Types of Thermocouple

Thermocouples are used in a range of industrial settings as a way of measuring the temperature of certain components. There are a number of different types of thermocouples, and they all have varying characteristics that make them suitable for a range of applications. The different elements of thermocouples include temperature range, vibration resistance, durability, application compatibility and chemical resistance. It’s important to understand the different characteristics of a particular type of thermocouple to ensure that it meets your specifications.

What Are Thermocouples?

Thermocouples are a type of sensor that can measure the temperature. They are manufactured from two different metal wire legs which are welded at one end to form a junction. The junction part is used to measure the temperature, as it creates a voltage when there is a temperature change. Thermocouple reference tables are then used to calculate what the temperature is based on the voltage that is produced.

They can be used in many different scientific, industrial and OEM settings. Thermocouples are common in many sectors, including power generation, pharmaceuticals, oil and gas, biotech, paper and pulp and cement. As well as these industrial applications, thermocouples can be found in many appliances that we all use every day, such as furnaces, toasters and stoves. They are popular due to their low cost, durability and wide range of temperatures.

Different Thermocouples

There are various types of thermocouples, which are identified by the differences in their characteristics. The most common thermocouples are types E, K, J and T, which are ‘Base Metal’ versions. ‘Noble Metal’ thermocouples – types B, R and S – are suitable for applications involving high temperatures.

Type K

This is the most common thermocouple option due to its low cost, accuracy, reliability and the wide temperature range it has. This type of thermocouple is produced from nickel-chromium and nickel-alumel. Type K has a temperature range from –454 to 2,300 F (–270 to 1260C).

Type J

Type J is another popular type of thermocouple and is manufactured from an iron and constantan mixture. This doesn’t have as wide a temperature range as the Type K, and when used at higher temperatures the lifespan is shorter. However, when comparing levels of reliability and the cost, it performs on an equal scale. Its temperature range is from -346 to 1,400 F (-210 to 760 C).

Type T

A Type T thermocouple is manufactured using copper and constantan. This is an extremely stable form and is particularly suitable for applications that require very low temperatures, including ultra-low freezers and cryogenics. Type T thermocouples have a temperature range of -454 to 700 F (-270 to 370 C).

Type E

The two metals that are used in Type E thermocouples are nickel-chromium and constantan. When used at temperature ranges below 1,000 F, Type E thermocouples have better accuracy and a stronger signal than Type J or Type K. The temperature range of the Type E is between -454 and 1600 F (-270 to 870 C).

Type N

The Type N costs slightly more than the Type K. However, it has identical temperature limits and accuracy levels. These thermocouples are made using nicrosil and nisil.

Type S

This is a base metal thermocouple and is designed for use at extremely high temperatures. It is manufactured from a combination of platinum rhodium and platinum and is commonly used across the pharmaceutical and biotech sectors. As it is a particularly stable and accurate thermocouple, it is also used for lower-temperature situations. The temperature range of Type S thermocouples is -58 to 2700 F (-50 to 1480 C).

Type R

These thermocouples are made using platinum rhodium (13%) and platinum and they are designed for applications involving very high temperatures. The Type R is manufactured using a larger rhodium percentage than Type S, which results in a higher cost. The performance levels of the Type R are similar to those of the Type S. It can be used for measuring lower temperatures due to the stability and accuracy of the thermocouple. The Type R temperature range is -58 to 2700 F (-50 to 1480 C).

Type B

The Type B can be used at the highest temperature of all these thermocouples. The wires are a combination of platinum rhodium (30%) and platinum rhodium (6%). Even at these high temperatures, it can achieve excellent levels of stability and accuracy. The temperature range of the Type B is 32 to 3100 F (0 to 1700 C).

Types of Junction

As well as different types of thermocouples, the junction styles can also vary. The style that you choose will depend on the application the thermocouple will be used in and the particular properties you require.

The grounded thermocouple is the style of junction that is most commonly used. This type is created by welding the wires to the sheath, which makes one junction. This creates a fast response time, but it is more prone to electrical interference.

Ungrounded thermocouples are made by welding the wires together, but they are separated from the sheath by a layer of insulation.

Exposed thermocouples have the wires welded and then inserted directly. This enhances the response time, but as the wires are exposed they can degrade and become corroded.

Ungrounded uncommon thermocouples are produced by insulating all of the elements from each other.

Understanding the temperature range and other variations that are particular to your application will enable you to select the most appropriate thermocouple for your requirements. This will create greater levels of accuracy and stability and ensure you gain an accurate temperature reading every time.

About Author
Osho is Tech blogger. He contributes to the Blogging, Gadgets, Social Media and Tech News section on TecheHow.