Thermocouple probe

A thermocouple probe is a sensor for measuring temperature. Thermocouples are versatile temperature sensors and are commonly used for a wide range of applications: from industrial thermocouples to standard thermocouples for domestic appliances and equipment.

The thermocouple probe

The thermocouple probe is an indispensable temperature measurement instrument in many industrial and scientific applications. Thanks to its robustness, accuracy and wide measuring range, it is widely used in fields as varied as metallurgy, power generation, the chemical industry and scientific research.

The device works on the principle of the Thomas Johann Seebeck effect, named after the physicist of the same name, where a voltage is generated between two dissimilar metals when they are exposed to a temperature difference. This voltage is then converted into a temperature measurement.

Despite their apparent simplicity, thermocouples require a thorough understanding of how they work, the types available, as well as mounting and wiring techniques to ensure reliable and accurate measurements.

Let's explore in detail the different types of thermocouples, the principle of operation, installation and wiring methods, and best practices for optimizing their use in various applications.

Thermocouple sensor operating principle

thermocouple temperature sensor operating principle

The thermocouple probe works by combining two dissimilar metals, connected at an end called the "hot junction". When a temperature difference occurs at this junction, an electric current is created in the circuit, generating an electromotive force (EMF). The value of this EMF depends on the properties of the metals used and the temperature difference between the hot junction and a reference junction, often referred to as the "cold junction".

The metals used in thermocouples are of homogeneous quality, enabling the device to withstand different temperature gradients along its entire length without altering the FEM. This means that the thermocouple can measure temperatures accurately, even under variable conditions.

Measuring devices such as digital displays, PLCs and temperature controllers are designed to accept thermocouples directly. They automatically manage cold junction compensation, essential for accurate readings.

The alloys used to manufacture thermocouples are generally available in wire form, making them easy to manufacture and handle.

To choose the most suitable thermocouple for your application, it is essential to :

  • Understanding the basic structure of thermocouples: Knowledge of materials and their configuration is essential.
  • Know how they work: how the temperature difference between the junctions generates an electrical voltage.
  • Controlling temperature ranges: Make sure the thermocouple can operate within the temperature range required for your application.
  • Know the different types of thermocouple probes: Types K, J, T, E, N, S, R and B, each with specific temperature ranges and applications.
  • Understand thermocouple probe assembly and wiring: follow good installation and connection practices to ensure accurate, reliable measurements.
  • Take into account the insulation resistance of a thermocouple probe: An adequate insulation resistance between the wires and the protector is essential to reduce interference and ensure accurate measurements, especially in environments subject to electrical and electromagnetic interference.

Thermocouple probe types

Each type of thermocouple probe has specific output (voltage) characteristics, and requires appropriate calibration and measuring equipment to ensure accurate and reliable readings.

Advantages and disadvantages of thermocouples

Thermocouples offer a number of advantages that make them indispensable in many industrial and scientific applications.

The main advantages of thermocouples include :

However, thermocouples also have a number of disadvantages:

By weighing up these advantages and disadvantages, it is possible to determine whether thermocouples are suitable for a specific application. For environments requiring fast, reliable temperature measurement at low cost, thermocouples are often the ideal solution.

Thermocouple probe mounting and wiring

Mounting and wiring a thermocouple probe are crucial steps in ensuring accurate and reliable temperature measurements. Here are the key steps and considerations:

By following these recommendations, thermocouple probe mounting and wiring can be carried out efficiently, guaranteeing optimum instrumentation performance.

Isolating a thermocouple probe


Thermocouples are often exposed to harsh environments, subject to electrical, electromagnetic and other types of interference. To compensate for temperature measurement disturbances caused by such interference, metal-sheathed thermocouples are fitted with an insulation resistor between the wires and the protector.

This insulation resistance, rigorously tested at the factory, ranges from 1000 to 5000 MW minimum at 100 to 500 Vdc, depending on thermocouple diameter and at ambient temperature. These datasheet values are valid for thermocouple lengths of less than 1 meter, in accordance with current standards.

