All About Thermocouples (part 1)
Accurate temperature control of heat-treating furnaces, ovens and baths depends in large part on the proper choice of thermocouples as well as on ancillary items such as extension (lead) wire, protection tubes and connectors. Specifications like the latest revision to AMS 2750 (Pyrometry) have helped by focusing attention on the critical role of these important temperature sensors. Let’s learn more.
What is a thermocouple?
Thermocouples (Fig. 1) are used to sense temperature in heat-treating furnaces and are a type of electrical sensor that consists of two dissimilar metals joined together that produce an output when subjected to a difference in temperature. The joined end that is placed inside the furnace is called the “hot” (or measuring) junction. The end attached to the connector outside the furnace is called the termination end, and the end attached to the instrumentation is commonly called the “cold” end (Fig. 2). An electromotive force (EMF) is generated. EMF is measured in millivolts that are proportional to the difference in temperature between the hot and cold ends. The different types of materials used to construct thermocouples produce different output signals.
Extension wire, which must match the thermoelectric characteristics of the thermocouple, is used to connect the termination end to the temperature instrumentation. This wire is generally specified by thermocouple type and, if calibrated, is done so at a specific set of temperatures – for example 0?C (32?F), 20?C (72?F) and 65?C (150?F).
Some audit organizations feel extension wire does not need to be calibrated since the wire is only transferring the signal input from the thermocouple assembly. What is especially important, however, is to avoid creating additional junctions by splicing wire lengths together. This can cause erroneous readings. It is necessary to provide a separate metal conduit housing for the extension lead wire since it isolates or shields the lead wire from stray voltage signals like those produced by other electrical or power wiring or nearby machinery. Shielded extension wire with a so-called drain wire is available if a separate metal conduit is not available.
What is thermocouple wire?
Thermocouple wire consists of the two dissimilar metals that are joined together at the sensing end of the thermocouple. Different thermocouple types (e.g., J, K, N) use different types of metals for these wires (Table 1, p. 18 and Table 3, online) and have different application uses (Table 2). These wires are enclosed in various insulation packages (Fig. 3, online) including fiberglass, vitreous silica and ceramic fiber to name a few.
What is extension wire?
Extension wire (Fig. 4 online) is used to extend a thermocouple’s signal from the connection head of the thermocouple back to the instrument reading the signal. The extension-grade wire typically has a lower operational temperature limit (normally ambient temperature) in which the wire may be used. PVC, Teflon®, Kapton® and fiberglass insulators are common. As a rule, thermocouple wire may be used as extension wire, but extension wire may not be used in the sensing point (or probe part) of the thermocouple.
Our discussion continues in part 2 next month. IH