Thermocouples and compensating cables
Thermocouples and compensating cablesDate of publication: 29-01-2025 Update date: 10-04-2026 🕒 4 min read
Thermocouples are commonly used temperature sensors that can measure very high temperatures, reaching even several hundred degrees Celsius. They operate based on the Seebeck effect, which is a thermoelectric phenomenon. They consist of two wires made from different metals or alloys, joined at one end, which serves as the temperature measurement point (the so-called measuring junction). The other end, called the reference junction, is maintained at a known and constant temperature. The temperature difference between these junctions generates an electric voltage, known as electromotive force (EMF), which is proportional to the temperature difference and ranges from a few to several tens of microvolts per degree Celsius. Galvanometers and calibration potentiometers are usually used to measure EMF.
- Types of thermocouples and division into groups
- Applications of thermocouples
- Checking the thermocouple
- How to choose a thermocouple
- Thermoelectric and compensating wires
Division of thermocouples into groups
According to the PN-EN 60584-1:2014-04 standard, different types of thermocouples can be distinguished, divided into three groups.
Group I (non-noble metals)
- Type K thermocouple (NiCr-Ni) - the most commonly used type of thermocouple. The temperature measurement range extends from -200°C to 1200°C.
- Type J thermocouple (Fe-CuNi) - characterized by the smallest temperature range and shorter lifespan due to iron corrosion. Measurement range from -40°C to 750°C.
- Type T thermocouple (Cu-CuNi) - guarantees stable measurement in environments with very low temperatures. Measurement range from -200°C to 350°C.
- Type E thermocouple (NiCr-CuNi) - distinguished by a stronger signal and greater accuracy than type K or J in moderate temperature ranges (below 1000°C). Measurement range from -200°C to 900°C.
- Type N thermocouple (NiCrSi-NiSi) - similar to type K in terms of accuracy and measurement range, but with greater resistance to oxidation.
Group II (platinum-rhodium)
- Type S thermocouple (PtRh10-Pt) - used in environments with very high temperatures (up to 1600°C).
- Type R thermocouple (PtRh13-Pt) - identical to type S but contains a higher percentage of rhodium, making it more accurate than type S but also more expensive. Used as a reference sensor.
- Type B thermocouple (PtRh30-PtRh6) - used in extremely high temperatures (up to 1800°C), maintaining high accuracy and measurement stability.
Group III (tungsten-rhenium)
- Type C thermocouple (W5%Re–W26%Re) - used in extreme temperatures reaching 2315°C. Used in chemically inert environments and vacuum to avoid failures caused by oxidation.
- Type D thermocouple (W3%Re–W25%Re) - with the same properties as type C. Differences in rhenium proportions only affect thermoelectric properties.
Applications of thermocouples
Thermocouples are used in various industries and sciences, even in the most demanding environments. They are used as measurement and monitoring devices in various processes and equipment, as well as safety sensors. Examples of thermocouple applications;
- food industry - thermocouples play a key role because precision and speed of measurement are exceptionally important for the quality and durability of food. The use of thermocouples is not limited to temperature monitoring but also to controlling processes such as burner ignition or gas flow control;
- metallurgical industry - thermocouples help monitor melting and casting processes to ensure optimal conditions in extreme environments;
- chemical and petrochemical industry - thermocouples are used to monitor temperature inside reactors, ensuring safety during chemical reactions;
- automotive industry - thermocouples are used to monitor the temperature of engine components, such as cylinder heads, pistons, or exhaust systems, during performance and durability tests. They are also used to control exhaust emissions and optimize the operation of catalysts.
How to check if a thermocouple is working correctly
To diagnose the correct operation of thermocouples, methods such as:
- resistance measurement - using an ohmmeter, verify the value of the resistance of the thermocouple wires. Deviations from the manufacturer's specification values may indicate malfunction;
- dynamic tests - involve subjecting thermocouples to rapid temperature changes and observing the reaction speed and accuracy of readings. Delays or incorrect readings may indicate problems with the thermocouple's operation.
What to consider when choosing a temperature sensor
Choosing the right temperature sensor is crucial for ensuring precise, reliable, and efficient measurements in various industrial or scientific applications. The main aspects to consider when selecting a temperature sensor are:
- measured temperature - thermoelectric sensors provide shorter measurement times and higher mechanical resistance than resistance sensors. Additionally, thermocouples can measure temperatures up to 1800°C. The range of measured temperatures also determines the materials used for the thermocouple's sheath;
- measured medium - the medium in which the measurement will be made also affects the choice of sensor with the appropriate sheath. For example, in the case of a sensor for measuring the temperature of liquid aluminum, the sheath should be made of a material resistant to wetting and corrosion;
- installation location - the location where the sensor will be used is equally important when selecting the diameter and length of the sheath. The smaller the sensor's diameter, the faster the reaction time. The sheath length should be at least 6-15 times the diameter, depending on the medium in which the measurement is made. In places where it is not possible to use such a long sheath (e.g., in small diameter pipelines), it is recommended to use the sensor at an angle to the flowing medium to ensure the largest possible sensor sheath surface area in contact with the medium.
Thermoelectric and compensating wires
Thermoelectric wires are made from the same materials as the wires used in thermocouples, while compensating wires are made from alternative materials whose thermoelectric properties are similar to those used in thermocouples for a specific temperature range. These wires are used to connect sensors to measuring devices over long distances. They are mainly used in industrial conditions where the temperature around the sensor fluctuates, making it impossible to place the reference junction in a stable environment. Moving this junction to an environment where the temperature is stabilized is done by using these wires.
In summary, thermocouples are versatile temperature sensors capable of measuring very high temperatures thanks to the Seebeck effect. There are different types of thermocouples, each with its specific application, temperature range, and features. When choosing a thermocouple or temperature sensor, consider the temperature range, medium, installation location, and material specifications. Thermoelectric and compensating wires play a key role in transmitting measurement signals, ensuring precise and reliable results. Proper selection and diagnostics of temperature sensors are crucial for ensuring optimal performance of measurement systems in various industrial and scientific applications.
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