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How does a thermocouple work and what are thermocouple types?
May 07,2020
For many of us who work in automation, especially process automation, questions like what is a thermocouple, how does a thermocouple work, or what are the types of thermocouple can seem like very basic questions. Most of them will just say it’s a thermometer and leave it to that.

When we ask about the thermodynamics behind it, people actually don´t talk about it. The article will tell you what is a thermocouple exactly, and everything else you need to know about thermocouples.

What is a thermocouple?
Thermocouples are electrical devices consisting of two dissimilar electrical conductors forming different electrical junctions at various temperatures.

Building on the thermoelectric effect, a thermocouple produces temperature-dependent voltage, which can be used to measure temperature.

In addition to thermocouples, 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 thermocouple types, let’s understand how exactly they work.

How does a thermocouple work?
The working principle of the thermocouple relies on a law of physics. We call it the Seebeck effect, after Thomas Johann Seebeck. This French scientist found out that if we join two different metals and heat them on one end, the temperature differential between the two ends produces an electromotive force (EMF). Let’s have a look at the picture below to understand this better:

This EMF depends on the type of metals used and the temperature. Therefore, if we know the characteristics of both metals, we can calculate the temperature change by measuring the millivoltage produced. To relate the voltage to the temperature change, we’ll need the help of a thermocouple table.

Each thermocouple type will have its own reference table, which brings us to the next topic. The following section discusses the types of thermocouples available in the market.

Thermocouple types
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 thermocouple types apart by the cable colors.

But one needs to be careful because those colors change between countries and standards. This table will give you an idea of the most common thermocouple types, their temperature ranges in some of the most common standards.

Thermocouple replacement
Thermocouples have very wide temperature ranges so they are often used in extreme conditions. This makes thermocouple replacement a key aspect of maintenance. Luckily, smart transmitters can monitor the health of your primary element as well as its own health. These devices can give you enough data on your process to plan preventive maintenance based on predictive analyses.

Furthermore, they can also give you data on the performance of different sensors, so you can choose sensors with better performance and life for your process.