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, which be 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.
(positive leg listed first)
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.