Measuring temperature has played a vital role in science, industry, and everyday life for centuries. What began as basic observations of heat has evolved into a field dominated by precise instruments like the thermocouple. But do you know where it all began? Discover the history of thermocouples.
What is a thermocouple?
A thermocouple is a temperature sensor made up of two different metals joined at one end, creating what is called a measuring junction. When this junction is subjected to a temperature difference relative to the free ends, an electrical voltage arises that can be measured and correlated with the process temperature.
Contrary to what many assume, this voltage does not form exclusively at the hot junction. It results from a difference between the thermoelectric forces generated along the wires, a direct consequence of the physical properties of the materials involved. The theory that describes this phenomenon is... Seebeck effect.
Thermocouples stand out in the industry due to essential characteristics:
- They cater to a wide temperature range, from cryogenic applications to over 2.300°C;
- They provide a quick response;
- They have a simple yet robust construction;
- They withstand harsh atmospheres with high pressure, vibration, and corrosion.
They are used in refineries, cement plants, steel mills, engines, turbines, chemical, food and aerospace industries.
In practice, a thermocouple transforms a temperature difference into an electrical signal, which can be read by measuring or control instruments. Because it operates over wide temperature ranges and is inexpensive, the thermocouple is one of the most widely used sensors in industry. It is present in furnaces, boilers, motors, refrigeration systems, and various applications that require continuous thermal monitoring.
Who invented the thermocouple??
The origin of the thermocouple is directly linked to Thomas Johann Seebeck, a German physicist who discovered the thermoelectric phenomenon in 1821. Seebeck observed that when different metals were joined and one of the junctions was heated, an electric current arose that was even capable of deflecting the needle of a compass. This current was the effect of the generation of electromotive force due to the thermal difference between the conductors.
The discovery was essential for the emergence of the first reliable method of converting heat into a measurable electrical signal. Since then, several scientists have contributed to transforming the phenomenon into an applicable physical instrument.
During one of his experiments, Seebeck observed that when strips of different metals were joined and one of the junctions was heated, an electric current formed in the closed circuit. Although he interpreted the phenomenon as something related to magnetism and the deflection of a compass needle used in his observations, the discovery of the thermoelectric phenomenon that would become the principle of the thermocouple was recorded. He also organized different metals into a thermoelectric series according to their behavior in the generation of electromotive force.
This discovery led to the development of the first reliable method for converting thermal energy into an electrical signal. The detailed theory of how electrons behave in materials came much later, with advances in quantum theory.
The use of the phenomenon described by Seebeck to measure high temperatures emerged in 1826, when A.C. Becquerel suggested the use of thermocouples to estimate temperatures higher than those possible with conventional thermometry. His experiments included metals such as platinum and palladium. He observed that the current generated varied with temperature, and this variation could be used as a measurement.
| Year / Period | Marco | Importance |
|---|---|---|
| 1821 | Seebeck discovers the thermoelectric effect. | Basis of how thermocouples work |
| Decade of 1820 | Becquerel and Nobili propose its use for measurement. | Start of practical application |
| 1880–1900 | Improvements in alloys, welding, and calibration. | First reliable and reproducible sensors |
| XX century | Industrial standardization and global expansion | Thermocouples are becoming standard in industrial processes. |
| Today | Digital integration, greater precision, miniaturization | Smart sensors for advanced process control |
History of thermocouples
The sensations of cold and heat have always existed, but finding ways to measure temperature has always been a major challenge. As far as we know, the history of temperature sensors began in the Renaissance.
Most methods were indirect, observing the effect that heat has on something and thus being able to deduce the temperature.
In 1664, Robert Hooke proposed that the freezing point of water be used as the zero point. Soon after, Ole Roemer saw the need for two fixed points for measurement: Hooke's freezing point and the boiling point of water.
Prior to that, in 1592, there are reports that Galileo Galilei built a device that showed changes in temperature. But the thermometer as we know it today was invented by Santorio Santorii in 1612.
We can also mention Thomas Seebeck who, upon discovering the thermoelectric effect (which was later named the "Seebeck effect"), introduced the world to the most widely used temperature sensor, the thermocouple, and also Michael Faraday, who made a decisive contribution to the study of electricity and materials, which later enabled the development of sensors such as thermistors. These sensors saw increased use in the food, automotive, medical, and HVAC industries during the 1980s and 1990s.
We have other important names in the history of temperature measurement such as: Anders Celsius, Lord Kelvin, Daniel Gabriel Fahrenheit, C.H. Meyers (who invented the resistance temperature detector, which measures the electrical resistance of a piece of platinum wire and is generally considered the most accurate type of temperature sensor).
| Year / Period | Marco | Importance |
|---|---|---|
| 1821 | Thomas Johann Seebeck discovers the Seebeck effect, by joining two metals with different temperatures, generating an electrical voltage. | The physical basis of thermocouples as temperature sensors. |
| Decade of 1820 | Nobili and other scientists begin to use the effect to temperature measurement. | First step towards transforming the phenomenon into a practical tool. |
| 1880–1900 | Improvements in alloys, welds, and calibration → first reliable thermocouples. | Thermocouples are beginning to be used to measure high temperatures. |
| XX century | Standardization of types of thermocouples and expansion in industry (steelmaking, furnaces, energy). | Technology becomes widely used and secure. |
| Today | Electronics, cold junction compensation, precision and miniaturization. | Thermocouples are standard sensors in various industrial applications. |
Operating principle of the thermocouple
The basis of how a thermocouple works is the Seebeck effect. When two different metals are joined and exposed to different temperatures, the system generates an electrical voltage. This variation is proportional to the temperature difference between the junctions and allows for simple and direct measurement.
