Fig. 1 Typical examples of sheathed thermocouples (TC’s). Note the TC that is welded to a metal square as an aid in attaching that TC to some component. (Photo courtesy of Precision Measurements, Inc., Atlanta, GA).
As we have seen in the last two articles, the fixturing used in furnace-brazing is a very important item that MUST be carefully considered if you want to minimize the time used in each brazing cycle (excess fixturing weight adds a lot of time to your brazing cycle) and maximize the productivity and profitability of each furnace-brazing run.
Another very important “fixturing” item that has to be understood and properly used are thermocouples since they are the only way you can monitor the temperature inside each furnace load. The ability to properly monitor the temperature at different locations within the furnace chamber will not only help to ensure proper brazing filler metal (BFM) flow but can also prevent part distortion. To accomplish this, multiple thermocouples (or “TC’s” as they are often called) need to be placed in strategic positions within each furnace load. Some typical TC’s are shown in Figure 1.
This will be the first of two articles on the use of TC’s in brazing, and you will hopefully quickly see that they are an important part of your furnace’s fixturing, and when properly understood and used, will help to give you better, more reliable brazes. Some of the information in this article has been previously printed on this website a number of years ago and is being brought forth again, since many readers may not have seen my previous articles on this topic.
To begin with, I’d like to briefly look at what TC’s are, and the types commonly available for use in brazing furnaces today (much more exhaustive discussions about TC’s can be found on the websites of a number of thermocouple manufacturers and suppliers). In next month’s article, I’ll describe the actual use and placement of these TC’s in different brazing furnace cycles.
A thermocouple (not “thermal-couple”) is a temperature measuring device that consists of two metal wires, called thermoelements, each of a different composition, and joined together at one end to form a measuring junction (MJ), “couple”, or “hot junction”. These two leads form a closed-loop electrical circuit when the free ends are attached to a voltage-measuring instrument. The ends attached to the instrument are called the “cold junction”, or reference junction (RJ). Figure 2 illustrates these items.