Thermocouple Application Note 3

What is the significance of a temperature gradient when there are “intermediate materials” in the thermocouple circuit?

For the purpose of this blog, “intermediate materials” is defined as a portion of a thermocouple circuit that has a different Seebeck Coefficient (thermoelectric response) than the normal thermocouple wires. This could be due to physical damage to a wire, chemical contamination of a wire or the existence in the circuit of a different material, such as in a connector.

The magnitude of the thermoelectric force (emf) generated in a thermocouple circuit is dependent upon both the Seebeck coefficients of the materials and the temperature gradient between the measuring and reference junctions. If there is zero temperature gradient within a portion of the circuit, the emf generated in this portion will be zero. Within this “zero temperature gradient “ zone any variation in Seebeck coefficient, such as would occur with an intermediate material, will have no effect on the net output of the thermocouple.

If a temperature gradient does exist across an “intermediate material” there will be an error that is dependent on the magnitude of the gradient and the thermoelectric properties (often unknown) of the intermediate material.

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Figure 1: Contaminated tungsten-rhenium thermocouple wire

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