A thermocouple has two main points:
Hot junctionThis is where the measurement takes place. The tips of two different metals are joined together and placed at the location where the temperature is to be monitored.
Cold junctionIt serves as a reference. In modern equipment, this compensation is done electronically to ensure the accuracy of the reading.
Due to the temperature difference between the two junctions, the material produces a voltage. The value depends on the type of metal and the temperature variation. The reading can be taken using instruments such as voltmeters or industrial control systems.
Therefore, the thermocouple provides a constant reading and can work in conjunction with control systems: when it detects that the temperature is outside the desired value, the equipment can activate or deactivate the heating or cooling. This direct operation, combined with its low cost, resistance to high temperatures, and durability in industrial environments, makes the thermocouple one of the most widely used sensors in processes that require continuous thermal monitoring.
Types of thermocouples
Os types of thermocouples The most common types are classified by letters, such as K, J, T, E, N, R, S, and B. Each uses a distinct pair of metals and exhibits its own electrical properties. They follow tables produced by institutions such as the IEC and NIST, which guide the relationship between temperature and voltage in controlled environments. The physical construction, the type of insulation, and the protection method vary according to the intended use, but the thermoelectric curves must follow the references published in the standards.
Base metal thermocouples operate between temperatures ranging from approximately -200°C to 1200°C. Noble metal thermocouples, on the other hand, cover higher ranges and are used in specific industries such as metallurgy, foundries, and aerospace.
Type J, composed of iron and constantan, offers greater electrical stability under certain conditions and is suitable for applications up to approximately 750°C. However, it is vulnerable to the action of sulfur at high temperatures, which restricts its use in certain atmospheres.
Type E, with alloys such as nickel-chromium and constantan, exhibits high electromotive force and allows for good temperature resolution, being used in lower temperature ranges.
In addition to these, various other materials are used in sensors employed in aviation, chemical processes, glass manufacturing, and thermal control of reactors, always in accordance with internationally recognized reference tables.
Thermocouples are essential in scenarios that demand both durability and rapid response, such as:
- industrial furnaces and metallurgical processes
- monitoring of motors, boilers and turbines
- oil refining and petrochemicals
- temperature control in plastic extrusion
- automotive and aerospace industries
- laboratories and scientific research
- food production and sterilization
Type K thermocouple is the most popular.
O K-type thermocouple It was developed in the 1960s. Its composition involves nickel-chromium and nickel-aluminum alloys. Over time, it has undergone refinements that have made it widely used in industrial environments. Its thermoelectric behavior allows it to reach high temperatures in varied atmospheres, provided that corrosive conditions are controlled.
The type K sensor exhibits hysteresis when subjected to thermal cycling in intermediate temperature ranges, which can generate deviations in situations requiring long-term stability. Even so, it has become standard in industries and laboratories due to its affordability, ease of replacement, and wide availability of compatible instruments.
The development of alternatives such as the type N thermocouple arose from the need for greater stability in harsh conditions and in environments with radiation. Even so, type K remains the most prevalent in global industrial applications.
Thermocouples in Brazil
Temperature measurement has driven industrial evolution in Brazil. For decades, many processes depended on imported equipment that was expensive and difficult to replace. The introduction of the first domestically produced thermocouples changed this scenario and accelerated the modernization of sectors such as steelmaking, oilenergy and chemicals. It was within this context of transformation that Alutal emerged.
Founded in 1994 in the interior of São Paulo, Alutal was born with a straightforward purpose: to supply thermocouples and measurement systems that keep pace with Brazilian industry. With local production and international technological collaboration, it has ensured precision, competitive costs, and close technical support for its clients – a key differentiator for those who cannot afford operational downtime.
Over time, it has become a benchmark in sensors capable of operating in extreme conditions, in environments where thermal control defines safety and performance. Continuous investment in engineering and certifications has solidified the brand as a strategic partner for companies that need to measure temperature reliably.
By expanding its portfolio and strengthening its production chain, Alutal has helped industries of different sizes reduce downtime, avoid waste, and make decisions based on accurate data. The company has come to represent a turning point in the history of thermal measurement in the country: it has shown that robust technology can also be Brazilian.
Today, Alutal continues to contribute to Brazil developing globally competitive solutions in essential sectors of the economy.
If you want to understand which type of thermocouple best suits your process, the Alutal technical team is ready to guide you in making the right choice and ensuring optimal performance in the field.
It is a robust, simple, and low-cost temperature sensor used in industry. It measures the temperature of environments, liquids, or gases, especially where temperatures are too high (or too low) for conventional thermometers.
The main difference lies in the physics that each one uses to "sense" the temperature:
While a thermocouple measures temperature through the electrical voltage (millivolts) generated at the junction of two different metals (Seebeck effect), the PT100 measures temperature through the variation in electrical resistance of a platinum (Pt) wire. It is a passive sensor; it requires a small excitation current to measure resistance.
