All You Need to Know about Thermocouples

In 1821, a physicist named “Thomas Seebeck” discovered that when 2 dissimilar metal wires are connected at both ends of a single junction in a circuit with a temperature applied to that particular junction, a current called an electromagnetic field (or EMF) would flow through the circuit. The energy thus produced by the circuit is called the Seebeck effect. Using the influence of Thomas Seebeck as a mentor, two Italian physicists, Leopoldo Nobili and Macedonio Mellon, collaborated in 1826 to design a thermoelectric battery called a thermal multiplier. It was based on Seebeck’s discovery of thermoelectricity by connecting a galvanometer, and a hot stack to calculate radiation. Because of his efforts, some people have identified Nobili as the discoverer of thermocouples. 


Thermocouples can be defined as the types of temperature sensors used for measuring the temperature at some specific point in the form of EMF or electric current. These sensors consist of 2 different metal wires connected together at a junction. At this junction, the temperature can be measured and the change in temperature of the metal wire stimulates the voltages. The amount of EMF generated in the device is very small (millivolts), so very sensitive equipment must be used to calculate the e.m.f generated in the circuit. The most common devices used to calculate e.m.f are the voltage equalization potentiometer and the standard galvanometer. Out of these two, a physically/mechanically balanced potentiometer is often used.

Thermocouple Working Principle

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

  • Seebeck Effect: This type of effect occurs between 2 dissimilar metals. When heat is applied to a metal wire, the flow of electrons moves from the hot metal wire to the cold metal wire. Therefore, the DC current stimulates the circuit. 
  • Peltier Effect: This Peltier effect is the opposite of the Seebeck effect. This effect means that a temperature difference can be created between two different wires by applying a potential change between them. 
  • Thompson-Effect: This effect states that when two dissimilar metals are attached to each other and form two junctions, a voltage is induced along the length of the conductor due to a temperature gradient. It is a physical word that indicates the rate and direction of temperature change at a precise location.

Thermocouple Construction

The device structure consists of two different metal wires joined at the connecting end. The intersection is thought of as a measuring head. The connection end or junction is divided into three types, namely grounded, ungrounded, and open or exposed. 

  • Ungrounded-junction: In this type of connection interface, the conductors are completely separated from the protective sheath. This junction is mainly used for high pressure applications. The main advantage of using this feature is to reduce the effect of the stray magnetic field. 
  • Grounded-junction: In this type of connection, the metal wires and the protective cover are connected together. This function is used for temperature measurement in acidic and noise-proof environments. 
  • Exposed-junction: This is suitable for areas where a quick response is required. This type of connection is used to measure gaseous temperature. The metal used to make the temperature sensor basically depends on the operating temperature range.

A thermocouple is usually constructed with two different metal conductors, namely iron and constantan, which identify the element by connecting it to a single junction called the hot junction. It consists of two junctions, one connected to a voltmeter or transmitter which is the cold junction, and the other junction is connected in a process called hot junction.

How Thermocouples Work? 

A thermocouple diagram is shown in the figure below. This circuit can be constructed from two different metals and they are connected together to form two junctions. The two metals surround the joint by welding. In the diagram above, the connections are labeled P and Q and the temperatures T1 and T2. When the temperature of the junction differs, an electromagnetic force is created in the circuit. When the mild temperature at the end of the junction becomes equivalent, as well as a reverse electromagnetic force is produced in the circuit and does not flow through it, the temperature at the end of the junction becomes unbalanced, causing a possible fluctuation in that circuit. The magnitude of the electromagnetic force induced in the circuit depends on the materials used in the manufacturing of the thermocouples. Meters count the total current in a circuit. The electromagnetic force induced in the circuit is calculated using the equation: E = a (∆Ө) b (∆Ө)2 (Where ∆Ө is the temperature difference between the hot thermocouple junction end and the cold thermocouple junction end. reference thermocouple, a and b are constants).

What Is A Probe Thermocouple & Its Applications?

Thermocouple probes are important in many different applications. They are typically used to measure temperatures in difficult or hard-to-reach places. For example, they may be used to measure the temperature of a molten metal, or the temperature inside a furnace. 

A thermocouple probe consists of two wires of different metals that are joined at one end. The probe consists of a metal tip that is placed in contact with the object to be measured. The tip is connected to the rest of the thermocouple by two wires. The other end of the wires is connected to a measuring device.

When the probe thermocouple is placed in a location where the temperature is different from the surrounding environment, a thermocouple voltage is generated. This thermocouple voltage can be measured and used to calculate the temperature. 

Thermocouple probes are often used in industrial applications, but they can also be used in scientific research or in medicine. For example, they may be used to measure the temperature of a patient’s skin or to monitor the temperature inside a laboratory. Thermocouple probes are also important in a variety of applications where accurate temperature measurement is critical. 

Thermocouple Thermometers!

They are very useful in medical devices such as thermometers and blood pressure monitors. They are also used in industrial settings to monitor process temperature, as well as in environmental applications such as weather stations.

Various Thermocouple Types Used In Different Industrial Settings!

Before discussing the types of thermocouples, it should be noted that the thermocouples must be protected in a protective case to isolate it from atmospheric temperatures. This coating significantly reduces the corrosion effect of the device.  There are several types of thermocouples. Let’s look at them in detail. 

