Gas turbine engines power most of our modern aircraft and ships. They also generate electricity around the world. A gas turbine engine can reach internal temperatures up to 2,732°F (1,500°C). Monitoring the internal temperatures of these complex machines is critical to maximizing service life and optimal performance.
Besides gas turbines, thermocouples accurately measure the temperatures of processes from extremely low to high heat. The American National Standards Institute (ANSI) and the American Society for Testing and Materials (ASTM) have compiled a list of nine primary thermocouple types. Each type meets specific temperature, reaction time, chemical and vibration resistance requirements. However, this article will focus on the type J and K thermocouple wires, their functionality, dissimilarities, and which suits your application.
What is a Thermocouple Wire, and What Are Its Uses?
Thermocouples are temperature sensors consisting of two wires made from different metals. When the two wires connect at one end, it generates a predictable voltage. The voltage varies with the difference in temperature between the measuring junction and reference junction, called a thermal gradient. The voltage is proportional to the temperature difference between the two ends. The thermocouple wires are usually encased in a protective sheath to ensure accurate temperature readings in harsh environments.
Thermocouples measure the temperature of gases, liquids, and solids. For example, they help monitor the temperature of food products during production, molten steel temperatures, chemical reactions in chemical plants, and industrial immersion heaters.
There is a difference between thermocouple grade wire and thermocouple extension wire. Thermocouple grade wire is the sensing point or probe part. In comparison, the thermocouple extension wire transmits a signal from the probe to the instrument panel.
Thermocouples can measure low or high temperatures depending on the physical properties of the wires.
There are nine types of thermocouple wires. See the table below.
Thermocouple Types |
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| Type | Positive Lead Wire | Negative Lead Wire | Max. Useful Temperature Range | |
| J | Iron | Constantin | 32 to 1,382°F (0 to 750°C) | |
| K | Chromel | Alumel | -328 to 2,282°F (-200 to 1,250°C) | |
| T | Copper | Constantin | -328 to 662°F (-250 to 350°C) | |
| B | Platinum 30%, Rhodium | Platinum 6%, Rhodium | 32 to 3,092°F (0 to 1,700°C) | |
| R | Platinum 13% | Platinum, Rhodium | 32 to2,642°F (0 to 1,450°C) | |
| S | Platinum 10% | Platinum, Rhodium | 32 to 2,640°F (0 to 1,450°C) | |
| E | Nickel-10 % chromium | Constantan | 32 to 1,472°F (0 to 800°C) | |
| N | Nickel-14 % chromium, 1.5 % silicon | nickel-4.5 % silicon-0.1 % magnesium | -454 to2,300°F (-270 to 1,260°C) | |
| C | Tungsten-5 % Rhenium | Tungsten26 % Rhenium | 32 to 2,640°F (0 to 1,450°C) | |
How Do Thermocouple Wires Work to Measure Temperature Accurately?
Thermocouple wires utilize the Seebeck effect, which states that when two different metals join at different temperatures, they generate a voltage. The voltage generated is proportional to the temperature difference between the two junctions. The thermocouple wires connect to a measuring device to convert the voltage into a temperature reading.
Thermocouple wires offer high accuracy and precision in temperature measurement. They are also fast and reliable, making them ideal for harsh environments where other temperature-sensing methods may not be practical. However, proper heater sensor placement can affect the accuracy.
What is Type J Thermocouple Wire?
J Type thermocouple wiring has a positive iron wire and a negative Constantan alloy wire. Constantan is a brand name for a 55% copper and 45% nickel alloy.
What are the advantages of Type J thermocouple wire? The J Type thermocouple’s working range is between 32 to 1,382°F (0 to 750°C). It resists oxidation and is suitable for high humidity and corrosive environments. They have a sensitivity of approximately 50 µV/°C and are suitable for low-temperature applications such as refrigeration and food processing. However, the iron wire will oxidize rapidly for temperatures above its working range.
Which Wire Is Positive On a Type J Thermocouple?
The positive wire on a type J thermocouple is color-coded white and made from iron. The negative wire is color-coded red and made with a copper-nickel alloy, Constantan. This link takes you to an ANSI and IEC color codes chart.
What is Type K Thermocouple Wire?
Type K thermocouple wire has a yellow positive chromel wire and a red negative alumel wire.
