Allied Encyclopedia: All About Thermocouple Wire – Part 1

What is thermocouple wire?

Thermocouple Wire

A thermocouple is a temperature sensor. The only thing absolutely necessary to create the simplest of thermocouples is thermocouple wire. Thermocouple wire consists of two dissimilar conductors, joined together at one end to complete the circuit and produce a voltage when heated or cooled. The measurement of the voltage produced can be converted into a relative temperature reading based on the predictable relationships between temperature and voltage of specific alloys. There are two junctions in a thermocouple; they are referred to by many different names, but the most common are the “hot junction” and “cold junction.” The hot junction does the “sensing” while the cold junction is maintained at a known temperature as a reference point. The difference in temperature between the two junctions is the absolute temperature of the location being measured.

Uses of Thermocouples

Thermocouples are popular as temperature sensors because they are generally inexpensive, reliable and practical for a wide temperature range, up to 2300°C. Because of this they are widely used in thermometers, diagnostic devices, probes and sensors in many industries. 

High temperature thermocouples are often used in the furnaces of the steel and iron industries to monitor the temperatures of the materials throughout the production process. Thermocouples are also useful in other manufacturing capacities—they can be used to test products to make sure they can withstand the heat levels at which they were designed to function.

Gas-powered heating appliances, like hot water heaters and ovens, can be made safer with a thermocouple. Many gas-fed devices use a pilot flame to ignite the gas burner. If the flame goes out, it poses a health hazard and the risk of explosion. A thermocouple placed strategically in the pilot flame, generates a voltage which powers the supply valve feeding gas to the pilot. So long as the pilot flame remains lit, the thermocouple remains hot, and the pilot gas valve is held open. The thermocouple temperature will drop if the pilot light goes out, causing the voltage produced by the thermocouple to plummet and the valve to close.

Thermocouple Grade Wire vs. Extension Grade Thermocouple Wire

Thermocouple grade wire is the wire used to make the probe part of the thermocouple. The extension grade wire is used to extend the signal from the probe back to the device interpreting the signal. Extension grade thermocouple wire will generally have a lower temperature limit than thermocouple grade wire, because it is not meant to be exposed to the same temperatures as the thermocouple wire. Extension grade wire merely carries a signal representing a higher temperature as received from the sensor. Extension wire may not be used as a component in the thermocouple itself, but is acceptable for thermocouple wire to be used as extension wire. This is a less common configuration, however, as extension grade wire is cheaper and generally used where permitted. While standard thermocouple wire part numbers follow a “type” and letter format, such as “Type K wire” or “Type N thermocouple wire,” part numbers for extension wire typically have an “X” in the prefix. For example, Type KX extension grade thermocouple wire is used to extend the signal of Type K thermocouple wire and Type NX extension grade wire is compatible with Type N thermocouple wire.

Type K Thermocouple Wire

Type KX Extension Grade Wire

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Want to learn more about the different types of thermocouple wire? Browse full thermocouple wire specifications online and check back later this week for the second half of our “All About Thermocouple Wire” series.