Type K, Type J, Type N – the list of thermocouple types seems like an alphabet soup. And as technology improves and manufacturers come up with more options, it can be hard to keep track of all the various names and classifications. This article outlines the 11 most common types of industrial thermocouples.
Thermocouplesare one of the most common devices used across industries and applications to measure temperature. These electrical thermometers are made of two wires of dissimilar metals joined together at the measuring point, also known as the hot junction. Due to the two metals’ differences in electronegativity, changes in temperature at the hot junction generate a voltage difference at the other end of the wires, called the connection point or cold junction. (See this videofor more details about what thermocouples are and how they work.)
Types of Thermocouples
What differentiates one thermocouple from another is the metals in its two wires: the positive leg and the negative leg. Because each thermocouple type has a different pairing, they differ in temperature limits, process conditions (inert, oxidizing, reducing atmospheres, heavy vibration), and so on. But just how many types of thermocouples are there?
The exact number is not easy to pin down. It increases as manufacturers develop novel compositions/pairings and as standardizing organizations recognize them, and decreases as certain thermocouple types fall out of favor and become obsolete. Then there are the thermocouples with different names but only slightly different variations of the same pairings.
But what we can safely say is that there are two basic groups of thermocouples. One uses materials like iron, nickel, copper, and chromium – base metals that produce high thermoelectric voltages when paired. The other group has more expensive noble metals, such as rhodium, platinum, rhenium, and tungsten, and are used at much higher temperatures.
Thermocouple Naming Conventions
Most thermocouples have just letter names, and the designations appear to be arbitrary. In other words, the letters do not correspond to the dominant metal’s chemical symbol, nor were the types standardized in alphabetical order. The American National Standards Institute (ANSI) and the ANSI-accredited American Society for Testing and Materials (ASTM) lists nine main thermocouple types: B, E, J, K, N, R, S, T, and C.
The one exception to the arbitrary designations are thermocouples that contain tungsten, whose chemical symbol is W, and rhenium. The number that follows the W states how much rhenium is in the positive leg. For example, Type W5 means the positive leg is 95% tungsten and 5% rhenium. If there’s no a number, there’s no rhenium in the positive leg. Tungsten thermocouples have the highest temperature limit of all types: up to 4,200°F (2,320°C)
While most thermocouples have one name, these three tungsten types have more:
- Type W5 or Type C
- Type W3 or Type D
- Type WR or Type W or Type G
Thermocouple Types, Compositions, and Applications
So, how many thermocouple types are there? The quick answer is “at least 11.” The most common ones are K, J, N, E, and T – ones with less expensive base metals. Here’s a quick guide to all the thermocouple types available at WIKA.
Type K Thermocouple
Type K thermocouples are among the most widely used, perhaps due to their relative affordability. With a maximum operating temperature of 2,300°F and a stable output, they’re ideal for a variety of applications, including in refineries and steel and iron production. Type K thermocouples are made up of two nickel-based alloys, Chromel and Alumel, which make them more resistant to corrosion except when exposed to sulfur.
Type J Thermocouple
Although it has a smaller operating temperature range (up to 1,400°F) and a shorter lifespan than other thermocouple types, Type J thermocouples are one of the few that can be used safely in reducing atmospheres. Their composition of iron and Constantan (a Cu-Ni alloy) make them an inexpensive choice, but the iron leg is susceptible to rust. Despite their limitations, Type J thermocouples are reliable and ideal for most general industrial applications, such as in boilers and furnaces.
Type N Thermocouple
Type N thermocouples have the same operating temperature range and level of accuracy as Type K thermocouples. However, the use of Nicrosil (a Ni-Cr alloy with small amounts of silicon and magnesium) and Nisil (similar to Nicrosil but without chromium) make Type N thermocouples more resistant to oxidation and oxidation-related drift issues. They also have a better repeatability compared to Type Ks. On the downside, Type Ns are more expensive than Type Ks.
Type E Thermocouple
One of the most notable features of Type E thermocouples is that they have the highest output among the common thermocouple types. Type Es can be used in inert and oxidizing atmospheres, and their use of Chromel and Constantan make them affordable. Although they’re recognized as being more accurate and stable than Type Ks, Type E thermocouples can be used only at temperatures up to 1,600°F. However, they make up for this by being effective in cryogenic applications.
Type T Thermocouple
Despite their maximum operating temperature of 700°F, Type T thermocouples are best suited for sub-zero temperatures, making them effective for use in cryogenics and freezer applications. Featuring legs made of copper and Constantan, they’re not subject to corrosion when exposed to moisture, and they can be used in oxidizing and reducing atmospheres.
Type R and Type S Thermocouples
With maximum operating temperatures of 2,700°F and expensive platinum alloy legs, Type R and Type S thermocouples have a lot in common. They’re both suitable for applications in extremely high temperatures, such as heat treating and biotech, and they should both be protected with a ceramic tube with a secondary tube for insulation. The positive legs of Type Rs feature a 13% rhodium and platinum blend, while the positive legs of Type Ss have 10% rhodium. For that reason, Type R thermocouples have a slightly higher output and higher stability.
Type B Thermocouple
Type B thermocouples are unique in that both legs are made up of costly platinum-rhodium alloys. The positive leg contains 30% rhodium, and the negative leg contains only 6%. They can withstand temperatures up to 3,100°F, which makes them suitable for applications like glass production and incinerators. While they provide an exceptionally high level of accuracy and stability, Type Bs have a lower output and are more susceptible to drift than Type R and Type S thermocouples.
Type WR, W3, and W5 Thermocouples
Thanks to their use of the refractory metals tungsten and rhenium, type WR, W3, and W5 thermocouples are capable of operating at up to 4,200°F. While their extreme heat resistance allows them to be used in solar and aerospace applications, they have no resistance against oxidation and, thus, can be used only in vacuum, hydrogen, or inert atmospheres.
At a Glance: Thermocouple types and application
refineries, steel, iron
cryogenics, freezers, food production
Platinum – 13% Rhodium
sulfur recovery units
Platinum – 10% Rhodium
high temperature furnaces, biotech, pharmaceutical, labs
Platinum – 30% Rhodium
Platinum – 6% Rhodium
Tungsten – 26% Rhenium
semiconductors, solar, aerospace
Tungsten – 3% Rhenium Tungsten – 25% Rhenium
Tungsten – 5% Rhenium Tungsten – 26% Rhenium
* also known as Type W
See WIKA’s Thermocouple Reference Guide for more details about the wires’ magnetism, colors, codes, and service conditions. For questions about which electronic temperature instrument is right for your application, contact our ETM experts.
As a trusted manufacturer of thermocouples and other temperature measurement devices like thermowells and RTDs, WIKA offers solutions for nearly every industry. For questions about which electronic temperature instrument is right for your application, contact our ETM experts.