How does a thermocouple work and its limitations?

Thermocouple

A thermocouple is a sensor used to measure temperature. Thermocouples consist of two wire legs made from different metals. The wire legs are welded together at one end, creating a junction. This junction is where the temperature is measured. When the junction experiences a change in temperature, a voltage is created. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect [explained below], and this voltage can be interpreted to measure temperature. The main limitation with thermocouples is precision; system errors of less than one degc can be difficult to achieve.

Seebeck effect:

This is known after the name of a German physicist Thomas Johann Seebeck. The Seebeck effect is a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances. When heat is applied to one of the two conductors or semiconductors, heated electrons flow toward the cooler one. The cause of this movement of electrons lies in the metallic bonding of metals and in particular in the free delocalized electrons. Delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond. Delocalized means that the electrons are free to move throughout the structure, and gives rise to properties such as conductivity.

If a metal wire is only heated at one end, the lattice oscillations and the movements of the free electrons increase there. Because of the heavy collisions, they begin to spread and diffuse to the cold end. There, the kinetic energies of the electrons are lower and the electrons are not repelled again by heavy collisions. The hot end of the wire thus has a smaller number of electrons than the cold end. As a result, an electrical voltage is obtained between the two ends, also known as thermoelectric voltage, and the process of conversion of temperature difference into voltage is called the thermoelectric effect.

The voltages produced by the Seebeck effect are small, usually only a few microvolts (millionths of a volt) per kelvin of temperature difference at the junction. If the temperature difference is large enough, some Seebeck-effect devices can produce a few millivolts (thousandths of a volt). Numerous such devices can be connected in series to increase the output voltage or in parallel to increase the maximum deliverable current.

A linear approximation of the Seebeck effect can be seen in the following equation: ?V = S x [Th – Tc], ?V- Voltage difference between two dissimilar =metals, S - Seebeck coefficient in V/K (commonly in μV/°C), [Th – Tc] = temperature difference between hot and cold junctions

The Seebeck coefficient is specific to the two conductors that are used to construct the thermocouple and has a non-linear dependence on the temperature. Using a linear approximation of the Seebeck effect can produce significant measurement errors. It is important to know that a temperature measurement cannot be determined solely from the EMF generated by the thermocouple. Instead, the following three parameters must be known: [1] the thermoelectric voltage due to thermal gradient between the hot and cold junctions [2] the thermocouple type [3] the cold junction temperature. With these information available, as explained above Temperature at hot junction Th = V [thermocouple] / [k x Tc] + Tc,

Th - the hot junction temperature in degc, Tc - the cold junction temperature in degc, [Th – Tc] is temperature difference between hot and cold junctions, k - Seebeck coefficient as a function of Tc in μV/°C

Thermocouple:

 If a simple wire such as the one described above were exposed to a heat source, the wire would heat up evenly. Due to the lack of temperature gradient between the ends, no thermoelectric voltage could be measured.

Therefore, two different conductors are required which differ in the strength of the Seebeck effect (e.g. copper and iron). The different wires are now connected at one end. This joint serves as a measuring junction (“hot junction”) and is exposed to the temperature to be measured. The other ends lead to the so-called reference junction (“cold junction”), the temperature of which is usually ambient temperature. There is now a temperature gradient between the measuring junction and the reference junction and thus between the ends of the respective wires. This results in a thermoelectric effect with the consequence of an electrical voltage. Since these are different metals, the thermoelectric effect is different in strength. For example, compared to copper, iron has a thermoelectric voltage around 6 times as high.

Since the two metals are connected to each other at the measuring junction, they are at the same electric potential. As a result, the electric potentials at the reference junction differ and an electrical voltage is caused that can be measured with a voltmeter.

At the hot junction, you get temperature Th as explained above.

Temperature at hot junction Th = V thermocouple / [k x Tc] + Tc,

Th - the hot junction temperature in degc, Tc - the cold junction temperature in degc, [Th – Tc] is temperature difference between hot and cold junctions, k - Seebeck coefficient as a function of Tc in μV/°C

Limitations of thermocouple: There are many. Only three limitations explained.

Poor hot junction thermal/electrical connection– If the two conductors are not properly joined together at the hot junction, the wrong thermoelectric voltage may be generated. For a bare wire thermocouple, there are a few different ways of joining two leads together. Leads can be twisted together, soldered together, or welded together. For applications with excessive mechanical vibrations, twisting leads together is not recommended. For high-temperature applications, the junction should not be soldered together due to the possibility of solder reflow. Cold welding leads together is often the best option.

For insulated junctions, there is a different concern. Due to the construction of the junction, it has the benefit of being more mechanically sturdy and corrosion-resistant than bare wire-type thermocouples. A drawback here is that since there is no metallic surface directly exposed to the measurement temperature, the thermal resistance of the hot junction increases. This slows down the thermocouple response to any change in temperature.

Achieving an accurate cold junction temperature measurement is crucial when using thermocouples to derive a hot junction temperature. Traditional cold junctions would be chilled in an ice-cold water bath in order to keep a very consistent and known temperature of 0°C, hence the name “cold junction.

Credit: Google