| Abstract | This paper describes the fabrication of a complete thermocouple assembly. Elements of construction and installation are discussed under the headings thermocouple current, fabrication, switch box fabrication, field installation of the cable and temperature readings. The principle of measurement is that the emf generated between a reference junction of known temperature and one of unknown temperature can give the unknown temperature. The two metals used are copper and constantan, and the wires must be protected electrically from short circuits. Twenty gauge duplex wire individually encased in polyvinyl chloride is satisfactory. The wires from the installation are usually terminated in a weatherproof switch box. Leeds and Northrup rotary switches, type 31-3 are used. Simple clamp type connectors and terminal blocks ensure positive connection. The copper wires are attached to rotary switches. The constantan wires are attached to a common constantan wire. The thermocouple cable's outer jacket is cut at the depths at which readings are required and the copper and constantan wires are soldered to give a thermocouple junction. The completed multi-wire cable is inserted in a suitable length of polyethylene hose. A lucite or brass plug seals the lower end. Steel switch boxes house the switches and wire connection from the elements and still provide accessibility for readings. The cable and switch box are filled with high quality transformer oil. Installation involves pushing the assembly into a bore hole or in the case of short cables, a test pit. Longer assemblies are usually lowered into bore holes with a 10-20 lb weight tied to the bottom. Finally the bore hole is backfilled so that no air pockets exist. |
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