What is a Thermocouple : Working Principle & Its Applications

In the year 1821, a physicist namely “Thomas Seebeck” revealed that when two different metal wires were linked at both ends of one junction in a circuit when the temperature applied to the junction, there will be a flow of current through the circuit which is known as electromagnetic field (EMF). The energy which is produced by the circuit is named the Seebeck Effect. Using Thomas Seebeck’s effect as his guideline, both Italian physicists namely Leopoldo Nobili and Macedonio Melloni were collaborated to design a thermoelectrical- battery in the year 1826, that is called a thermal multiplier, it drew from the discovery of Seebeck’s thermoelectricity by merging a galvanometer as well as a thermopile to calculate radiation. For his effort, some people identified Nobili as the discoverer of the thermocouple.


What is a Thermocouple?

The thermocouple can be defined as a kind of temperature sensor that is used to measure the temperature at one specific point in the form of the EMF or an electric current. This sensor comprises two dissimilar metal wires that are connected together at one junction. The temperature can be measured at this junction, and the change in temperature of the metal wire stimulates the voltages.

Thermocouple
Thermocouple

The amount of EMF generated in the device is very minute (millivolts), so very sensitive devices must be utilized for calculating the e.m.f produced in the circuit. The common devices used to calculate the e.m.f are voltage balancing potentiometer and the ordinary galvanometer. From these two, a balancing potentiometer is utilized physically or mechanically.

Thermocouple Working Principle

The thermocouple principle mainly depends on the three effects namely Seebeck, Peltier, and Thompson.

See beck-effect

This type of effect occurs among two dissimilar metals. When the heat offers to any one of the metal wires, then the flow of electrons supplies from hot metal wire to cold metal wire. Therefore, direct current stimulates the circuit.

Peltier-effect

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This Peltier effect is opposite to the Seebeck effect. This effect states that the difference of the temperature can be formed among any two dissimilar conductors by applying the potential variation among them.

Thompson-effect

This effect states that as two disparate metals fix together & if they form two joints then the voltage induces the total conductor’s length due to the gradient of temperature. This is a physical word that demonstrates the change in rate and direction of temperature at an exact position.


Construction of Thermocouple

The construction of the device is shown below. It comprises two different metal wires and that are connected together at the junction end. The junction thinks as the measuring end. The end of the junction is classified into three type’s namely ungrounded, grounded, and exposed junction.

Thermocouple Construction
Thermocouple Construction

Ungrounded-Junction

In this type of junction, the conductors are totally separated from the protecting cover. The applications of this junction mainly include high-pressure application works. The main benefit of using this function is to decrease the stray magnetic field effect.

Grounded-Junction

In this type of junction, the metal wires, as well as the protection cover, are connected together. This function is used to measure the temperature in the acidic atmosphere, and it supplies resistance to the noise.

Exposed-Junction

The exposed junction is applicable in the areas where a quick response is required. This type of junction is used to measure the gas temperature. The metal used to make the temperature sensor basically depends on the calculating range of temperature.

Generally, a thermocouple is designed with two different metal wires namely iron and constantan that makes in detecting element by connecting at one junction that is named as a hot junction. This consist of two junctions, one junction is connected by a voltmeter or transmitter where the cold junction and the second junction is associated in a process that is called a hot junction.

How Does a Thermocouple Work?

The thermocouple diagram is shown in the below picture. This circuit can be built with two different metals, and they are coupled together by generating two junctions. The two metals are surrounded by the connection through welding.

In the above diagram, the junctions are denoted by P & Q, and the temperatures are denoted by T1, & T2. When the temperature of the junction is dissimilar from each other, then the electromagnetic force generates in the circuit.

Thermocouple Circuit
Thermocouple Circuit

If the temperate at the junction end turn into equivalent, then the equivalent, as well as reverse electromagnetic force, produces in the circuit, and there is no flow of current through it. Similarly, the temperature at the junction end becomes imbalanced, then the potential variation induces in this circuit.

The magnitude of the electromagnetic force induces in the circuit relies on the sorts of material utilized for thermocouple making. The entire flow of current throughout the circuit is calculated by the measuring tools.

The electromagnetic force induced in the circuit is calculated by the following equation

E = a (∆Ө) + b (∆Ө)2

Where ∆Ө is the temperature difference among the hot thermocouple junction end as well as the reference thermocouple junction end, a & b are constants

Thermocouple Types

In before going with a discussion of thermocouple types, it has to be considered that thermocouple needs to be protected in a protective case to isolate from the atmospheric temperatures. This covering will significantly minimize the corrosion impact on the device.

