The furnace-TC is often accompanied by a second, sheathed “over-temperature” thermocouple (also called “high-limit TC”). The function of this TC is to cause an automatic furnace alarm or shutdown should the furnace exceed the maximum temperature set on the over-temp controller. As with all sheathed TC’s, be sure the sheath material is compatible with the atmosphere and temperature requirements for the furnace load.
Finally, the TC’s attached directly to parts that are being brazed are called “load-thermocouples” or just “load-TC’s”. For most brazing-furnaces operating from 1000-2300F (540-1260C), the Type-K (Chromel-Alumel) thermocouple is still the most popular load-TC. It is relatively inexpensive, is quite reliable, and can have a long life when properly used. The newer Type-N (Nicrosil-Nisil) thermocouple is a more stable TC for use in similar applications as Type-K, and should be considered when greater life and stability is needed. As will be discussed in greater detail next month, I recommend that brazing shops use at least a minimum of three (3) Type-K (or Type-N) load-TC’s in each batch-furnace (vacuum or atmosphere) brazing run. Continuous-belt furnaces have a number of different options for placing/using TC’s in their brazing atmospheres, which will also be discussed in next month’s article.
Important Note: TC wires do deteriorate with time, depending on the furnace conditions, which can result in significant accuracy loss. Therefore they should be changed or recalibrated on a regular basis to insure maximum reliability. Some companies and specs require that new TC’s be used for each furnace brazing run. Many other plants change them daily or weekly, or only when they notice deterioration.
Finally, for commercial and aerospace brazing today, the latest “D” revision of the Aerospace Materials Specification AMS 2750 (“Pyrometry”) is being required more and more. This is a comprehensive document, including the details of TC usage, which brazing companies need to use in order to be in compliance with today’s more rigid commercial and aerospace requirements for brazing furnaces.
Next Month: We will look at correct placement of TC’s in furnace brazing loads, and how, together with correct furnace heating/cooling rates, they can help to maximize uniformity of temperature throughout each brazing load and minimize any distortion of components that are being brazed together.