Thermocouples are economical temperature sensing devices that measure the temperature difference at the junction of two dissimilar metals. These units are often called thermocouple temperature probes and they use the Seeback effect in which certain metals generate voltage when exposed to temperature gradients. They are made of two wires of different metals that are joined to form a measuring junction. The other ends of the wires are attached to a measuring device to form a reference junction. When the two junctions are at different temperatures, voltage flows between them. This voltage can be read off against a reference to determine what the temperature is.
Thermocouples produce an output voltage which depends on the temperature difference between the junctions of two dissimilar metal wires. It is important to realize that thermocouples measure the difference in temperature between the two points they don’t measure the absolute temperature. Thermocouples are generally selected to make sure that the measuring equipment does not limit the range of temperatures that can be measured.
The metal pairs have designated letters like: E for Chromel/Constantan, J for Iron/Constantan, etc. Thermocouples are available as grounded, where the junction tips are physically attached to the walls of the probe; ungrounded, where the junction is not attached to the walls of the probe; exposed type, where the junction is exposed to the environment. The exposed type gives you the best response time, but they limited in use to non corrosive and non pressurized applications. They are available as single or double elements and the larger the number of elements, the higher the accuracy.
There are various types of thermocouples used for things such as scientific research, medical research and the food industry. There are about a dozen thermocouple temperature probe types that are commonly used in industrial applications. Base metal thermocouple temperature probes are helpful for measuring temperatures under 1000·C. Noble metal thermocouples are good to approximately 2000°C and refractory metal thermocouples are useful to about 2600°C. While selecting thermocouples, you have to consider: the range of temperatures that will be sensed; the diameter of the probe, the sensing length; the type of connection (such as bayonet, threaded, soldered, press fitted); and the termination type.