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#WishIknew -What is a thermocouple?

For many of us who work in automation, especially process automation, “What is a thermocouple?” can seem like a very basic question. But I bet if you ask, most of us will just say it’s a thermometer and leave it at that. When you ask about the thermodynamics behind it, you’ll get a lot of people pausing. We just don’t think about it!

The market has a wide variety of temperature sensors – resistance temperature detectors (RTDs), infrared sensors, thermistors, silicon diodes – just to name a few. The pros and cons of each make them more suited for some applications and less ideal for others.

But before we start talking about different types of thermocouples, let’s understand how exactly they work.

The Seebeck effect

The thermocouple working principle relies on a physical law, as usual. We call it the Seebeck effect, after Thomas Johann Seebeck. This French scientist found that if you join two different metals and heat them on one end, the temperature differential between the two ends produces an electromotive force (EMF). This picture will help you see it better.

Seebeck Effect Thermocouple
Courtesy of termopares.com.br

This EMF depends on the type of metals used and the temperature. Therefore, if you know the characteristics of both metals, you can calculate the temperature change by measuring the millivoltage produced.

To relate the voltage to the temperature change, you’ll need the help of a thermocouple table. If you want to learn more about that, we have an entire article talking about it here.

Each thermocouple type will have its own reference table, which brings us to the next topic.

Thermocouple types

As we just learned, a thermocouple consists of two metals that produce an EMF when one end experiences a temperature change.

However, one thermocouple can’t handle all temperature ranges, so we use different metals to measure all the ranges we need. You can tell the types apart by the cable colors. But be careful, because those colors change between countries and standards. This table will give you an idea of the most common types, their temperature ranges, and the colors in some of the most common standards.

thermocouple types
Courtesy of Temperature Sensors

We have a lot more we can talk about when it comes to thermocouples, but we won’t go over all that today. In the Visaya archives, you can find posts about compensating cables and extension cables, thermocouple loops, thermocouple tables, and more. If you want more, check out some of these other articles.

For now, we will leave it as is. And next time someone asks you “What is a thermocouple?” you can say more than just “It’s a temperature sensor.”

These articles have more information:

Your friendly neighborhood temperature sensor! – Is it hot in here, or is it just me? Regardless, let’s talk temperature sensors! This basic variable is necessary to a variety of processes and segments. In most cases, you need to monitor exact temperatures, and in some, you need precise control. When you learn about temperature sensors from the maintenance point of view, you’ll find that you only need to consider certain points to choose the right sensor for your application.

Temperature measurement and control systems! – Today, you have tons of ways to connect your instrumentation to your control system, but as always, all the options have good points and bad. With an application where you have to connect temperature sensors to your system, which option(s) will work best for you?

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