It is important to note that these resistance values decrease when the thermocouple is subjected to high temperatures, until they become almost zero at very high temperatures. Effective insulation of thermocouples is therefore crucial to ensure accurate and reliable measurements, particularly in extreme temperature conditions and disturbed environments.

This is why it is always preferable to use measuring instruments with galvanic isolation input / output or input / output / power supply.

FAQs on thermocouple probes

What is a thermocouple?

A thermocouple is a temperature sensor consisting of two wires of different metals welded together at one end, called the hot junction. When this junction is exposed to a temperature, it generates an electrical voltage proportional to that temperature, due to the Seebeck effect.

What are the different types of thermocouples?

The main types of thermocouples are :

    • Type K sensor (Nickel-Chromium/Nickel-Aluminium): Temperature range from -200°C to 1260 degrees Celsius.

    • Type J sensor (Iron/Constantan): Temperature range -210°C to 760 degrees Celsius.

    • Type T sensor (Copper/Constantan): Temperature range from -200°C to 370 degrees Celsius.

    • Type E sensor (Nickel-Chromium/Constantan): Temperature range from -200°C to 900 degrees Celsius.

    • Type N sensor (Nicrosil/Nisil): Temperature range from -200°C to 1300 degrees Celsius.

    • Type S sensor (Platinum-Rhodium 10%): Temperature range from 0°C to 1600 degrees Celsius

    • Type R sensor (Platinum-Rhodium 13%): Temperature range 0°C to 1600 degrees Celsius

    • Type B sensor (Platinum-Rhodium 30% / Platinum-Rhodium 6%): Temperature range 0°C to 1700 degrees Celsius

How does a thermocouple work?

A thermocouple works by measuring the electrical potential difference between the hot junction and a reference junction, often maintained at a known temperature. The voltage produced is converted into a temperature measurement using calibration tables.

What is cold junction and cold junction compensation?

Cold junction refers to the reference junction of a thermocouple, where the wires are soldered to metal terminals. Cold junction compensation is a technique used to compensate for temperature variations at this junction to ensure accurate measurements.

What are the advantages and disadvantages of the different types of thermocouples?

    • Type K: Wide measuring range, low cost, good durability.

    • Type J: Inexpensive, suitable for low temperatures.

    • Type T: Excellent precision at low temperatures.

    • Type E: High sensitivity.

    • Type N: High stability, oxidation resistance.

    • Type S, R, B: Accurate and stable at high temperatures, but more expensive.

How do I choose the right type of thermocouple for my application?

The choice of a thermocouple depends on several factors:

    • Temperature range: Make sure the thermocouple can measure in the range required for your application.

    • Durability and resistance to environmental conditions: Some thermocouples are more resistant to corrosion, vibration or chemical environments.

    • Response time: Some types of thermocouple have a faster response time.

    • Cost: Cost may vary according to materials and thermocouple characteristics.

What materials are used in thermocouples?

Thermocouples are manufactured from various combinations of metals and alloys, such as :

    • Nickel-Chromium and Nickel-Aluminium (Type K)

    • Iron and Constantan (Type J)

    • Copper and Constantan (Type T)

    • Nicrosil and Nisil (Type N)

    • Platinum-Rhodium and Platinum (Types S, R, B)

What is the Seebeck effect?

The Seebeck effect is the phenomenon whereby a temperature difference between two different materials generates an electrical voltage. This effect is the basis of thermocouples.

How to calibrate a thermocouple

Calibrating a thermocouple involves comparing its measurements with those of a known standard at different temperatures, and adjusting the readings accordingly to guarantee accuracy.

What are the most common applications for thermocouples?

Thermocouples are used in many sectors and applications, including :

    • Industry: Control of manufacturing processes, furnaces, engines, boilers, reactors, turbines, incinerators, transformers, pumps, compressors, radiators.

    • Laboratories: Scientific experiments, cryogenics, biology, chemistry, physics, materials.

    • Electronics: Thermal monitoring of components such as processors, circuits, batteries, sensors, thermal management.

    • Medical: Temperature monitoring in thermotherapy, incubators, cryogenics, surveillance, sterilization.
    • Plastics: temperature control for injection, extrusion, molding, blow molding, calendering.

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