Overview: This article explains thermocouples, their working principle, types, applications, and factors for accurate temperature measurement, highlighting the Seebeck effect and sensor selection.

Thermocouples are extensively utilized in engineering and various industrial applications. They are recognized for their simple construction, robustness, durability, and cost-effectiveness. They can measure a wide range of temperatures and provide rapid response times, making them highly reliable for temperature measurement.

J-HD-150X (IEC) Thermocouple heavy duty{width="3.65625in" height="1.840340113735783in"}

Fig. 1 J-HD-150X (IEC) Thermocouple Heavy Duty. Source: oemsecrets

What Is a Thermocouple?

A thermocouple, as shown in Fig. 2, is made by joining two wire leads composed of different metals. These wires are connected at one end to form a junction, a temperature-sensing element commonly called the hot junction. The other ends of the wires are usually attached to a measuring device, known as the cold junction. Heat exposure at the sensing junction produces a thermocouple voltage that can be used to determine the temperature.

Diagrammatic illustration of the working of the thermocouple

Fig. 2 Diagrammatic Illustration of the Working of the Thermocouple. Source: Rakesh Kumar, Ph.D.

Working Principle

The operation of a thermocouple is fundamentally based on the Seebeck effect.

The Seebeck Effect

When one end of a metal bar is heated, a voltage, called the Seebeck voltage, appears along the bar. The magnitude of this voltage changes with temperature and depends on the type of metal. If there is a temperature difference between the two ends of a conductor, the Seebeck effect causes a thermal electromotive force to be generated, which is directly proportional to the temperature difference.

Thermocouple Construction and Voltage Generation

A thermocouple is made by connecting two wires composed of different metals. When these dissimilar metals, which have distinct Seebeck voltages, are joined at the sensing junction and exposed to heat, they generate a thermocouple voltage. This voltage is a function of the temperature difference between the hot and cold junctions.

The Seebeck effect offers a straightforward method for determining temperature electrically. The voltage produced is usually interpreted using thermocouple reference tables or through polynomial equations specified by standards such as ITS-90, which establish the relationship between temperature and voltage output.

Types of Thermocouple

They are classified based on various criteria

Based on the Combination of Metals

Different thermocouple types are created using various combinations of dissimilar metals and are classified with letter codes like

  • J Iron-Constantan](https://www.oemsecrets.com/electronic-components/sensors-and-transducers/sensors/temperature-sensors/thermocouples?page=16) (Fe-CuNi)

  • K Chromel-Alumel (NiCr-NiAl)

  • T Copper-Constantan](https://www.oemsecrets.com/details/mwtcrec1915n) (Cu-CuNi)

  • E Chromel-Constantan](https://www.oemsecrets.com/details/5tctte3036) (NiCr-CuNi)

  • S Platinum-Rhodium/Platinum](https://www.oemsecrets.com/details/mwtcrec5915n)

  • R 13% Rhodium/ > Platinum

Each classification has established thermoelectric characteristics and specific operating temperature ranges.

Based on Metal Properties

The two types include

  • Base metal thermocouple utilizes common metals.

  • Noble metal thermocouples are typically employed for > high-temperature measurements.

Based on the Sensing Junction

The construction of thermocouple sensing junctions varies among the three main types:

  • Ungrounded. Ungrounded thermocouples provide enhanced protection and electrical isolation but have slower response characteristics.

  • Grounded. Both grounded and exposed junctions typically respond more rapidly because of direct metal contact, though they may experience increased susceptibility to electrical interference from ground loops.

  • Exposed. Exposed junction designs deliver the fastest response times > due to direct heat transfer capability, making them ideal for > applications such as monitoring battery surface temperatures.

Applications

Thermocouples are used as temperature sensors in various applications.

Industrial and process control: Commonly used in industries such as metallurgy, chemicals, power generation, turbines, boilers, reactors, and paper manufacturing for high-temperature measurement, remote monitoring, and automated control.

Aerospace and automotive: Testing engines, exhausts, and spacecraft components. Integrated into micro unmanned aerial vehicles to monitor chemical cell temperatures, preventing overheating and improving safety.

Nuclear power plants: Used for fluid temperature measurement in reactor coolant systems, especially where harsh conditions or placement inside reactor vessels make other sensors unsuitable.

High temperature measurement: Preferred for measuring fluid and gas temperatures in high-temperature environments due to their simplicity and reliability.

Battery temperature monitoring: Essential for safe operation and accurate modelling of lithium-ion batteries, particularly in electric vehicles.

Factors Influencing the Working of Thermocouple

To achieve accurate temperature measurements using this principle, it is important to consider several factors. These include

  • Non-linear relationship between voltage and temperature

  • Necessity of cold-junction compensation

  • Typically small output voltage

  • Effects of physical construction

  • External conditions on the thermocouple's performance

TIPD109 is a reference design from Texas Instruments that provides a simple and accurate solution for thermocouple temperature measurement. This design demonstrates how to achieve less than 1°C accuracy using a circuit, making it suitable for precise temperature sensing applications.

The TIPD109 design incorporates essential components such as anti-aliasing filters and biasing resistors to ensure reliable sensor diagnostics and signal integrity. It typically utilizes the ADS1118, a 16-bit analog-to-digital converter with an integrated temperature sensor, which facilitates both thermocouple signal measurement and cold-junction compensation in a compact and power-efficient manner.

Overall, TIPD109 serves as a reference for engineers looking to implement accurate thermocouple measurement systems with minimal complexity.

Summarizing the Key Points

  • Thermocouples generate voltage based on the Seebeck effect, enabling precise temperature measurement across wide ranges with simple, durable construction suitable for industrial environments.

  • Selection of the right thermocouple type depends on factors such as temperature range, response time, and environmental conditions to ensure accurate and reliable measurements.

  • Grounded, ungrounded, and exposed junction thermocouples offer different response times and electrical isolation, making them suitable for specific applications like high-temperature or fast-response needs.

  • Texas Instruments' TIPD109 reference design exemplifies a simple, > cost-effective solution that achieves less than 1°C accuracy by > incorporating filters and ADC technology for precision > measurements.

Reference

Han, Y., Feng, Y., Lou, H., & Zhang, X. (2018). Thermocouple-based temperature sensing system for chemical cell inside micro UAV device. Journal of Physics Conference Series, 986, 012012. https://doi.org/10.1088/1742-6596/986/1/012012

Hwang, I., Cheon, S., & Park, W. (2020). Consideration factors in application of thermocouple sensors for RCS temperature instrumentation. EPJ Web of Conferences, 225, 03005. https://doi.org/10.1051/epjconf/202022503005

Koshkouei, M. J., Saniee, N. F., & Barai, A. (2024). Thermocouple selection and its influence on temperature monitoring of lithium-ion cells. Journal of Energy Storage, 92, 112072. https://doi.org/10.1016/j.est.2024.112072

A Basic Guide to Thermocouple Measurements- Texas Instrumentshttp://www.ti.com/lit/pdf/sbaa274

Oemsecrets. (n.d.-c). TIPD109 - Compare distributor prices & inventory | OemSecrets.com.https://www.oemsecrets.com/compare/TIPD109

TIPD109 reference design | TI.com. (n.d.). https://www.ti.com/tool/TIPD109

The Engineering Mindset. (2020, September 13). How Thermocouples Work - basic working principle + RTD [Video]. YouTube. https://www.youtube.com/watch?v=v7NUi88Lxi8