A thermocouple is a device that is used to sense temperature in your process. It has two wires that are made of different metals they are Joined together at one end. In the Type J thermocouple one wire (or leg) is made of Iron, the other wire is made of Constantan. You may hear of the refered to as "an Iron Constantan thermocouple" or just Type J.
There are several types of thermocouples used to sense temperature, but for our purpose, we will concentrate on the most common thermocouple type in the plastics market in the United States... the Type J.
Type J thermocouples are used in Extrusion, Blow Molding and Injection Molding. They are used because of the relatively low cost and acceptable accuracy in the 0-1600 deg. F temperature range.
When the thermocouple is connected to a temperature controller, it enables the temperature controller to measure the process temperature.
HOW DOES A THERMOCOUPLE WORK:
This is the short version of how a thermocouple works. As we know, a thermocouple is made up of two different metals, then joined at one end, this forms the hot junction, sometimes referred to as "the hot end".
When the temperature changes (increases for example) a small dc voltages is produced in proportion to the temperature because of the two dissimilar metals. If both wires were Iron or Constantan, there would not be a voltage produced, so you would not have a thermocouple.