So, there are many types of thermocouples. Let us have a detailed look at those.

Type K – This is also termed as Nickel-Chromium/Nickel-Alumel type of thermocouple. It is the most generally used type. It has the features of enhanced reliability, preciseness, and inexpensive and can operate for extended temperature ranges.

Type K Thermocouple
Type K Thermocouple

The temperature ranges are:

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%

Type J – It is a mix of Iron/Constantan. This is also the most used type of thermocouple. It has the features of enhanced reliability, preciseness, and inexpensive. This device can be operated only for lesser temperature ranges and has a short lifespan when operated at a high range of temperatures.

J Type
J Type

The temperature ranges are:

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%

Type T – It is a mix of Copper/Constantan. The T type thermocouple holds increased stability and is generally implemented for lesser temperature applications like ultra-low temperature freezers and cryogenics.

T Type Thermocouple
T Type Thermocouple

The temperature ranges are:

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%

Type E – It is a mix of Nickel-Chromium/Constantan. It has a greater signal ability and improved accuracy when compared with that of Type K and J thermocouples when operated at ≤ 1000F.

E Type
E Type

The temperature ranges are:

Thermocouple grade wire – -454F to 1600F (-2700C to 8700C)

Extension wire (00C to 2000C)

This T-type has an accuracy level of

Standard +/- 1.7C or +/-0.5% and the special limits are +/- 1.0C or 0.4%

Type N – It is considered as either Nicrosil or Nisil thermocouple. The temperature and accuracy levels of type N are similar to type K. But this type is more expensive than type K.

N Type
N Type

The temperature ranges are:

Thermocouple grade wire – -454F to 2300F (-2700C to 3920C)

Extension wire (00C to 2000C)

This T-type has an accuracy level of

Standard +/- 2.2C or +/-0.75% and the special limits are +/- 1.1C or 0.4%

Type S – It is considered as either Platinum/Rhodium or 10%/Platinum thermocouple. The S type of thermocouple is extremely implemented for high-temperature range applications such as in Biotech and pharmacy organizations. It is even used for lesser temperature range applications due to its increased preciseness and stability.

S Type
S Type

The temperature ranges are:

Thermocouple grade wire – -58F to 2700F (-500C to 14800C)

Extension wire (00C to 2000C)

This T-type has an accuracy level of

Standard +/- 1.5C or +/-0.25% and the special limits are +/- 0.6C or 0.1%

Type R – It is considered as either Platinum/Rhodium or 13%/Platinum thermocouple. The S type of thermocouple is extremely implemented for high-temperature range applications. This kind is included with a higher amount of Rhodium than Type S that makes the device more costly. The features and performance of type R and S are nearly similar. It is even used for lesser temperature range applications due to its increased preciseness and stability.

R Type
R Type

The temperature ranges are:

Thermocouple grade wire – -58F to 2700F (-500C to 14800C)

Extension wire (00C to 2000C)

This T-type has an accuracy level of

Standard +/- 1.5C or +/-0.25% and the special limits are +/- 0.6C or 0.1%

Type B – It is considered as either 30% of Platinum Rhodium or 60% of Platinum Rhodium thermocouple. This is widely used in the higher range of temperature applications. Of all the above-listed types, type B has the highest temperature limit. At the increased temperature levels, the type B thermocouple will hold increased stability and accuracy.

B Type Thermocouple
B Type Thermocouple

The temperature ranges are:

Thermocouple grade wire – 32F to 3100F (00C to 17000C)

Extension wire (00C to 1000C)

This T-type has an accuracy level of

Standard +/- 0.5%

The types S, R, and B are considered noble metal thermocouples. These are chosen because they can function even at high-temperature ranges providing great accuracy and a long lifetime. But, when compared with base metal types, these are more expensive.

While choosing a thermocouple, one has to consider many factors that suit their applications.

  • Check what are the low and high temperature ranges necessary for your application?
  • What budget of the thermocouple to be used?
  • What percentage of accuracy to be used?
  • In which atmospheric conditions, does the thermocouple is operated such as inert gaseous or oxidizing
  • What is the level of response that it is expected which means that how quickly the device needs to respond to the temperature changes?
  • What is the lifetime period that is required?
  • Check before the operation that the device is immersed in water or not and to which level of depth?
  • Will the utilization of the thermocouple either to be intermittent or continuous?
  • Will the thermocouple be subjected to twisting or flexing all through the device lifetime?

How Do You Know If You Have a Bad Thermocouple?

