Designing with Thermocouples: Get the Most from Your Measurements

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More than 60 percent of all industrial temperature measurement applications in the U.S. use thermocouples. Despite their widespread use, there are many misconceptions about thermocouples. This paper will discuss some of the basic technical issues that engineers need to consider when applying thermocouples.

Why use thermocouples?

Some common reasons for using thermocouples include:

  • They're already used in your facility
  • The application requires a sensor that can withstand a lot of physical stress or one that is physically small
  • The expected high and low temperature ranges and/or total span exceeds the limitations of other sensor types
  • Thermocouples are relatively inexpensive compared to some other sensors on the market

So how do you get the best possible thermocouple measurements? Although the thermocouple is a simple device, its small voltage signal is easily corrupted, and wiring one requires care.

How do thermocouples work?

Heat and electrical energy are related when it comes to electrical conductors, including thermocouple wires. A common misconception is that the thermocouple junction creates the voltage signal, similar to a battery. This is not true; rather, the signal is generated along the entire length of wire where there is a temperature gradient.

If you've ever held one piece of a wire, and then heated the other end, you quickly discovered that the heat moves up the wire. Why?

Heat moves toward the cold end because the higher kinetic energy of hot atoms imparts some of their energy to their colder neighbors, making them vibrate. The atoms vibrate faster toward the hot end. They don't move, however, because the solid structure holds the nucleus in place.