  • Type K Thermocouple– Also called a nickel-chromium/nickel-alumel thermocouple. This is the most commonly used type. Its features include better reliability, accuracy and affordability, and it can operate in a wider temperature range.
  • Type J Thermocouple – This is an Iron/Constantan mix. It is also the most commonly used type of thermocouple. Its features include better reliability, accuracy and affordability. This device can only be used at lower temperatures and its service life is short when used at higher temperatures.
  • Type T Thermocouple – This is a copper/Constantan mix. The T-type thermocouple has better stability and is generally used in lower temperature applications such as ultra-low temperature freezer and cryogenic parts.
  • Type E Thermocouple – This is a nickel-chromium/constantan mix. It has better signal capability and better accuracy compared to type J and Type K thermocouples when used at ≤ 1000 0 F.
  • Type N Thermocouple – This is considered either as a Nicrosil thermocouple or a Nisil thermocouple. The temperature as well as the accuracy levels is almost similar to that of type K thermocouple, but this one is more costly than that one.
  • Type S Thermocouple – Considered as either platinum/rhodium or 10%/platinum thermocouple. The S-type thermocouple is widely used in high temperature applications such as biotechnology and pharmacy. Due to its higher accuracy and stability, it is used even in lower temperatures.
  • Type R Thermocouple – Considered as either platinum/rhodium or 13%/platinum thermocouple. Type S thermocouples are for high temperature applications. It contains more rhodium than Type S, making the device more expensive. The features and performance of the R and S are almost identical. Due to its higher accuracy and stability, it is used even in lower temperatures.
  • Type B Thermocouple – Considered as either a 30% platinum rhodium or a 60% platinum rhodium thermocouple. It is widely used at higher temperatures. Of all the above types, type B has the highest temperature limit. At higher temperature levels, the B-type thermocouple maintains better stability and accuracy.

Advantages of Thermocouples

Thermocouples have the following advantages. 

  • It has very high accuracy 
  • It is durable and can be used for example in severe and high vibration conditions. 
  • Thermal response is fast 
  • Operating temperature range is wide. 
  • Costs are low and they are very consistent 

Disadvantages of Thermocouples

Disadvantages of thermocouples are as follows. 

  • Low Voltage 
  • Nonlinearity 
  • Least Stability 
  • Least Sensitivity 
  • Harder Thermocouple Recalibration 

What Are Thermocouples Used for?

Some applications of thermocouples include: 

  • As temperature sensors in thermostats (offices, homes, businesses). 
  • Industry to control metal temperatures in iron, aluminum & other metals. 
  • Cryogenic & low temperature applications in food industries
  • Used as heat pumps for thermoelectric cooling. 
  • Temperature testing in chemical plants and petroleum plants. 
  • For detecting the ignition switch in gas engines.

How Are Thermocouples Different From RTDs?

There are some key differences between thermocouples and RTDs that you should be aware of as follows:  

  • Thermocouples are made of two different metals, while RTDs are made of a single metal.
  • Thermocouples generate a voltage when there is a difference in temperature between the two metal wires, while RTDs measure resistance to determine the temperature.
  • Thermocouples are less accurate than RTDs, but they are faster and more responsive.
  • Thermocouples can be used in a wider range of temperatures than RTDs.
  • Thermocouples are less expensive than RTDs.

Lifespan of Thermocouples

The life of the thermocouple depends on its use. Therefore, the service life of a thermocouple cannot be accurately predicted. If the device is properly maintained, it will have a long service life. After continuous use, they can be damaged due to the aging effect after which they need to be replaced in order to ensure continuity and safety of the process. 

How to Discover If You Have a Bad Thermocouple 

To know if the thermocouple is working perfectly, the device must be tested. Before replacing the device, it is necessary to check whether it really works or not. A multimeter and a basic knowledge of electronics is enough for this. There are mainly three approaches to testing a thermocouple with a multimeter and they are explained as follows: 

  • Resistance Test: To perform this test, the device must be placed in line with the gas device and the necessary equipment is a digital multimeter and alligator clips.
    • Procedure – Connect alligator clips to multimeter parts. Attach clips to both ends of the thermocouple where one end is folded into the gas valve. Now turn on the multimeter and note the reading options. If the multimeter reads ohms in the low order, the thermocouple is fully functional. Or if the reading is 0 ohms or more, it’s not in good shape. 
  • Open Circuit Test: Alligator clips, switch and digital multimeter are used here. Here voltage is calculated instead of resistance measurement. Now the lighter heats the other end of the thermocouple. If the multimeter shows a voltage in the range of 25-30 mV, it is working correctly. Or else, if the voltage is close to 20mV, the device must be replaced. 
  • Closed Loop Test: It uses alligator clips, a thermocouple adapter, and a digital multimeter. Here the adapter is placed inside the gas valve and then the thermocouple is placed on the other end of the adapter. Now turn on the multimeter. If the reading is between 12-15 mV, the device is good. Or if the voltage drops below 12mV, it means a broken device. 

The above test methods can therefore be used to determine whether a thermocouple is working properly or not.

Contact Heatcon Sensors for Best-Quality Thermocouples!

Heatcon Sensors is a leading manufacturer and global supplier of all types of high-quality temperature sensing devices and thermocouples. If you have an industrial process with specific temperature measurement requirements in even most difficult conditions, not only you can get the most suitable and reliable thermocouple for you, but their engineers can even design a custom-made thermocouple that will work best with your process with greater stability and accuracy at lost operational costs.