Nickel-chromium and nickelalumel are the metal pairs most commonly used in what is called the “Type K” thermocouple.
The Type K is inexpensive and can be used across a temperature range from -200 to 1250°C (-328 to 2282°F).
Measuring temperatures inside a furnace can present several challenges: high temperatures, temperature cycling, and hostile atmospheres exceeding the limits of many measurement devices while others have greatly reduced lifetimes and poor accuracy.
This article approaches mainly two specific challenges related to temperature measurement in furnaces: types of oxidizing and reducing atmospheres in furnaces used in microelectronics fabrication.
The need to heat is common to many manufacturing processes.
Rubbers and adhesives are cured, metals are annealed to modify their metallurgy and properties, coatings are dried, metals are melted, and ceramics are fired or vitrified.
Many of these processes are carried out in ovens, heated either by electricity or gas.
An oven that can heat to above 1000°C (1832°F) is termed a furnace.
A kiln is a particular type of furnace used in ceramics.
At high temperatures many materials start to react with the surrounding atmosphere.
If that atmosphere is very short of oxygen, it may pull oxygen from the material being heated. Gas heating usually results in an oxygen-deficient atmosphere.
If the atmosphere is oxygen-rich, the material being heated will capture a proportion, forming an oxide layer.
Such an atmosphere is termed “oxidizing.” This is the process employed in diffusion furnaces used in microelectronics fabrication to produce Si O2.