Thermocouples – robust sensors for a wide range of temperatures

A thermocouple is a temperature sensor that consists of two metal wires, both of which are connected to each other at one end (the measuring tip, or hot junction) to form a closed electrical circuit. When the temperature at the measuring tip of the thermocouple changes, thermoelectric voltage is generated due to the differing thermoelectrical properties of both metals. This phenomenon is otherwise known as the Seebeck effect. The exact voltage build-up depends on the materials used and the temperature. If you know the properties of the materials, you can calculate the prevailing temperature by measuring the thermoelectric voltage – even at extreme temperatures.

Different metals need to be joined in order to use the Seebeck effect to measure temperatures. The corresponding combination for standard industrial applications is standardized in DIN EN 60584-1. Thermocouples may consist of ignoble metals and alloys, including nickel / nickel-chromium (type K), iron / copper-nickel (type J) or noble metals such as platinum / platinum with 13% rhodium (type R).

Ignoble thermocouples provide a uniquely strong signal, while noble metal thermocouples tend to be significantly more accurate and temperature-resistant.

In order to ensure the thermocouple’s measuring signal is correct, the temperature first needs to be determined as a reference junction at the junction between the thermocouple and the measuring device (cold junction). All measuring devices for thermocouples account for this cold junction compensation (CJC) with their factory settings.

A thermocouple relies on the Seebeck effect.

The freely moving electrons in a metallic conductor with a difference in temperature between the two ends are shifted from the hot end towards the cold end due to the excess energy. The resulting shift in charge leads to electrical potential between the two ends. This material-specific effect occurs in all metals and semiconductors.
To use the Seebeck effect, different metals need to be joined together. The junction between the two metals is brought to the measuring temperature with suitable thermal coupling. Due to the difference in temperature between the measurement point, namely the hot junction, and the connection to the measurement device, the cold junction, the charge shifts for both metals. The voltage build-up typically varies strongly due to the different thermoelectric properties of the metals. The difference in potential between the two thermo materials can be measured as a thermoelectric voltage.

In order to standardize the thermoelectric voltage, the material pairings and the reference junction, i.e., the cold end where the signal is measured, have been standardized at 0°C. Reference junctions are typically at ambient temperature for the majority of measuring devices used for industrial applications. As a result, the thermocouple produces less thermoelectric voltage than required to calculate the exact temperature according to the standard. To account for this, the measuring devices measure the terminal temperatures and computationally compensate for the missing thermoelectric voltage with 0°C in a process otherwise known as cold junction compensation (CJC).

Thermocouples are electrical contact thermometers that can, in principle, be used in all measuring systems for industrial and research-related purposes.
They are primarily used in applications involving high temperatures, strong vibrations or high oscillations, or where quick response times are required. Greater deviation is expected than with resistance thermometers, for example.
Thermocouples measure using points, making them ideal for sensitive temperature profile measurements.
Thermocouples can either be designed as immersion sensors or surface sensors, or as a hand-held measuring device with two isolated test probes, for example. As a result, they are suitable for a wide range of applications in the following industries:

• Industrial furnace construction
• Plant/machine construction
• Automotive
• Chemicals
• Power plan engineering
• Glass
• Food
• etc.

Thermoelectric voltage refers to the difference in electrical potential between the positive and negative conductor of a thermocouple, which builds up due to a difference in temperature between the measuring tip and the connecting contacts. The resulting voltage is generated by the thermoelectric effect, otherwise known as the Seebeck effect. The standard voltage tables for individual types of thermocouples are standardized in DIN EN 60584-1 with 0°C as the reference junction. If the reference junction is at another temperature, the missing or excess thermoelectric voltage needs to be arithmetically compensated for.

Generally speaking, a thermocouple can be laid with its base material from the measuring point to the control room. However, the sensor may not be flexible enough or simply too expensive, depending on the diameter, design and type of thermocouple. Just imagine the costs and work required to lay a type R platinum thermocouple over 100 meters across the factory floor, or bending an 8.0mm thermocouple to follow the cable routing.
For this reason, thermocouple or compensation cables are used to bridge the distance between the measuring point or thermocouple and the measuring card. Beyond purely transmitting the signal, the cable is also required for signal generation to ensure the temperature difference between the cold end of the thermocouple and the intrinsic temperature of the measuring card can be compensated. The thermocouple and compensation cables therefore need to possess the same thermoelectric properties as the thermo material in the thermocouple.
While the thermocouple cable has the same thermo material as the inner conductor, only materials that possess identical thermoelectric properties as the thermo material are used for the compensation cable.
Using the same thermo material in the thermocouple cable improves the accuracy of the measuring chain.
That being said, the use of compensation cables remains justified due to their cheaper purchase price. For instance, noble metal thermocouples in particular only use compensation cables, as producing a thermocouple cable is simply not economical.
The color coding for thermoelectric and compensation cables is standardized in DIN EN 60584-3.

Type K thermocouples are one of the 10 types of thermocouples standardized in DIN EN 60584-1. The type describes the thermomaterial combination. In a type K thermocouple, a nickel chromium alloy is used as the positive conductor and nickel as the negative conductor.
In Europe, this type is marked in green in accordance with DIN EN 60584-3. It is one of the most commonly used types for industrial applications.

The following types of thermocouples have been standardized: