) compensating leads, (c) law of intermediate metals, (d) law of intermediate temperature.

Answer 1

Thermocouples:

a)Extension leads

Tthermocouples are typically only a few centimeters long. It is clearly impractical to connect a voltage-measuring instrument at the open end of the thermocouple to measure its output in such close proximity to the environment whose temperature is being measured, and therefore extension leads up to several meters long are normally connected between the thermocouple and the measuring instrument.

A near-zero junction voltage is most easily achieved by choosing the extension leads to be of the same basic materials as the thermocouple, but where their cost per unit length is greatly reduced by manufacturing them to a lower specification.

b)Compensating leads

To use lower specification materials of the same basic type as the thermocouple, copper compensating leads are also sometimes used with certain types of base metal thermocouples. in this case the law of intermediate metals has to be applied to compensate for the emf at the junction between the thermocouple and compensating leads

Thermoelectric behavior of compensating leads is only valid for a limited range of temperatures that is considerably less than the measuring range of the thermocouple that they are connected to

c) Law of intermediate metals

The law of intermediate metals states that a third metal may be inserted into a thermocouple system without affecting the emf generated, if, and only if, the junctions with the third metal are kept at the same temperature.

When thermocouples are used, it is usually necessary to introduce additional metals into the circuit This happens when an instrument is used to measure the emf, and when the junction is soldered or welded. It would seem that the introduction of other metals would modify the emf developed by the thermocouple and destroy its calibration. However, the law of intermediate metals states that the introduction of a third metal into the circuit will have no effect upon the emf generated so long as the junctions of the third metal are at the same temperature, as shown in Above Figure

d) Law of intermediate temperatures

The law of intermediate temperatures states that the sum of the emf developed by a thermocouple with its junctions at temperatures T1 and T2, and with its junctions at temperatures T2 and T3, will be the same as the emf developed if the thermocouple junctions are at temperatures T1 and T3.

This law, illustrated in above Figure, is useful in practice because it helps in giving a suitable correction in case a reference junction temperature other than 0 °C is employed. For example, if a thermocouple is calibrated for a reference junction temperature of 0 °C and used with a junction temperature of 20 °C, then the correction required for the observation would be the emf produced by the thermocouple between 0 °C and 20 °C.