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High Temperature Thermocouples
 

HIGH TEMPERATURE THERMOCOUPLES

Once your application requires measuring temperatures above 2246º F (1250º C), you will need to move from metal sheathed thermocouples to platinum-platinum/rhodium, ceramic sheathed thermocouples. (Note, there are other technologies, such as infrared measurement but there are some significant reliability and accuracy issues with these technologies in most applications. We are focusing on contact measurement instruments only.)
For extreme applications, above the range for platinum thermocouples, a series of tungsten/rhenium thermocouples are available although they are restricted to us in vacuum or reducing environments.
 
The three types of platinum-platinum/rhodium thermocouples are letter designated “S, R, and B.” Both the S and R types consist of a pure platinum leg and a second leg using varying percentages of platinum and rhodium. The type S is considered the “standard” and the type R is considered to be slightly more robust than the type S.
 
The B type consists of two legs with alloys of platinum and rhodium. A type B thermocouple will allow the measurement of higher temperatures than the R or S, 3308ºF (1820º C). However, a type B sensor will not provide accurate measurements for temperatures under 425º C. If you have an application where you need the performance of a B type thermocouple but need accurate readings under 425º C, please refer to the SEVA temperature systems.
At high temperatures contaminants become more active and will attack the platinum if it is not properly protected. The standard protection tube for platinum sensors is alumina. As with other components, there are a variety of quality levels. AccuTru uses a very high density alumina and then treats the material using a proprietary process to improve the life of the tube and reduce the risk of thermoelement contamination.
 
Platinum thermocouples require special care. Platinum “like” oxygen so in a reducing environment, it is important to provide access to oxygen in the sheath to prevent early thermocouple failure.
 
Contrary to platinum based thermocouples, tungsten-rhenium (Types W and C) will fail in a matter of hours if exposed to oxygen at high temperatures. It is imperative that the sheath materials in these sensors are impervious to oxygen. Various acidic gasses, sulfur and many other process environments are known to penetrate the protection tube. This causes drift or premature failure due to embrittlement.
AccuTru has developed methods from simple purging of the element to new technologies that greatly improve the life and durability of platinum based sensors.

 

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