Mineral-insulated sheath thermocouples for the high-temperature range are used in industry and research to measure very high temperatures, sometimes under particularly difficult conditions. The selection of the respective thermocouple is significantly influenced by the environmental influences, temperature requirements and mechanical stress. The thermocouple (inner wires) and the sheath material of mineral-insulated high-temperature thermocouples are made of high-quality precious metals. With the exception of a few designs, these are basically flexible and can measure temperatures of up to 2300°C, depending on their composition. They offer the usual advantages of high measuring accuracy, short response times and a long service life. An indispensable product with a wide range of applications in industry and research.
There are a number of different types available for selection in the high temperature range for thermocouples according to DIN EN 60584. Among other things, the temperature range and the limiting deviation are of decisive importance for the selection of the type.
| Color code | Type | Inner core / thermocouple + / - | Application temperature range |
 | Type S | Platinum-Rhodium10% / Platinum | 0 to 1600°C |
| Type R | Platinum Rhodium13% / Platinum | 0 to 1600°C |
 | Type B | Platinum-Rhodium30% / Platinum-Rhodium 6% | 800 to 1700°C |
 | TYPE C | Tungsten-5%Rhenium / Tungsten-26%Rhenium | 0 to 2315°C |
 | TYPE A | Tungsten-5%Rhenium / Tungsten-20%Rhenium | 0 to 2300°C |
The selection of the sheath material for sheathed thermocouples is of central importance; it protects the measuring point and the thermocouple from external influences. In the high-temperature range, extreme temperatures prevail and there is a high level of interaction between sheath material, temperature, atmosphere and application area.
Especially for high temperature thermocouples type A and type C, in inert, reducing or oxidizing atmospheres, specific sheath materials are required for the respective application. The correct design increases the resistance and thus also the service life of the thermocouple.
When selecting the right sheath material for thermocouples for the high temperature range, it should be noted that some sheath materials are flexible and bendable in their design, while others can only be used in a rigid design. We will be happy to help you develop a solution for your application.
| Jacket material | Application temperature range | Minimum bending radius | Areas of application |
| Inconel600 (I) | 0 to 750°C | 5x diameter | Inert and oxidizing atmosphere, vacuum |
| Platinum (PR) | 0 to + 1550°C | 5x diameter | Inert and oxidizing atmosphere |
| Tantalum (TA) | 0 to + 2200°C | 12x diameter | Inert atmosphere, vacuum |
| Molybdenum (MO) | 0 to + 2000°C | Rigid version | Reducing atmosphere, vacuum, inert atmosphere |
| Rhenium (RE) | 0 to + 2200°C | Rigid version | Inert and oxidizing atmosphere |
| Tungsten (W)* | 0 to + 2300°C | Vacuum, very low outgassing | |
| Molybdenum, 50% Rhenium * | 0 to + 2300°C | Inert, hydrogen-containing, nitrogen-containing and ammonia-containing atmosphere, vacuum | |
| *On request only |
| Jacket material | Outer diameter in mm | |||||||
| 1,0 | 1,5 | 1,6 | 2,0 | 3,2 | ||||
| Platinum (PR) | x | x | x | x | x | |||
| Tantalum (TA) | x | x | x | |||||
| Molybdenum (MO) | x | x | ||||||
| Rhenium (RE) | x | x | ||||||
| Molybdenum 50% Rhenium (MR) | Upon request | |||||||
High temperature thermocouples also differ from conventional thermocouples in the choice of insulator, i.e. insulation material. Usually, for thermocouples up to 1300°C, magnesium oxide is used as insulation between the thermocouple and the sheath of the thermocouple. However, magnesium oxide as well as aluminum oxide show decreasing insulation resistance and ductility as the temperature increases. For high temperature thermocouples type A and type C, hafnium oxide and beryllium oxide are often used as insulation material up to temperatures of 2500°C.
Hafnium oxide as insulation material in high temperature thermocouples tends to be rather coarse-grained. Its ductility is lower than that of beryllium oxide, which is why the material is often used for high-temperature thermocouples in rigid designs. Beryllium oxide as an insulator in high temperature thermocouples, on the other hand, is very suitable for flexible designs.
| Type | Material designation | Application temperature range | Features |
| MgO | Magnesium oxide | 0 to 1600°C | Very hygroscopic, common insulation material in sheath thermocouples. Only limited resistance behavior at high temperatures |
| AL2O3 | Alumina | 0 to 1550°C | Very good application behavior using platinum |
| HFO2 | Hafnium oxide | 0 to 2200°C | Good properties at high temperature, very good alternative to berryllium oxide, non-toxic, but with poor bending properties |
| BeO* | Berryllium oxide | 0 to 2200°C | Very good conductivity in the high-temperature range, very good resistance behavior |
| *Caution should be exercised when using berrylium oxide. Beryllium oxide is considered a toxic material. Incorrect or improper handling can lead to health problems |
Uninsulated fine wires consist of a positive leg and a negative leg of a thermocouple. They are available in type S, type B, type R, type C and type A thermocouples. The wire thickness of the thermocouples is available in 0.25mm, 0.35mm, 0.51mm and 0.81mm. The respective legs are wound separately on a roll and can be provided in individual lengths. The fine wires correspond to the thermocouples of DIN EN 60584.
| Type | Thermocouple + / - | Wire diameter in mm | |||
| 0,25 | 0,35 | 0,51 | 0,81 | ||
| Type S | Platinum-Rhodium10% / Platinum | x | x | x | x |
| Type R | Platinum Rhodium13% / Platinum | x | x | x | x |
| Type B | Platinum-Rhodium30% / Platinum-Rhodium 6% | x | x | x | x |
| TYPE C | Tungsten-5%Rhenium / Tungsten-26%Rhenium | x | x | x | x |
| TYPE A | Tungsten-5%Rhenium / Tungsten-20%Rhenium | x | x | x | x |
