Practical thermocouples
The reasons for the popularity of thermocouples are not just the existence of a range of types designed to cover almost all temperature, environmental and accuracy requirements, or the fact that they are small. Others include the ease with which they can be made and applied, and the availability of a vast assortment of housings and special packages to match almost every imaginable application.

For simple applications, thermocouples can easily be made from lengths of bare thermocouple wire or insulated cable, the insulation material being selected for compatibility with the application, and likewise the cable itself. As for wire diameter, anything from 0.1mm OD to 3mm or more (especially for industrial use) is the norm. The measuring junction is best constructed by welding the two wires together.

Soldering or twisting are less satisfactory, although with the aid of a clamping screw in a connecting block, greater security can be obtained. The key to success is a good electrical connection which does not disrupt the composition of the thermocouple wires themselves. Bear in mind the expected operating conditions for the measuring junction.

Base metal thermocouples are usually welded electrically in an argon atmosphere, while platinum thermocouples can be welded using a small oxy-hydrogen flame. Beyond this, base metal thermocouple wires are normally supplied ready annealed and are thus prepared for use directly after welding. The same is not usually the case with the platinum equivalents which therefore have to be annealed after inserting the wires into the insulators and making up the junction.

At the other end of the cables, each thermocouple wire can be joined to a copper wire to form the reference junction. Again, welding is the best bet, but silver soldering using a very small quantity of solder in paste form together with a miniature flame is a reasonable alternative - as long as all traces of corrosive flux are removed. The junctions can then be fitted into closed end tubes or potted for immersion in an ice-water mixture.

This method of thermocouple construction is simple, versatile and fine for experimentation in the laboratory. Accuracy will be good since, on the one hand, each element can be placed close to the requisite site, and on the other, the device approaches the theoretical ideal as outlined in Part 1, Section 2.

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