What is a Thermocouple? | Thermocouple Working Principle

In the year 1821, a physicist named “Thomas Seebeck” revealed that when two different metal wires are connected in a circuit at both ends of a junction, the temperature applied to the junction is the current flowing through the circuit. Is known as the electromagnetic field (EMF). The energy generated by this circuit is known as the Sibek effect.

Under the guidance of “Thomas Seebeck“, two Italian physicists named Leopoldo Nobili and Macedonio Maloney designed a thermoelectric battery in the year 1826 called a thermal multiplier. In today’s article, we will see what is the working principle of what is thermocouples and we will see the discussion of its benefits and types in today’s article.

What is a Thermocouple?

 

Inside this, two different metal wires are used which are connected at a junction. Temperature is measured at this junction. Stimulates voltage due to changes in temperature of metal wire.

The amount of emf generated in the device is very small so it is very important to use a very sensitive device to know the amount of emf in this device. The device used to calculate e.m.f is a normal galvanometer or balancing potentiometer. Either of these is used physically or mechanically.

Construction of Thermocouple:

The construction of a thermocouple is as shown in the figure below which consists of different wires and a meter is placed to measure it. There are three main types of thermocouple construction which are as follows.

• Ungrounded Junction.
• Grounded Junction.
• Exposed Junction.

#1. Ungrounded Junction-

In the Ungrounded-Junction system, the conductor is completely covered by a protective cover. Ungrounded-Junction is mainly used in applications with high pressure. The main advantage of doing so is that the effect of the wandering magnetic field is reduced.

#2. Grounded Junction-

In this type of junction, the metal wire and the safety cover are connected together. Such a junction is used to measure the temperature in a corrosive environment. This junction provides noise resistance.

#3. Exposed Junction-

The exposed junction is used in environments where a very fast response is required. Such a junction is used to measure gas temperature. The material used in the manufacture of thermocouples depends mainly on the range of measurement of temperature.

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Thermocouple Working Principle:

The working principle of the thermocouple depends on the main three methods which are as follows:

• See Beck Effect.
• Peltier Effect.
• Thompson Effect.

#1. See Beck Effect-

This type of effect occurs between two different metals. When heat is given to anyone metal, electrons flow from the hot side to the cold side, thus starting the current flowing in the circuit.

In short, the phenomenon that the temperature difference between two different metals stimulates the potential differences between them. The sea back effect produces a small voltage for the Kelvin of temperature.

#2. Peltier Effect-

This type of effect is the exact opposite of the See beck effect. The Peltier effect shows that the temperature difference can be created between any two different types of conductors by applying the potential difference between them.

#3. Thompson Effect-

Thompson effect creates the effect that when two different types of metals join, it forms a junction, that is, the voltage induces the length of the total conductor due to the gradient of temperature. This is a physical term that Indicates a change in temperature rate and direction in position.

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How Does a Thermocouple Work?

The thermocouple consists of two different types of metal plates. These two plates are connected to each other and form a junction. The junction of these two plates is placed on top of which we want to measure the temperature. This junction is known as the hot junction. And the remaining ends are kept at normal temperature. This normal junction is known as a cold junction.

The difference in temperature between two different metals due to the Seabeck effect causes a potential difference between the two points of the thermocouple plates. So we can easily calculate the temperature of the hot junction by measuring the voltage.

Thermocouple Types:

One of the important things before considering the type of thermocouple is that it needs to be protected protectively to separate it from the ambient temperature. This protection cover will properly reduce the corrosion on the device.

There are different types of thermocouples available in the market which are as follows:

• Type K Thermocouple.
• Type T Thermocouple.
• Type J Thermocouple.
• Type E Thermocouple.
• Type N Thermocouple.
• Type S Thermocouple.
• Type R Thermocouple.
• Type B Thermocouple.

#1. Type K Thermocouple-

This type of thermocouple survey is the most common type of thermocouple. Also known as nickel-chromium/nickel-alum type thermocouple. It provides features like high reliability, accuracy, and cheapness. It has a wide range of temperature measurements.

The positive lead of this type of thermocouple is made of about 90% nickel and 10% chromium. Its negative lead is made up of about 95% nickel, 2% aluminum, 2% manganese, and 1% silicon.

The color of the positive lead is yellow and it is non-magnetic. When the color of the negative lead is red and it is a magnetic material. And the overall jacket is yellow.

The temperature range of the K Thermocouple is as follows:

• Thermocouple grade wire – -454F to 2300F (-270⁰C to 1260⁰C)
• Extension wire (0⁰C to 200⁰C)
• This K-type has an accuracy level of
• Standard +/- 2.2C or +/-0.75% and the special limits are +/- 1.1C or 0.4%.

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#2. Type T Thermocouple-

The Type T Thermocouple is perfect for measuring low temperatures. This is because its positive lead is made of copper while its negative lead is made of Constantine.

