range of thermocouple choices gets smaller as the temperature rises.
Table 1. Types and characteristics of commercially available thermocouples.
In the nearly 200 years since
the Seebeck effect* was quantified, thermocouples
have steadily multiplied in type and application. Today, there are literally
thousands of combinations of thermocouple calibrations, sheath and insulator
materials, mounting styles, and protection tubes. Thermocouples are not
super-accurate, but they are comparatively cheap and suitable for applications
ranging from kitchen ovens to electric arc furnaces.
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,
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
Table 2. Operating data for materials used in sheaths and insulators.
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.
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.
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.
Compared 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
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.
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.