What are the Different Thermocouple Types?
Thermocouples come in many shapes, sizes, and materials. The distinct properties of different metals are the key to thermocouple operation. Since we’re really measuring how two different wires react in the same environment and computing those differences to a standard temperature reference. We’ll examine the various metals found in some common thermocouples and dig a little deeper into why this instrument is classified into different types. Read our primer on “What is a Thermocouple?” to give you an idea of the basics. Otherwise, grab your Periodic Table of the Elements and let’s get started.
Figure 1: A K-type thermocouple with a fastenable eyelet in lieu of a probe
What Metals make up a Thermocouple?
All metals have different properties; characteristics that set them apart from each other. With a thermocouple, we use these innate differences to help determine the temperature difference that we’re trying to measure. It is important to remember that there are always two dissimilar metals within thermocouples; it is intrinsic to how they work.
Some common metals used in thermocouples are iron (Fe), copper (Cu), nickel (Ni), and platinum (Pt). Metal alloys, or combinations of metals, are also used in thermocouples. Nichrome is an alloy consisting of nickel and chromium (Cr). Constantan, a mixture of copper and nickel, is also prevalent. There is even alumel, which is a trademarked alloy consisting of mostly nickel with small percentages of aluminum, manganese, and silicon.
Each type of metal brings certain advantages to thermocouple temperature measurement. Some combinations of metal may work well for relatively hot temperatures, but not for cold ones. Other combinations might give very accurate temperature readings but only within a specific and narrow range of measurements. Still another combination can be resilient to the corrosive environment where it is employed; therefore adding a longer lifespan to the thermocouple before it needs replacement.
Different Thermocouple Types
When a certain pair of metals are discovered to cooperatively produce a reliable voltage signal they are standardized and given their own classification. This way a specific “type” of thermocouple has properties that are now regulated. Meaning, those voltage measurements are reproducible exactly the same from one thermocouple to another. Let’s take a look at some common thermocouples types, the metals that they consist of, and areas where they excel.
Figure 2: A cross-section showing the two differing metal thermocouple wires
The J is the most common thermocouple out there. Made from the common metals iron and constantan (Fe-Con), the type J thermocouple is a low-cost option. The temperature range is from 0ºC to +750ºC and is accurate to within a few degrees. This type is most accurate in the mid to low ranges since iron will rapidly oxidize toward the higher measurable temperatures and the standard voltage readings can be permanently altered. You’ll find type J thermocouples in a wide variety of manufacturing processes.
Also a common thermocouple, type K can measure from -270ºC to +1200ºC, and is most accurate to within a couple of degrees towards the higher side. It is comprised of nichrome-nickel (NiCr-Ni) or nichrome-alumel (NiCr-Alumel). These metals are common, relatively inexpensive, and typically used above the 600ºC mark. It is comparable to a type J in price but with a less refined voltage to temperature output. Type K thermocouples are arguably the most widely used type and are found throughout many different industries.
Figure 3: A type K (NiCr-Ni) thermocouple with a standard connector on the end
A thermocouple that is often used to measure colder temperatures, type T is comprised of copper and constantan (Cu-Con). This combination of metals gives a reliably measurable voltage from -270ºC to +350ºC, perfect for food-related uses and scientific research, like cryogenics. Below -200ºC even the type T thermocouple begins to lose its accuracy. You may remember from high school science class; that the coldest known temperature possible is -273ºC; otherwise known as 0 Kelvin or absolute zero. It’s pretty impressive that our simple thermocouple can plumb those depths of temperature measurability.
Let’s go the other way and measure something extremely hot. The type B thermocouple can measure from 200ºC all the way up to 1700ºC. To put that in perspective, steel and iron melt at around 1400ºC and 1500ºC, respectfully, so this type of thermocouple would be an ideal fit for specialized industrial use. Unlike, type J or K materials which would be at their melting points at this high of a temperature. The two very rare elements of rhodium (Rh) and platinum (Pt) are used in type B, and these metals don’t come cheap. Also, depending on the spot price, platinum is around the same price as gold. Rhodium, on the other hand, is currently worth twice the price of gold. Due to this, along with its specialized use, means you won’t see too many type B thermocouples out there.
Figure 4: A chart depicting some common thermocouple type ranges
And those are just a few of the many different thermocouple types employed in all sorts of processes. Keep in mind, the temperatures and ranges we list are just a rough guideline; the accuracy of a thermocouple can be better within a certain sub-section of its range in comparison to its whole temperature envelope. For a more practical discussion on usages for different types of thermocouples; check out: “What are Thermocouples used for?” where we investigate some of the ways this sensor is employed.
As you can see, not all thermocouples are alike; they are based on the same scientific principles but different materials lead to specializations within this area of temperature sensing. It is this sort of customizability that makes thermocouples so versatile. With so many different options this instrument has great flexibility. Therefore making it an ideal choice for an endless list of possible uses. Whether your deciding factor is: temperature range; cost; or longevity; there is a thermocouple out there for every job. Thirsty for more discussion about thermocouples or did we gloss over something you want more detail on? Check out the “Question and Answer Guide: Thermocouples” for more on this amazing sensor.