Chromel is an alloy made with 90% nickel and 10% chromium. Alumel consists of 95% nickel, 2% aluminum, 2% manganese, and 1% silicon.
The effective K Type thermocouple range is between -200 to 2502°F (-129 to 1372°C). They have a sensitivity of approximately 41 µV/°C. Type K thermocouples work best for high-temperature applications such as furnaces and kilns.
They have a high resistance to corrosion and oxidation. However, it is unsuitable for low-temperature applications below -200°C, as the alumel wire will start to become magnetic.
Can You Solder Type K Thermocouple Wire?
Yes, you can solder type K thermocouple extension wire, and it’s the preferred method for connecting to other wires or terminals. Use a suitable flux to ensure a strong and lasting bond when soldering this wire.
When soldering Type K wire, it is crucial to avoid overheating it as this can lead to inaccurate temperature readings.
Differences Between Type J and Type K Thermocouple Wires – Accuracy, Precision, and Applications?
Although Type J and Type K thermocouples are entirely different wires, the accuracy of both J and K cables are the same, with a variation of +/- 2.2°C or +/- .75% in their temperature zones.
Type K cables can withstand continuous exposure to temperatures around 2,012°F (1,100°C), with a max temp of 2,501°F (1,372°C).
Type J cables operate best in the 700 to 1000°F (371 to 538°C) temperature range. Above 1,400°F (760°C) can cause a permanent change in the magnetism of the iron, affecting the accuracy of temperature readings.
Temperature limits for cable can vary by manufacturer and the wire gauge. For example, this table shows the upper-temperature limits of J and K cables and how they change by wire diameter.
| Type | 8AWG .128” |
14 AWG 0.064” |
20 AWG 0.032” |
24 AWG 0.020” |
28 AWG 0.013” |
30 AWG 0.010” |
36 AWG 0.005” |
| J | 1400°F (760°C) | 1100°F
(590°C) |
900°F
(480°C) |
700°F
(370°C) |
700°F
(370°C) |
600°F
(320°C) |
590°F
(315°C) |
| K | 2300°F
(1260°C) |
2000°F
(1090°C) |
1800°F
(980°C) |
1600°F
(870°C) |
1600°F
(870°C) |
1400°F (760°C) | 1100°F
(590°C) |
[Source]
Type K thermocouple wires have a wider measuring range and can measure temperatures up to twice as high as Type J.
Which Thermocouple Wire is Better for Your Needs?
Determining which thermocouple wire best suits your application depends on several factors, including the temperature range of the application, the sensitivity required, and the working environment.
If your process is relatively low temperature, such as food processing, pharmaceuticals, and ovens, then Type J thermocouple wire may be the best option. It has a wide temperature range between -40 to 750°C and a high oxidation resistance.
Type K thermocouples are best suited for high-temperature applications, corrosive environments, and processes where the cable is prone to oxidation. Applications include wet environments such as immersion heaters submerged in tanks.
For high-temperature processes such as furnaces, kilns, and gas turbines, Type K thermocouple wires are the best choice.
Installation and Maintenance Tips for Thermocouple Wire
When installing a thermocouple, ensure that the two junctions are in contact with the surface to be measured. The thermocouple must be grounded to prevent electrical interference.
Regular maintenance, cleaning, and inspection for wear and tear of thermocouple wires ensure their accuracy and reliability. Replace worn or questionable thermocouples immediately to avoid costly quality problems.
Thermocouple Wire Suppliers and Cost
There are many suppliers of thermocouple wires, including Omega Engineering, RS Components, and Thermocouple Technology. The cost of thermocouple wires will depend on several factors, including the type of wire, the type of insulation, the length, and the supplier.
A 100-foot Type J, 24 AWG wire spool costs less than $100. One hundred feet of Type K, 24 AWG runs between $125 and $150, depending on the wire and insulation.
Type J and Type K thermocouple wires are readily available, making them a cost-effective solution for temperature sensing in many applications.
Type J and Type K Thermocouple Wires – Reliable and Accurate Temperature Sensors
While they differ in their respective temperature range and composition, both offer high accuracy and precision in temperature measurement.
Determining which thermocouple wire is better for your needs will depend on the temperature range of the application, the sensitivity required, and the working environment.
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