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Thermocouples are comprised of two electrical conductors, typically dissimilar base or noble metals. When subjected to a thermal gradient, each conductor generates a voltage that varies depending on the metal used. The difference between the voltage generated by one conductor (the hot junction) and the other (the cold or reference junction) increases with temperature and can range from 1 to 70 microvolts/°C. This differential is electronically calibrated to indicate the actual temperature being measured, within standard or special limits of error.
The most commonly used thermocouple is the Type K, which employs conductors made from chromel and alumel. Type K thermocouoples cover a wide thermal range (-194 to 1371°C) and are reasonably accurate at low- to mid-range temperatures. The vast majority of installed thermocouples measure temperatures of less than 1000°C. However, a fair number of heating applications exceed this range, including certain ceramics, carbon black, reducing vacuum furnaces, hot isostatic pressing, sintering operations, etc.
Table 1 lists the types and characteristics of commercially available thermocouples, while Table 2 shows operating data for materials used in sheaths and insulators. Four thermocouple types are designed specifically for elevated thermal processes
* The Seebeck effect refers to the “production of an electromotive force (emf) and consequently an electric current in a loop of material consisting of at least two dissimilar conductors when two junctions are maintained at different temperatures.”1
Platinum-rhodium alloys are used for each conductor. Commonly used in the glass industry, Type B thermocouples produce stable readings up to 1815°C. They can be sensitive to contamination and should never be inserted in metal protection tubes.
Types R and S
These types are often specified as alternatives to Type K due to their greater stability at high temperature. Conductors are pure platinum and a platinum-rhodium alloy. The maximum reading for each is 1760°C. As with Type B, these thermocouples function best in a contamination-free environment and should not be used with metal protection tubes.
Also known as Type W5, the C calibration has a peak operating range of 2315°C and is currently the most popular choice for vacuum furnaces and other ultra-high-temperature applications. The conductors are tungsten/5% rhenium and tungsten/26% rhenium. Type C thermocouples should not be used in oxidizing atmospheres and are not practical at temperatures below 400°C.
Continuing ResearchCompared to standard base metal thermocouples, high-temperature calibrations use more costly noble metal conductors and have limited durability. Embrittlement of the conductor wires is more likely at elevated temperatures, and it is commonly recommended that thermocouples be replaced after every heating cycle of 1700°C or more. Ongoing development with a variety of material combinations aims to address the durability issue. For example, one project is currently researching a new molybdenum alloy sheath with a significantly higher recrystallization temperature.
New and different measurement techniques are also being explored. A new proprietary all-graphite mechanical measuring system has been developed that is capable of repeated sensing of temperatures as high as 2600°C. Because of embedded pressure-compensating software, this system is suitable only for hot isostatic pressing (HIP) environments and is not commercially available. It is, however, an indicator that technical solutions are on the horizon to serve the demanding needs of the steadily growing high-temperature measurement market.
For more information regarding thermocouples, contact Avure Technologies, Inc., 3721 Corporate Dr., Columbus, OH 43231; (614) 891-2732; fax (614) 891-4568; e-mail Kristi.firstname.lastname@example.org; or visit the website at www.avure.com.