Thermocouples: What You Need to Know

Thermocouples are the most common type of temperature measurement instrument. These are used in industrial machinery and many different types of temperature and pressure sensing equipment.

They allow you to measure the temperature of an object, a fluid, or air.

Thermocouples come in various types, including glass-electrolyte ones, gold-plated ones, and more.

Let’s explore the types of thermocouple guide to see how they are used in the industrial world.

What Does a Thermocouple Do?

Thermocouples are used across various applications and purposes. They are versatile sensors and can measure a wide range of temperatures. They come in many models and specifications to determine how they work and where you could use them.

First, you need to know the differences in thermocouple types and how they function to understand better how to use them. Generally, a thermocouple’s purpose is to detect temperature changes and provide quick responses during a certain process. 

For example, for furnace maintenance, a thermocouple is used to read and help maintain a specific temperature. If the furnace temperature changes, it will detect it and allow for an appropriate response to be made. 

Thermocouple Sensor and Thermocouple Types

For the most part, a thermocouple sensor is made up of two different metallic wires that are welded together at one end. When a temperature change occurs on the other end, a continuous current flow is sent throughout the thermoelectric circuit. 

Therefore, a voltage is produced due to the different metal wires and junction temperatures. 

The voltage is then read by a thermocouple thermometer that translates it into the appropriate values for temperature. The thermocouple can also be connected to any compatible device, depending on the intended application. 

Moreover, there are different types of thermocouples based on different components, metal compositions, design, etc. let’s begin understanding the differences in the thermocouple wire types – Base Metal thermocouples and Noble Metal Thermocouples. 

The Base Metal thermocouple types are K, J, N, E, and T. Noble Metal thermocouple types are C, S, R and GB. The noble metal thermocouples are best for applications that require high calibrations and wider temperature ranges. 

Each thermocouple type has unique characteristics based on its temperature range, vibration resistance, chemical resistance, durability and application compatibilities.

Let’s further discuss the types of thermocouples, their characteristics and their applications. 

Thermocouple types and their applications

Let’s go through the most common types – the base metal thermocouples. 

The type J thermocouple comprises Iron and Constantan wires and is very commonly used. Their temperature range is slightly towards the smaller side. Moreover, at very high temperatures, its lifespan can be pretty short compared to a Type K thermocouple. 

Nevertheless, a Type J thermocouple matches the Type K reliability and expenses. 

On the contrary, a type E thermocouple (Nickel and Chromium or Constantan) has a higher accuracy and stronger signal than both types K and J thermocouples. Generally, it is true at moderate temperature ranges around 1000˚F or lower. 

A Type T thermocouple is made of Copper and Constantan wires, making it extremely stable. It is often used for very low-temperature environments and applications such as ultra-low freezers and cryogenics. 

A type N thermocouple is made with Nicrosil and Nisil, but it shares similar temperature ranges and accuracy as type K thermocouples. However, it is on the more expensive side. 

Click here to read more about specifications, applications, and Noble Metal thermocouples. 

K Type thermocouple: The Most Common Thermocouple Type

It is the most common type of thermocouple because it’s the least expensive. It also has a wide temperature range is reliable and accurate for many applications.

It is made up of Nickel-Chromium or Nickel-Alumel wires. A Type K thermocouple has a temperature range of -454˚F to 2,300˚F (grade wire) and a 32˚F to 392˚F (extension wire). The standard accuracy stands at +/- 7.5%, whereas its special limit accuracy is +/- 0.4%.

Application

Type K thermocouples are typically used for temperatures above 1022˚F (550˚C) and the thermocouple’s maximum working pressure. They are widely used in steel and iron-making industries to monitor temperatures and chemistry throughout different processes. 

These thermocouples are also used to test temperatures in process plants’ chemical productions and petroleum refineries. More uses include heat appliance safety testing and nuclear applications due to its relative radiation hardness. 

For further details on Type K thermocouples, check out this article.

How To Identify the Type of Thermocouple

To identify a thermocouple type, you must first have a good idea, or at least find a chart, of the thermocouple wire color-codes. 

Each thermocouple type comes in different color-coding, and there are variations in each type’s color-coding. 

It sounds all too complicated, but once you understand or find a reliable guide on the color codes, you will easily identify the types. If you need to identify a thermocouple in any appliance, machinery or device, there are 3 steps to follow. 

Firstly, you need to locate the temperature probe. Then, you will follow the probe cable from its tip until its end. The end will often lead to the machine’s temperature display’s backside or inside a control tower. 

You can check this article for their color-coding graph!

Further Variations in Thermocouple Types: Junction Types, Probes, and Other Characteristics

The junction is when the thermocouple wires are welded together and connected to a thermocouple thermometer (or other compatible devices). The different junction types include exposed grounded and ungrounded junctions. 

Depending on the junction type, you can determine the response time of your thermocouple. It also affects the thermocouple’s ability to function in high pressure, corrosive or various other environments. 

Before choosing any thermocouple type, you need to consider other characteristics such as:

• vibration resistance
• chemical reactivity
• abrasion
• pressure
• temperature limitations
• insulation, etc. 

There are also different thermocouple probe types, including:

• penetration probes
• air/gas probes
• flexible insulated-wire probes
• surface probes
• scientific needle-tip probes
• food probes, etc.

To learn more about probe types, follow this link. 

Conclusion

Simply put – there is a large variety of thermocouple types, but you can design them in such a way to suit the exact requirements of your applications. 

That’s the beauty of a thermocouple! 

It truly is versatile and reliable enough to be used in so many ways across different industries.