# How does a thermocouple work?

• Daniel2244
In summary, a thermocouple works by two dissimilar metals being connected at each end. When heat is applied to the hot junction, electrons move between the hot junction of the different metals at a faster rate causing the two different juctions to have a different charge producing a voltage.

## Homework Statement

descibe the operational principle of a thermocouple

## The Attempt at a Solution

A thermocouple works using the seebeck affect. Two dissimilar metals joined together at one end forming a "hot" junction this is where the temperature is measured. At the other end the wires aren't joind this is the reference end (cold end). As the hot junction is heated, the heat is conducted at different speads in the two dissimlar metals. The electrons at the hot junction "diffuse" and move to the cooler end (reference end) of the wires making one of the wires more positively charged and the other more negatively charged as more electrons will be in the reference end of the better conductor wire creating a potential difference

Last edited:
You are confusing two things.
In the Seebeck effect, only one metal is needed and no circuit. Electrons diffuse from the hot end to the cold, creating a potential difference.
In a thermocouple, strips of two different metals are connected at both ends, creating a circuit. With one end heated, a current flows.
See https://www.explainthatstuff.com/howthermocoupleswork.html

Daniel2244
So are you looking to see if you are correct? It's been awhile since I have worked with these, so I looked it up.
https://en.m.wikipedia.org/wiki/Thermocouple
So it looks like you sort of got it. The Seebeck Effect is the thermoelectric effect where temperature difference produces a voltage.

haruspex said:
Nice article

Daniel2244
haruspex said:
You are confusing two things.
In the Seebeck effect, only one metal is needed and no circuit. Electrons diffuse from the hot end to the cold, creating a potential difference.
In a thermocouple, strips of two different metals are connected at both ends, creating a circuit. With one end heated, a current flows.
See https://www.explainthatstuff.com/howthermocoupleswork.html
So, A thermocouple works by two dissimlar metals being connected at each end. One end is called the hot junction and the other end is a cold junction. When heat is applied to the hot junction, electrons move between the hot junction of the different metals at a faster rate causing the two different juctions to have a different charge producing a voltage.

Yes, thermocouples operate on the Seebeck effect.

A typical setup is three junctions: T, T0 and TR where
T is temp. to be measured;
T0 is a reference temperature, often an ice bath;
and TR is the temperature of the measuring device, typically a potentiometer at room temperature so no current flows across the TR junctions. See attached "Seebeck effect.jpg" taken from Heat and Thermodynamics by M W Zemansky of CCNY, hopefully visible.

Each wire type A, B and C (note the same wire type C connecting T0 to TR) has its own characteristic "entropy transport parameter" S* and depends on the material and temperature of the wire. The emf's developed are across the wires so there are Δemf's developed between junctions a to c, c to e, e to d and d to b. Then the respective transport parameters are integrated over temperature over these four wires; the result after cancellation of the S*c parameters is
Δemfa,b ≈ (T - T0)(S*A - S*B) as desired.

Clearly we want to use wires A and B with ΔS* as different from one another as possible. I seem to remember copper and constantan.

There are other thermoelectric effects, e.g. Peltier and Joule. They're different phenomena.

#### Attachments

• seebeck effect.jpg
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## 1. How does a thermocouple produce electricity?

A thermocouple produces electricity through the Seebeck effect, which is the conversion of temperature differences into electrical potential. This occurs when two different metals are joined together, creating a temperature gradient. The difference in temperature between the hot junction (where the thermocouple is exposed to heat) and the cold junction (where the thermocouple is connected to a measuring device) creates a voltage that can be measured as an electrical current.

## 2. What materials are used in a thermocouple?

The most commonly used materials in a thermocouple are two dissimilar metals, typically copper and constantan. However, other metal combinations such as iron and constantan, or chromel and alumel, can also be used depending on the temperature range and sensitivity needed.

## 3. How accurate is a thermocouple?

The accuracy of a thermocouple depends on several factors, including the materials used, the temperature range, and the quality of the junction. Generally, a thermocouple can have an accuracy of around 1-2% of the measured temperature. However, with proper calibration and high-quality materials, some thermocouples can achieve accuracies of 0.1% or better.

## 4. How does a thermocouple measure temperature?

A thermocouple measures temperature by detecting the voltage created by the Seebeck effect. The voltage is then converted into a temperature reading using a reference table or equation that correlates the voltage to a specific temperature. In some cases, the voltage is also amplified and converted into a digital signal for more accurate and precise readings.

## 5. What are the advantages of using a thermocouple?

Thermocouples have several advantages, including their wide temperature range, fast response time, and durability in harsh environments. They are also relatively inexpensive, easy to use, and can be made in various sizes and shapes, making them suitable for a wide range of applications. Additionally, since they produce their own electrical energy, they do not require external power sources.