Fig. 1. Illustration of the elements of a single TC circuit (Adapted from Fabian, ed., Vacuum Technology: Practical Heat Treating and Brazing (OH: ASM International, 1993), p. 141, fig. 8

In this first of two articles on TC’s and their use in brazing, 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. 1 illustrates these items.

A thermocouple (TC) can technically, therefore, be 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 “milli-volts”, but rather as a “temperature”, the numerical value of which has been computed to correspond to that particular voltage reading.

Table-1 Lists a Number of the Commonly Used TC’s Available Today

The TC that is inserted into the furnace chamber to monitor the temperature of the atmosphere and furnace hot zone is often called the “furnace-thermocouple”. This TC is often a Type-R or S thermocouple (Platinum vs. platinum-rhodium), sheathed in protective alumina or molybdenum to provide long life and maximum protection from high-temperature oxidation up to approx. 2700F (1480C). The tip (measuring junction) of this furnace TC should extend a minimum of at least 2″ (50mm) beyond the heating elements into the hot zone. This “furnace-TC” is usually then connected to the furnace microprocessor (reference junction) to control the rate of heat input into the furnace during each cycle.

Thermocouple Types Commonly Available Today

Table 1: Thermocouple Types Commonly Available Today

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.

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