Fig. 1. Placement of TC on surface of part
First of all, remember that good furnace control means you need to know the temperature of the parts inside a brazing furnace, and this can only be done in batch-type furnaces via well-placed thermocouples (TC’s).
I recommend that brazing shops use a minimum of at least three (3) Type-K (or perhaps Type-N) load-TC’s in each furnace brazing run, and even more TC’s than that, if possible, if their furnace will allow. The more TC’s used, the better will be the overall control of the cycle variables.
The tip of each load-TC should be firmly touching the part it is monitoring, as shown here in Fig. 1.
Please note that a TC measures the temperature at the point where the two TC-wires first touch each other. Thus, as seen in Fig. 1, the twisted TC lead does not actually measure the temperature at the surface of the part it is touching, but actually measures the temperature almost ½” above that surface at the point where the TC-wires first touch each other. Therefore, do not twist wires to make a TC. Instead, weld or braze the ends of the TC-wires instead.
It is also very important that only calibrated thermocouple wire be used when making TC’s. Each TC-wire has a correction factor that needs to be applied to the results measured in your furnace. If you do not use calibrated wire, your temperature readings could have significant error.
When there is a significant difference in cross sectional mass within a part (thin sections and very heavy sections within the same part), then TC’s should be attached to both the heavy and thin section (of at least one of the parts in each load). Then not only is the temperature of the full load being monitored, but also the temperature differential (delta T) between the thin and thick section of a single part can be closely measured and controlled in order to prevent distortion of that component.