Thermocouples

A thermocouple is comprised of two dissimilar materials (usually metallic wires) bonded together. The junction between the wires forms a microscopic region where voltage is induced between the wires. As the temperature of the junction changes, the voltage will vary in a repeatable, controlled manner, and can be measured at the other end of those wires.

For building science purposes, T-type thermocouples, made of Copper and Constantan with a temperature range from -200 to 350°C, are always appropriate. Shielded thermocouple wire is strongly preferred, to limit noise from power wires being inferred over the thermocouple’s microvolt signal.

Accuracy of a thermocouple depends on the material properties, and uniformity of those properties. It is strongly recommended that researchers always purchase thermocouples and extension wire with lowest uncertainty. The preferred extension wire will be labeled "Special Limits of Error" or SLE, and typically has a ±0.5°C accuracy limit. This is half the error range of standard thermocouple wire. SLE wire cost is minimally higher than standard, but may require a slightly longer lead time in purchasing.

Accuracy of a thermocouple measurement also depends on the temperature, and thus location, of any junctions in the signal wiring. Often, a thermocouple is purchased with a connector attached from the factory. That connector creates two additional thermocouples (one for each pin/wire), called cold junctions. Where possible, the connector should be located to maintain similar temperatures to the data logger to minimize the impact of this intermediate cold junction on the measurement.

Several forms of manufactured thermocouples are available, for use in different applications.