Thermocouples: All You Need To Know About Thermocouples

Thermocouples

Have you ever thought of a versatile temperature measuring device with high levels of accuracy and consistency? Well, thanks to the recent industrial and technological advancements, we now have a thermocouple sensor.

A thermocouple sensor is a temperature measuring device used for residential, commercial and industrial applications.

While the use of thermocouples has been on the rise, most people do not seem to understand what they are and how they work.

Therefore, this guide seeks to uncover all the essential information you need to know concerning thermocouples.

What Are Thermocouples?

A thermocouple is a temperature measuring device used over a wide range of applications. It has two dissimilar metal wires joined to form two junctions, a measuring end and a referencing end.

The measuring end, commonly known as the hot junction, is enclosed in a metal sheath and is usually connected to the heat source. The other end is the referencing end or cold junction, which provides a reference point for the electromotive force.

What Is a Thermocouple Used For?

Due to their versatility, thermocouples can be used for a wide range of domestic, industrial and commercial applications such as:

  • Pharmaceutical industry
  • Paper and pulp industry
  • Oil and gas production
  • Power generation
  • Food production
  • Cement industry
  • Toasters
  • Stoves and furnaces

Thermocouple Diagram

Typically, a thermocouple consists of two dissimilar metal strips, say A and B, conjoined to form two junctions, p and q. The temperature at the two junctions are maintained at different levels, say T1 and T2, respectively.

A heat source is then placed at the measuring end, p. Additionally, the adjacent end, q, acts as the referencing junction and there is also a voltmeter to measure the resulting electromagnetic force.

How Does a Thermocouple Work?

As earlier mentioned, a thermocouple sensor has two ends; the measuring end and the reference end. When reading temperatures, the measuring end is connected to a heat source to initiate potential current flow.

Temperature changes between the two ends create a thermal gradient. As a result, an electron flow arises, causing an electromotive force, EMF measured in millivolts.

Thermocouples work by applying the Seebeck effect.

During his studies, Thomas Seebeck (1770-1831) found that if two ends of a conductive metal were put in environments with different temperatures, an electric current would efficiently flow through them. However, if he conjoined the two metal ends, no electric current would flow through. The same case would apply if the two ends were set at the same temperatures.

He furthered his studies and discovered that the current flow was more effective if two loops of different metals were used and different temperatures were maintained at both ends. Seebeck made this discovery in 1821 and it has been the idea behind the working of thermocouples.

This mechanism gives temperature readings based on high sensitivity and accuracy levels. The high accuracy and effectiveness make thermocouples one of the most accurate temperature measuring devices.

What Is the Principle of Thermocouple?

Thermocouples adopt the principles of Seebeck, Peltiers and Thomson.

The Seebeck principle states that when two dissimilar metal elements are joined to make a closed circuit and different temperatures set at the two ends, an electromotive force (EMF) is produced in the closed circuit.

Additionally, the Peltier Principle, emphasises that when two different metals are conjoined, forming two junctions, the emf generated within the circuit results from the temperature differences at the two ends.

Lastly, the Thomson Principle, further emphasises that if two, unlike metals, are joined to form two junctions, a potential electric current exists within the circuit. The electric current results from the temperature gradient along the length of the conductors.

Types of Thermocouples

There are different types of thermocouples based on the type of metal used and their calibrations. Each of the different thermocouples is given a code to denote the alloy composition and temperature range. The most common ones are the base metal thermocouples, type J, K, E, N and T and the high-temperature calibrations or noble metal thermocouples; type C. R, S and GB.

The different alloys and calibrations determine the temperature range for each thermocouple. However, the overall temperature range for thermocouples lies between -200°C to +1800°C.

Here is a brief look at the most common thermocouples; their alloys and temperature ranges:

Type Alloy combination Temperature range

  • K +ve Nickle Chromium, -ve Nickle Aluminium 0 to 10000C
  • J +ve Iron, -ve Copper Nickle 0 to 7500C
  • E +ve Nickle Chromium, -ve Copper Nickle 0 to 8000C
  • T +ve Copper, -ve Copper Nickle -185 to 3000C

Of all thermocouple types, K is the most common.

Thermocouple Colour Code

Different thermocouples have different colour codes depending on the origin of the cables. The insulation cables should be coloured for easy identification of the phase, earth and neutral cables. The operating standards of colour codes vary from one region to the other.

Thermocouple Type K

Thermocouple type K, also known as Chromel-Alumel due to its Nickle Chromium vs Nickle Aluminium alloy, is the most used thermocouple today. Such thermocouples are mainly used for standard temperature measurements under normal atmospheric conditions.

The maximum continuous temperature reading for type K thermocouple extends to slightly above 10000C, although oxidation over 8000C causes drift and decalibration. However, under short exposures, it can extend up to 12000C.

Although widely used due to their affordability and versatility, they are not as stable as other base metal thermocouples. The various types of type K thermocouples available are:

  • Mineral insulated thermocouples
  • Swaged tip thermocouples
  • Miniature thermocouples
  • General-purpose thermocouples
  • Heavy-duty thermocouples

Type K Thermocouple Chart

Type k thermocouple chart contains predetermined output readings in millivolts under different temperatures. The chart has temperature readings in its first column, showing an increment of 100C. The other columns marked 0-10 contain mV readings.

How to Know if You Have a Bad Thermocouple?

The best way to know if you have a bad thermocouple is by conducting a voltage test using a millivolt meter. Before testing, first, understand the millivolts your thermocouple should generate under standard conditions. If the millivolt readings do not tally, then your thermocouple could be faulty.

What Are the Advantages of Thermocouples?

The use of thermocouples has numerous advantages, including:

  • High accuracy and consistency
  • Fast thermal reaction
  • Have a vast number of applications
  • Highly adaptable and can be used under harsh conditions

Where to Buy Thermocouples

If you want to buy a quality thermocouple, visit Thermocouples – Process Parameters for a wide collection of thermocouples. The thermocouples here are made under meticulous craftsmanship from leading thermocouple manufacturers, UK.

Conclusion

Thermocouples are your to-go deal if you want a reliable temperature measuring device for your residential, commercial and industrial. Get a thermocouple that suits your intended usage and enjoy the accuracy and consistency of temperature readings.


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