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All About Thermocouple

Wednesday, August 7, 2019

What is a thermocouple?
This is a sensor used to measure temperature in a number of processes. Thermocouples are constructed of two wire legs made from dissimilar metals which are fixed together at one end, creating a junction. When this junction experiences a change in temperature a voltage is created, this voltage can then be measured and referenced back to the temperature.

What is a thermocouple wire?
This is the wire used in the manufacture of a thermocouple temperature sensor or the sensing point (or probe part) of the thermocouple. It functions even in exceptionally high service temperatures. There are multiple types and grades of thermocouple wire. Each type of thermocouple wire has a specific combination of metal alloys. This combination is what defines the type of thermocouple. Allied Wire & Cable’s stock of thermocouple wires includes 
Type K
Type E 
Type T 
Type N 
Type R
Type J
We also have a full range of wire insulation and conductor specifications in stock. 

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A brief history of thermocouple
The thermocouple was discovered in 1821 by the German physicist Thomas Johann Seebeck (1770—1831), but the physical principle was explained much later by using Quantum Mechanics.
This refers to the most important application of the "Seebeck Effect", which is the thermocouple to use in thermometry as a temperature sensor. The thermocouple acts as a transductor that receives a temperature difference and produces the corresponding voltage difference.
One more important application of the Seebeck Effect is in the Radioisotope Thermoelectric Generators (RTGs), to generate electrical energy from heat generated by a radioisotope decay. This is very useful, for example, as an energy source in a space probe that works away from sunlight.

How does a thermocouple work?
When the ends of two metallic wires of different metals (let's say "A" and "B", pure and/or alloys) are put in contact, between the unconnected ends an electrical potential difference ("voltage") Vab(T) is generated. This voltage is determined by the pair of metals (A and B) and the temperature T at which the AB union is located. This voltage Vab(T) that is generated at the ends is approximately proportional to the temperature T of the union AB.

Why Vab(T) is generated?
Each metal at a given temperature T is composed of atoms that have an electronic distribution of different energy (calculated by using Quantum Mechanics). This distribution is like a fingerprint: each element is different.
Due to the difference in distribution, when A and B come into contact there are electrons that migrate from one metal to the other. This means that one loses electrons (and becomes electrically positive) and the other wins electrons (and becomes electrically negative), which generates an electric field that points to the negative metal, the metal that receives the electrons.
The process does not continue indefinitely, because on each electron there will be an electric force whose will oppose migration. Then, the process ends when the electric field generated by the charge difference reaches equilibrium.
Consequently, putting the metals in contact generates an electric field that determines the voltage Vab(T) between this pair of metals A and B at the temperature T.

Why do we use a thermocouple?
They are self-powered device so they are very economical, making them useful in various fields.
Thermocouples are widely used in temperature measurement as they are cheap, interchangeable, are made with available connectors, and can measure a wide range of temperatures, up to 2300 °C.
They also used as a temperature sensor. That is they are used to sense the temperature difference.
An interesting application of the thermocouple is, it often used to convert a temperature difference into electricity. More easily, they are used to convert heat energy into electric power.
What are the applications of a thermocouple?
Thermocouples are applied in the temperature measurement of furnaces, gas turbine exhausts, diesel engines, thermoelectric generators, industrial plants, laboratories, plants, stoves, and, etc.

Filed Under: Allied Encyclopedia

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