Thermocouples are an integral part of experimental and industrial landscapes. These temperature sensors are used to provide reliable and accurate temperature data on scales wider than traditional thermometers.
However, thermocouples take different shapes and forms, with each type presenting unique measuring capabilities. Elements such as applicability, vibration resistance, abrasion, installation needs, and chemical resistance define the choice of a thermocouple.
Here is a quick guide to the different types of thermocouples.
How a Thermocouple Functions
To better understand the various classifications of thermocouples, you must have basic knowledge of what a thermocouple is and how it functions.
The basic structure of a thermocouple includes two dissimilar thermocouple wires, a hot junction, a connection point, and a cold junction.
The two different metal wires are connected to a hot junction. This is the point where temperature measurements are taken. The opposite side of the connected wires creates a terminal known as the cold junction or reference junction, which is linked to a voltage reader.
The two critical junctions of a thermocouple create varying temperatures. Essentially, thermocouples function on the principle of this temperature difference.
Classification of Thermocouples
Thermocouples are classified on two major factors, including metal pairings and styling.
1. Metal Pairings
The dissimilar metals of a thermocouple are made up of two different materials. Each thermocouple type features a unique metal that significantly differs from the other in terms of electronegativities. Here are the various types of thermocouples based on metal pairings.
• Type-K Thermocouple
This is one of the most sought-after thermocouple types due to its wide temperature range. Type-K thermocouples are made up of a positive element of 90% Nickel – 10% Chromium and a negative element of 95% Nickel – 2% Manganese – 2% Aluminum – 1% Silicon. With a service temperature range of -330°F and 2300°F, these thermocouples are best suited for inert atmospheres.
• Type-B Thermocouple
The thermocouple wires of Type-K consist of a positive lead of 70% Platinum – 30% Rhodium and a negative lead of 94% Platinum – 6% Rhodium. Type-B thermocouples are shielded with a ceramic protection tube. Protection tubes and alumina insulators are used to prevent silica contamination against which Type-B thermocouples are prone.
• Type-R Thermocouple
Type-R thermocouples consist of a positive element of 87% Platinum – 13% Rhodium and a negative element of 100% platinum. The sensor has a service temperature scope of 32°F and 2700°F.
Like all platinum type thermocouples, Type-R should be insulated to protect them against silica contamination induced by Mullite ceramics. When type R thermocouple sensors are consistently exposed to high temperatures, excessive grain growth is induced on its elements. This results in mechanical breakage on the thermocouple’s body.
• Type-S Thermocouple
This is another platinum type thermocouple consisting of a positive lead of 90% Platinum – 10% Rhodium and a negative element of 100% Platinum. Type-S thermocouples have the highest measurement accuracy, making them the preferred option for applications with extremely high temperatures.
However, like other platinum thermocouples, Type-S sensors must not be used in applications exposed to metallic vapors.
• Type-J Thermocouple
This is the most used thermocouple due to its cost-effectiveness. Type-J thermocouples consist of a positive element of 100% iron and a negative lead of 45% Nickel – 55% Copper.
A type J thermocouple is best suited for maximum temperature applications and oxidizing atmospheres.
• Type-E Thermocouple
Type E thermocouples comprise a positive element of 90% Nickel – 10% Chromium and a negative lead of 95% Nickel – 2% Manganese, 2% Aluminum, and 1% Silicon. This thermocouple type is more stable than type K, providing higher accuracy and faster response.
Type E thermocouples are highly resistant to the corrosion effect of high-moisture atmospheres, making them a preferred option for sub-zero applications.
• Type-N Thermocouples
A type-N sensor is made up of a positive thermocouple wire of Nicrosil (Nickel, Chromium, and Silicon) and a negative wire of Nisil (Nickel and Silicon). These thermocouples are recommended for similar applications as type K sensors.
Type N thermocouples have a wide range of temperature scopes running from -330°F to 2300°F.
• Type-T Thermocouple
Commonly known as Copper Constantan, a Type-T thermocouple is made up of a positive lead of 100% Copper and a negative lead of 45% Nickel – 55% Copper. The sensor is best suited for an oxidizing atmosphere.
2. Styling
Where an additional protective sheath is added to insulate a thermocouple, different styles are adopted. There are three styles of thermocouple probes, exposed, grounded, and ungrounded.
• Exposed
Exposed thermocouples have the sensory wires joined and extended outside the protective metal tube.
• Grounded
When the edge of the sensory wires is built in contact with the protective sheath, the sensor is known as a grounded thermocouple.
• Ungrounded
Ungrounded sensors have the thermocouple wire joined within the protective sheath without coming into contact with the body of the sheath.
David Pisse, a seasoned software developer and AI enthusiast, brings over a decade of experience in innovative technology solutions. With a passion for blending AI with traditional development practices, David offers unique insights into the future of software engineering.
