How Thermocouples Measure Temperature

Overview

A thermocouple is made of two dissimilar conductors joined at one end. When the measuring junction and the reference junction are at different temperatures, the loop generates a small voltage — the Seebeck effect — which the instrument converts into a temperature reading.

Key Points

The reference junction matters. The voltage depends on the temperature difference between the two junctions, so the cold end must be measured and compensated.
Extension vs compensating cable. Extension cable uses the same conductors; compensating cable uses cheaper substitutes with matching characteristics over a limited range.
Polarity. Reversing the wires produces an error roughly twice the temperature difference.
Type selection. K for general use, N for stability in air, S/R/B for high temperatures, T for low temperatures and moist atmospheres.

Summary

Most “sensor” problems are actually circuit problems: wrong cable type, reversed polarity, or an uncompensated reference junction.

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