Thermocouple Types, Insulation Materials and Sheath Materials
Temperature Sensors > High Temperature Thermocouple Assemblies -
Operating Temperatures up to 4200°F (2315°C)
Our high temperature thermocouples are specifically designed for durability and reliability in high temperature applications to 4200°F (2315°C). Such applications involve oxidizing, reducing, inert gas or contamination atmospheres that challenge the life of the thermocouple.
Thermocouple Types
Choose the thermocouple type from the temperature range table below. Minimum requirements for all Conax Technologies thermocouples are standard limits of error and are denoted by a single letter (eg. S). Special limits of error are also available and are designated by a double calibration letter (eg. SS).
|
Catalog
Type
|
Calibration
|
Upper Service Temperature
|
Remarks
|
| S |
Platinum-10% Rhodium
vs. Platinum |
2700° F (1480° C) |
For use in oxidizing, inert and vacuum atmospheres |
| R |
Platinum- 13% Rhodium
vs. Platinum |
2700° F (1480° C) |
For use in oxidizing, inert and vacuum atmospheres |
| B |
Platinum- 30% Rhodium
vs. Platinum- 6%Rhodium |
3100° F (1700° C) |
For use in oxidizing, inert and vacuum atmospheres |
| C* |
Tungsten- 5% Rhenium
vs. Tungsten-26%Rhenium |
4200° F (2320° C)
3450° F (1900° C)
|
For use in hydrogen or inert atmospheres
For use in vacuum atmospheres |
*Formerly Conax Technologies Type W5, Type C not an ANSI official designation.
Note: • Upper Temperature Limits per ASTM E230 for 24 AWG wire for Types S, R and B.
• For Type C, special limits of error wire is not available (ref. ASTM E988).
• Platinum calibration not recommended for direct immersion in vacuum atmosphere, use with protection tube.
Insulation Materials
|
Material
|
Upper Service Temperature
|
Melting Point
|
Remarks
|
|
Alumina
(Al2O3) |
3450° F
(1900° C) |
3686° F
(2030° C) |
High-purity aluminum oxide is the standard insulation material supplied for Conax Technologies
high temperature thermocouple assemblies. Alumina offers high thermal conductivity
and high electrical resistivity. |
|
Hafnia
(HfO2) |
4352° F
(2400° C) |
5054° F
(2790° C) |
Hafnium oxide is usually present in zirconium ores in amounts ranging from .5 to 2 percent.Refined high-purity hafnia offers a higher electrical resistivity at elevated temperatures than yttria or zirconia. Hafnia’s resistivity is comparable to that of beryllium oxide, but hafnia does not present the potential health problems associated with beryllium oxide. Hafnia is therefore replacing beryllium oxide in many applications. Hafnia is the commonly recommended insulation for Conax Type C thermocouples.
|
Other Insulation Materials
|
Material
|
Chemical
Symbol
|
Upper Service
Temperature
|
Remarks
|
| Beryllium Oxide |
BeO |
4200° F (2315° C) |
High electrical resistivity, toxic dust, special
handling required |
| Magnesium Oxide |
MgO |
2500° F (1370° C) |
Used primarily with compacted sheathed
thermocouples |
| Thorium Oxide |
ThO2 |
4532° F (2500° C) |
Low electrical resistivity, good resistance to
reducing environments |
| Yttrium Oxide |
Y2O3 |
3632° F (2000° C) |
Low electrical Low electrical resistivity,
stabilizer material for zirconium |
Sheath Materials
|
Material
|
Upper Service
Temperature
|
Melting
Point
|
Remarks
|
|
Molybdenum
(Mo) |
3400° F
(1871° C) |
4730° F
(2610° C) |
Molybdenum offers excellent mechanical properties at elevated
temperatures and is exceptional for use in reducing or neutral
atmospheres. Molybdenum offers superb thermal shock
resistance and has potentially unlimited life at high temperatures
in a vacuum. Pure hydrogen, argon and helium atmospheres are
completely inert to molybdenum. Performance is poor in oxidizing
atmospheres. Above 800° F (427° C), molybdenum will react with
oxygen. |
|
Tungsten
(W) |
4200° F
(2315° C) |
6100° F
(3380° C) |
Tungsten has the best strength at high temperatures of all the
common refractory metals. Tungsten provides good abrasion
resistance. It can be used in hydrogen at all temperatures,
and in dry argon and helium. Tungsten is not recommended for
use in oxygen bearing environments, as oxygen is known to
cause physical degradation in only a few hours
depending on temperature and oxygen content. |
|
Tantalum
(Ta) |
4500° F
(2482° C) |
5425° F
(2996° C) |
Tantalum is best used in reducing or neutral atmospheres and in
a vacuum environment. Its performance is poor in oxidizing
atmospheres, and it must be protected from gases such as
oxygen and nitrogen at temperatures above 570° F (300° C).
Tantalum is outranked by tungsten for high temperature strength.
Tantalum is excellent for thermal cycling applications and is the
most corrosion resistant of the refractory materials. It is almost
completely immune to attack by acids except hydrofluoric acid. |
| Alumina (Al2O3) |
3450° F
(1900° C) |
3686° F
(2030° C) |
High-purity aluminum oxide offers high thermal conductivity, high
electrical resistivity and good resistance to thermal shock.
It also offers excellent resistance to chemical attack. Aluminum
oxide functions well in oxidizing, reducing or high vacuum
applications, but is not recommended for high vacuum
environments in the presence of graphite at temperatures
exceeding 2372° F (1300° C). |
*Molybdenum, Tungsten and Tantalum sheath assemblies are backfilled with argon and sealed to prevent oxidation.
