Thermocouples are largely used in the fields of science and electronics as a temperature sensor, which are quite easy to handle and use. The functioning is based on the Seebeck effect, common in case of electrical conductors, that experiences a temperature gradient along their length. These sensors are simple, rugged, do not need batteries, and have the ability to measure very wide temperature ranges. They consist of two different metals, joined together at one end. Whenever the junction of the two metals is heated or cooled, a voltage can be obtained, that is proportional and can be correlated to the temperature, that was given when the sensor was heated. Thermocouple alloys are usually available as a wire. They are quite cheap, but their main drawback is their accuracy.
How it Works
To understand the working of thermocouples, the Seebeck effect has to be understood, which states something to this effect, that whenever a conductor is exposed to a thermal gradient, it generates a voltage. Now, if we attempt to measure the voltage, then that would require connecting another conductor to the end of the already hot conductor. The conductor that has been attached later would also experience the temperature gradient, and a voltage of its own would be generated, which would oppose the voltage of the original conductor. The magnitude of the effect would certainly depend on the metal in use. Using different metals would help complete the circuit, which creates yet another circuit which consists of two legs that can generate two different voltages, thus, leaving a small difference voltage available for measurement. As and when the temperature is increased, the difference in the voltage also increases.
It is essential to understand that generally thermocouples measure the difference in temperature between two points, not the exact temperature. In some of the traditional applications, one of the junctions, preferably the cold junction, was maintained at a known and fixed temperature which would work as the reference temperature, while the other end was attached to the probe. This method is certainly good for laboratory applications, but when it comes to being a part of the control instruments, it is really difficult to work with. In control devices, there is already an arrangement made to reduce the temperature gradient if it rises above a certain level, in between the gradients. So, the voltage and the temperature of the cold part can be simulated and maintained. This whole process of controlling the temperature is called cold junction compensation.
Thermocouples are available in different combinations of metals and calibrations. There are four calibrations that are found commonly, and those are J, K, T, and E, which are classified on the basis of temperature, range, and sensitivity. There are high temperature calibrations also available like the R, S, C, and GB, which are less sensitive and have low resolutions comparatively. Every calibration has its own particular temperature range and environment, although the maximum temperature depends on the diameter of the wire used. The classification can also be based on the inertness of the conductor and its magnetic strength.
Thermocouples are the best option when the need is to measure a large range of temperature. But, the efficiency of the device decreases as and when it is used to measure smaller differences in temperature. They are generally used in the steel industry, where it is used to measure temperature throughout the entire process of making steel. It is also used in radioisotope thermoelectric generators to generate electricity, though in a series which is commonly termed as a thermopile.