In order to know whether a thermocouple is working perfectly, one needs to perform testing the device. Before going with the replacement of the device, one has to check that it is actually functioning or not. To do this, a multimeter and basic knowledge of electronics are completely enough. There are mainly three approaches to testing the thermocouple using a multimeter and those are explained as below:

Resistance Test

To perform this test, the device has to be placed in a gas appliance line and the equipment required is digital multimeter and crocodile clips.

Procedure – Connect the crocodile clips to the sections in the multimeter. Attach the clips on both ends of the thermocouple where one end will be folded into the gas valve. Now, switch on the multimeter and note down the reading options. If the multimeter displays ohms in small order, then the thermocouple is in perfect working condition. Or else when the reading is 40 ohms or more, then it is not in good condition.

Open Circuit Test

Here, the equipment used is crocodile clips, a lighter, and a digital multimeter. Here, instead of measuring the resistance, voltage is calculated. Now, with the lighter heat up one end of the thermocouple. When the multimeter displays voltage in the range of 25-30 mV, then it is working properly. Or else, when the voltage is close to 20mV, then the device has to be replaced.

Closed Circuit Test

Here, the equipment used is crocodile clips, thermocouple adapter, and digital multimeter. Here, the adapter is placed inside the gas valve and then the thermocouple is placed to one edge of the adapter. Now, switch on the multimeter. When the reading is in the range of 12-15 mV, the device is in proper condition. Or else when the voltage reading drops below 12mV, it indicates a faulty device.

So, using the above testing methods, one can find out whether a thermocouple is working properly or not.

What is the Difference Between Thermostat and Thermocouple?

The differences between thermostat and thermocouple are:

Feature Thermocouple Thermostat
Range of Temperature -454 to 32720F -112 to 3020F
Price Range Less High
Stability Provides less stability Provides medium stability
Sensitivity Thermocouple has less sensitivity Thermostat offers the best stability
Linearity Moderate Poor
System cost High Medium

Advantages & Disadvantages

The advantages of thermocouples include the following.

  • Accuracy is high
  • It is Robust and can be used in environments like harsh as well as high vibration.
  • The thermal reaction is fast
  • The operating range of the temperature is wide.
  • Wide operating temperature range
  • Cost is low and extremely consistent

The disadvantages of thermocouples include the following.

  • Nonlinearity
  • Least stability
  • Low voltage
  • Reference is required
  • least sensitivity
  • The thermocouple recalibration is hard

Applications

Some of the applications of thermocouples include the following.

  • These are used as the temperature sensors in thermostats in offices, homes, offices & businesses.
  • These are used in industries for monitoring temperatures of metals in iron, aluminum, and metal.
  • These are used in the food industry for cryogenic and Low-temperature applications. Thermocouples are used as heat pumps for performing thermoelectric cooling.
  • These are used to test temperature in chemical plants, petroleum plants.
  • These are used in gas machines for detecting the pilot flame.

What is the Difference Between RTD and Thermocouple?

The other foremost thing that has to be considered in the case of the thermocouple is how it is different from the RTD device. So, the tabular explains the differences between RTD and thermocouple.

RTD Thermocouple
RTD is extensively suitable for measuring less range of temperature which are between (-2000C to 5000C) The thermocouple is suitable for measuring a higher range of temperature which are between (-1800C to 23200C)
For a minimal range of switching’s, it exhibits increased stability These have minimal stability and also results are not precise when tested multiple times
It has more accuracy than a thermocouple Thermocouple has less accuracy
The sensitivity range is more and can even calculate minimal temperature changes The sensitivity range is less and these cannot calculate minimal temperature changes
RTD devices have a good response time Thermocouples provide a quick response than that of RTD
The output is linear in shape The output is non-linear in shape
These are more expensive than thermocouple These are economical than RTDs

What is the Life Span?

The lifespan of the thermocouple is based on the application when it is utilized. So, one cannot specifically predict the thermocouple life period. When the device is maintained properly, the device will have a long life span. Whereas, after continual usage, they might get damaged because of the aging effect.

And also, because of this, the output performance will be lowered and the signals will have poor efficiency. The price of the thermocouple is also not high. So, it is more suggested to modify the thermocouple every 2-3 years. This is the answer to what is the lifespan of a thermocouple?

Thus, this is all about an overview of the thermocouple. From the above information finally, we can conclude that the measurement of thermocouple output can be calculated by using methods like a multimeter, potentiometer, and amplifier by output devices. The main purpose of the thermocouple is to build consistent & direct temperature measurements in several different applications.

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