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Field Test Best Practices: A Resource for Practical Residential Building Science

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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.

Immersion Thermocouples

Immersed thermocouples are the preferred method for measuring the temperature of liquid flow in a pipe. They should be mounted in a plumbing tee where an elbow would normally be used. If there is no convenient spot where an elbow would normally be used, a u-shape can be plumbed-in to allow the installation of the tee.

Surface-Mount Thermocouples

Surface-mount thermocouples are used when equipment cannot be penetrated. Measuring refrigerant temperature is an example of appropriate surface-mount thermocouple usage; there is no reasonable method for immersing a thermocouple in the refrigerant flow path. Surface-mount thermocouples are also used to measure solid surface temperatures.

Bare Thermocouples

In building applications, bare thermocouples are used for measurement of air temperatures. While they can be used for other measurements, the best practice is to use one of the other thermocouple types (immersion or surface-mounted thermocouples) for measuring liquid or solid temperatures. Bare thermocouples are just that – a soldered or welded junction of Copper and Constantan where the sheathing has been stripped or pulled back from T-type thermocouple wire.