The temperature range of a Type T Thermocouple is as follows:

• Thermocouple grade wire – -454F to 700F (-270⁰C to 370⁰C)
• Extension wire (0⁰C to 200⁰C)
• This T-type has an accuracy level of
• Standard +/- 1.0C or +/-0.75% and the special limits are +/- 0.5C or 0.4%

#3. Type J Thermocouple-

The Type J Thermocouple is the most widely used thermocouple in the market. Its positive lead is made of iron while its negative lead is made of Constantine (45% nickel and 55% copper).

The color of the positive lead is white while the color of the negative lead is red. And the overall jacket is colored black.

The temperature range of a Type J Thermocouple is as follows:

• Thermocouple grade wire – -346F to 1400F (-210⁰C to 760⁰C)
• Extension wire (0⁰C to 200⁰C)
• This J-type has an accuracy level of
• Standard +/- 2.2C or +/-0.75% and the special limits are +/- 1.1C or 0.4%

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#4. Type E Thermocouple-

E Thermocouple type provides higher accuracy and stronger signal compared to types K and J in the medium temperature range. It is a more stable type of thermocouple compared to type K. Type thermocouples produce the highest emf per degree than other types of thermocouples.

The positive lead of this type of thermocouple is made of nickel chromium (90% nickel and 10% chromium). While its negative lead is made from Constantine (95% nickel, 2% aluminum, 2% manganese, and 1% silicon). The color of the positive lead is purple and the color of the negative lead is red.

The temperature range of a Type E Thermocouple is as follows:

• Thermocouple grade wire – -454F to 1600F (-270⁰C to 870⁰C)
• Extension wire (0⁰C to 200⁰C)
• This E-type has an accuracy level of
• Standard +/- 1.7C or +/-0.5% and the special limits are +/- 1.0C or 0.4%

#5. Type N Thermocouple-

The N-Type Thermocouple has been designed by the Defense Science and Technology Organization (DSTO) of Australia. The positive lead of the N-Type Thermocouple is made of Nicrosil. Nicrosil is a combination of nickel, chromium, and silicon. While the negative lead is made of nisil. Nisil is a combination of nickel and silicon.

The temperature range of a Type N Thermocouple is as follows:

• Thermocouple grade wire – -454F to 2300F (-270⁰C to 392⁰C)
• Extension wire (0⁰C to 200⁰C)
• This N-type has an accuracy level of
• Standard +/- 2.2C or +/-0.75% and the special limits are +/- 1.1C or 0.4%

#6. Type S Thermocouple-

S Type Thermocouple is used for high-temperature applications but in some cases due to high accuracy and stability, sometimes it is also used for measuring low temperatures.

Such a thermocouple is commonly used in applications such as the pharmaceutical and biotech industries as it requires high temperature and precision.

The temperature range of a Type S Thermocouple is as follows:

• Thermocouple grade wire – -58F to 2700F (-50⁰C to 1480⁰C)
• Extension wire (0⁰C to 200⁰C)
• This S-type has an accuracy level of
• Standard +/- 1.5C or +/-0.25% and the special limits are +/- 0.6C or 0.1%

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#7. Type R Thermocouple-

This type of thermocouple is made of platinum/rhodium, its positive lead is made of 87% platinum and 13% rhodium while the negative lead is made of platinum.

The range of R-type thermocouples is slightly longer than S-type thermocouples. The features and effects of the R and S types are almost identical.

The temperature range of the Type R Thermocouple is as follows:

• Thermocouple grade wire – -58F to 2700F (-50⁰C to 1480⁰C)
• Extension wire (0⁰C to 200⁰C)
• This R-type has an accuracy level of
• Standard +/- 1.5C or +/-0.25% and the special limits are +/- 0.6C or 0.1%

#8. Type B Thermocouple-

A combination of platinum and rhodium is used in the manufacture of Type B Thermocouple. Its positive lead is made up of 70% platinum and 30% rhodium, while its negative lead is 94% platinum and 6% rhodium.

This type of thermocouple is widely used to measure high temperatures. It has the highest temperature measurement limit of all the thermocouples mentioned above.

The temperature range of the Type B Thermocouple is as follows:

• Thermocouple grade wire – 32F to 3100F (0⁰C to 1700⁰C)
• Extension wire (0⁰C to 100⁰C)
• This B-type has an accuracy level of
• Standard +/- 0.5%.

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Difference Between Thermostat and Thermocouple:

Advantages of Thermocouple:

The benefits of Thermocouples are as follows:

• The thermal reaction is instantaneous.
• The measuring range of temperature is wide.
• Wide operating temperature capability.
• Cost low and highly compatible.
• High accuracy.
• It is tough and can be easily used in harsh as well as high-vibration environments.

Disadvantages of Thermocouple:

The disadvantages of thermocouples are as follows:

• The stability of this is minimal.
• Low voltage.
• Reference required.
• Least sensitive.
• Thermocouple recovery is difficult.

Application of Thermocouple:

The following are several applications of Thermocouples:

• It is used as a temperature sensor in thermostats in home offices and businesses.
• It is used in industries to measure the temperature of iron, aluminum, and metals.
• This is used to check the temperature in chemical plants and petroleum plants.
• Thermocouples are used to detect pilot flames in devices used to generate heat from gases such as water heaters.
• This is used in gas machines to detect pilot flames.

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