Describe chromel alumel thermocouple extension wire?

Answer 1

Sometimes prior to June, 1953, Professors E.A. Farber and J. R. Akerman, of the University of Wisconsin, observed in a particular instance that two thermocouples made of hte same material and exposed to the same temperature simultaneously, gave different electromotive force (EMF). A natural question to arise from this observation was "Does the method of making the thermocouple junction have an influence on the EMF produced?". This experimental work was undertaken in an endeavor to find an answer. Preliminary experimental work indicated that there were reproducible differences between thermocouples manufactured by various methods. For more extensive experimental work, various heat treatments were investigated in addition to various methods of junction manufacture. Chromel-alumel and iron-nickel were the two metal combination used. Chromel-alumel was chosen because a combination of pure metals might be affected differently than a combination of alloys. The extremes of temperature under which the thermocouples were tested were -300¡ãF and 1000¡ãF. These temperatures were somewhat arbitrary, but the object was to subject the measuring junctions to temperatures considerably above and below the reference junction temperature of 32¡ãF. Tests were conducted with thermocouples whose junctions were formed by (1) twisting, (2) pinch welding, (3) silver soldering, (4) aceylene welding. Thermocouples made by these four methods were tested as manufactured, that is, no heat treatment of any kind, after subjection to 650¡ã F for twelve hours followed by slow cooling in the furnace to 300¡ã F or below. Briefly, the differences between EMF developed by thermocouples made by different methods, at a given temperature, are small and not reproducible. Thus was unexpected in view of the preliminary experimental work. At any temperature between 32¡ã F and 1000¡ã F the greatest EMF difference between chromel-alumel thermocouples made with various junction types corresponds to about 3.5¡ãF; between 32¡ã F and -300¡ã F the greatest difference corresponds to 5¡ã F. The corresponding figures for iron nickel are 6¡ã F and 10¡ã F. The commercially pure metals iron and nickel do not produce thermocouples less affected by (or, sensitive to) junction type than do the alloys chromel and alumel, if sensitivity is based on a comparison of differences in EMF generated. If comparison of temperature differences corresponding to EMF differences is the basis, iron-nickel thermocouples are more sensitive than those of chromel-alumel. Heat treatment had some effect on EMF-temperature relationship, but not very much. In instance where heat treatment had a definite effect, it was to increase difference between the maximum and minimum EMF by about 20% at 1000¡ã F for chromel-alumel thermocouple. At -300¡ã F apparently metallurgical changes occurred which affected the EMF-temperature relationship, but at a slow rate. Two to four hours were necessary for some of the chromel-alumel thermocouple to give an EMF that was unchanged with time, at a given temperature. At a temperature of approximately -300¡ãF, the temperature indicated by several of the chromel-alumel thermocouples change approximately 3¡ãF in one and a half hours, while other chromel-alumel thermocouples indicated no change at all or indicated a change in the opposite direction. In general, the difference in EMF between chromel-alumel thermocouples of various junction types and best treatments are considerably less than tolerance allowed by most manufactures of thermocouple wire. Since there was not much difference between the EMF generated by thermocouples with different type junctions, an endeavor was made to find out what would produce a decided difference in EMF. A number of chromel-alumel thermocouples were made with both twist and silver solder junctions, the junctions of each type being made as nearly alike as possible. A number of thermocouples were made with both twist and silver solder junctions, the junction of each type being of different quality. The difference in quality of twist thermocouples was produced by varying the tightness of twist, by not cleaning the wire before twisting, and by peening the junction after twisting. The difference in quality of the silver solder thermocouples was produced by not cleaning the wire before soldering, by peening the junction after fabrication, and by heating the wire to a temperature much higher than necessary during soldering. The twist thermocouples with junction of non-uniform quality produced EMF with no greater differences than EMF produced by twisting thermocouples of non-uniform quality produced EMF with no greater differences than EMF produced by silver solder thermocouples with uniform quality junction. It was thought that a smaller wire size might be more sensitive to quality of junctions than the wire size used in all the above work. Silver solder thermocouples were made of 28 B & S gauge chromel and alumel wire, and both uniform and non-uniform quality junctions were formed. For the 28 B * S gauge wire, the uniform quality silver solder junctions had a maximum EMF difference between thermocouples of 0.005 mv (equivalent to 0.55 ¡ãF) and 0.013 mv (equivalent to 0.055¡ãF) at -300¡ãF and 1000¡ãF respectively. The non-uniform quality silver solder thermocouples made of 28 B &S gauge wire had a maximum EMF difference between thermocouples of 0.009 mv (equivalent to 1¡ãF and 0.008 mv (equivalent to 0.34¡ãF) at -300¡ãF and 1000¡ãF respectively. From this limited data, it appears that 28 B & S gauge wire is more sensitive to junction quality than is 20 B & S gauge wire, which was used for all the rest of the experimental work. Copyright 1957

University of Wisconsin-Madison