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Temperature Q&A with JR Madden: Thermocouples

Temperature Q&A with JR Madden: Thermocouples

Q: A Type K thermocouple is being used in a high temperature furnace environment for temperature measurementHowever the Type K thermocouple is drifting too much. Is there something I could do so I don't have to change it out frequently or is there another type of thermocouple I could use?


A: You could use a Type N with Inconel 600 Material, which is designed to perform well at high temperatures (over 2000F) and helps prevent corrosion in environments where atmospheres are reducing or alternately oxidizing and reducing. This would be particularly helpful for a furnace process that includes heating and cooling with moisture present, creating potential for oxidation to form from the process. Other high temperature environments that benefit from Type N with Inconel 600 Material include boilers and cookers processes or extreme environments where the thermocouple cannot be replaced easily.


In terms of their basic function, Type K and Type N thermocouples are similar. Type K is one of the most common type of general purpose thermocouple. However, the environmental setting that your thermocouple will be used in can affect your choice. Type N thermocouples provide a higher level of stability and durability in adverse environments than Type K thermocouples. 


To be more specific, Type K thermocouples are susceptible to a phenomenon called Green Rot, or oxidation around the thermocouple. Green Rot is oxidation inside the thermocouple sheathing caused by operation at certain low oxygen environments, creating large negative calibration drifts leading to curve changes over time. Potential IR failure can happen, where the internal elements short and touch the outer sheathing due to oxidation (Green Rot)


Type N is also a viable choice when adverse environmental factors and high temperature requirements are an issue and there is difficulty in frequently changing out the thermocouple. While a Type N with Inconel 600 Material is only 20% higher in cost than Type K, the Type N can perform longer without the drift that oxidation causes, preventing the need to shut down the process to change out the oxidized thermocouple.


If you decide that Type N thermocouple isn't for you and are using a Type K thermocouple where your environment is the key factor instead of temperature, you can reduce the chances of Green Rot by increasing the oxygen supply through the use of larger diameter protecting tubes. The opposite approach can also work by inserting a mechanism to absorb oxygen inside a tube which can bring oxygen levels below what will promote oxidation.

Moore Industries can help when you are deciding if Type N is viable replacement for your Type K thermocouple application. You can contact us with your questions or if you need to know more about a product, price or availability.

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*Image of Type K Thermocouple and The Worm, Moore Industries' Flexible RTD and Thermocouple Sensors for Thermowell Temperature Assemblies

Published: May 22, 2017
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