Why is the manufacturing process for type C thermocouples wires so difficult?

Type C thermocouples have the unique advantage that they allow for extremely high temperature measurement with excellent accuracy (up to 2300°C). They are suitable for applications in vacuum, high purity hydrogen, or pure inert atmospheres. However, it is essential to note that this thermocouple possesses inherent brittleness, and thus can be very challenging to work with.

What is an ideal high temperature sensor setup with Type D and Type C wires inside?

When designing a Type C and Type D thermocouple set up, the first step is to consider both what the (working) temperature will be and whether the atmosphere is oxidizing, reducing, inert or a vacuum?

The second step is to choose the appropriate sheath material and back fill gas according to the influencing factors in the first step. The sheath material needs to be able to protect the thermocouple wires from the effects of the temperature and atmosphere. Some considerations with regard to the sheath include:

• In a vacuum it is better for the sheath to be metallic than ceramic
• Some sheath materials are porous, does this affect the particular setup?
• Is the sheath compatible with the thermocouple wires?
• Sheath vaporization can occur at certain temperatures and can result in deposits on the thermocouple wires causing contamination and failures.
• If your setup is at risk of vaporization due to very high temperatures then you can consider a tungsten tube for the thermocouples. However, the tungsten tube will itself need a sheath of ceramic materials to protect it from the application environment.

In addition, the manufacturing process for type C thermocouple wires is considered challenging due to the following factors:

Junctioning of the type C thermocouple wires

One significant difficulty lies in the junctioning process, where extreme care is required to prevent wire damage. Incorrect handling can lead to wire burning or oxidation, which could impact the thermocouple’s performance and reliability. Furthermore, there is the issue of the ductility and recrystallization temperature increase towards the positive type C electrode due to the addition of rhenium. Rhenium is necessary to reduce the issue of brittleness.

Cutting the type C thermocouple wires to required lengths

Cutting type C thermocouple wire to the necessary length presents another challenge. Standard cutting methods are inadequate as they may cause the wire to split, necessitating the use of special cutters to ensure precise and accurate cuts.

Cleanliness

There are strict cleanliness requirements during assembly of type C thermocouples and this adds to the complexity. To ensure the highest quality, all our ARi ultra high temperature type C thermocouples are assembled in a cleanroom environment.

Due to the complexities involved and the potential risks in producing type C thermocouples, some manufacturers have type C production. To produce a high quality, reliable and accurate type C product also requires significant investment in the necessary equipment and trained personnel.

At Kamet we work closely in a long-standing relationship with the top-class manufacturer ARi. ARi has been an innovative market leader in thermocouple production since the very early days of development in the 1950s. Thanks to our partnership, we are able to also offer various Type C customization options, for example in sheath materials.

These are a few examples of what needs to be considered in designing an ideal high temperature Type C or Type D thermocouple set up.

For more technical information and specifications on our exotic thermocouples with type C wire you can access our brochure.

Kamet is well equipped with knowledge and extensive experience with which to assist your engineers in choosing which thermocouples and sheaths have the specs that will match your specific situation. Please feel free to contact us for advice.