Nicrobraz Technical Articles Library

The Dynamics of Temperature Control Using TC's in High-Temp Furnace Brazing

This article was adapted from our Data Sheet 3.4,"Thermocouple Use in Furnace Brazing," which also contains a detailed listing of TC specifications and key handbooks on TC's. Call for a free copy.

Accurate temperature measurement during a furnace cycle is essential to the success of the furnace run. This is accomplished by the use of thermocouples.

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). See Figure 4 (1).

A thermocouple (TC) is technically called a "thermoelectric pyrometer," because when the temperature of the wires at MJ is different than at RJ, a voltage is produced in the closed loop circuit, proportional to that temperature difference. This "Seebeck Effect," named in honor of its discoverer in 1821, J.T. Seebeck, is fundamental to understanding TC operation. The voltage, or "emf" (electromotive force), in this circuit is measured by the recording instrument; however it is not reported as "millivolts," but rather as a "temperature," the numerical value of which has been computed to correspond to that particular voltage reading.

The polarity of the thermoelements (the two wires) in the TC are determined by the direction of the current flow at the cooler junction (RJin brazing operations). Thus, when current flows from wire A to wire B, then A is considered "positive" relative to B, as shown in Figure 4, and that particular thermoelement is then designated with a "P," as in "Type NP" wire, shown in Table 1. Wire B is thus considered to be the "negative" thermoelement, and so designated, e.g., "Type NN."

Table 1 describes several features of the different types of important TC's used in furnace brazing. Figure 5(2) shows the typical millivoltage emf's developed by each type of TC, and the typical temperature ranges in which they can effectively operate.

For most brazing furnaces operating from 1000-2300° F(538-1260° C), the Type K thermocouple is still the preferred load-TC. It is relatively inexpensive, reliable, and has a long life when properly used. The newer Type N thermocouple is a slightly modified, more stable version of the Type-K TC and should be considered when greater stability is needed.

Important note: TC wires do deteriorate with time, depending on the furnace conditions, which results in significant accuracy loss. Therefore, they should be changed or recalibrated on a regular basis to insure maximum reliability. Some companies and specifications require that new TC's be used for each furnace run. Many other plants change them daily or weekly, or only when they notice deterioration.

An easy way to check for deterioration (loss in accuracy of readings), and to determine the proper timing for replacing TC's in accordance with a specific furnace, atmosphere, and cycle combination, is to:

a. Run four (4) TC's through five (5) furnace cycles.
b. On the sixth furnace cycle replace one of the TC's with a new one.
c. Replace an additional TC on each fifth cycle thereafter.
d. After twenty (20) cycles, compare the accuracies of each TC against a master TC standard.

Since you will have data for TC's that have now been through 20, 15, 10, and 5 cycles, develop a replacement schedule for TC's based on this data.

For commercial and aerospace brazing and heat treating, the Aerospace Materials Specification AMS 2750 on Pyrometry may be required. This is a comprehensive document including the details of TC usage. Those involved with the aerospace industry are strongly urged to become familiar with this document.

(1) Adapted from Fabian, ed., Vacuum Technology: Practical Heat Treating and Brazing (OH: ASM International, 1993), p. 141, fig. 8.8.

(2) Marlin Manufacturing Corporation, "Temperature Instrumentation for Research and Industry", Catalog No. 109 (OH: Marlin,1989), p. H- 1.

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