Instrumentation

Calibration of Thermocouples Thermocouple calibration is intended to improve accuracy. Conventional calibration might not. Calibration of a sample from a uniform set of as-delivered thermocouples to confirm conformity to standard tolerance is appropriate. However, despite individual calibration, the approximated standard scaling relation, rather than the actual calibration, usually is used in thermometry regardless of calibration […]

Active Tests of Thermocouple Manufacturers routinely apply quality control to their thermocouple products, yet users are responsible locally to assure that no occasional defect, error, or change has occurred in manufacture or installation. Several simple and routine test procedures are available to the user. Symptom and Occurrence of Inhomogeneity Thermoelectric inhomogeneity, though now widely misunderstood,

Thermocouple Junction Styles Junction designs vary widely for specialized application. Most junctions are very localized. Some, to average surface temperature along a line, have widely extended a junction between foil thermoelements. Measuring and reference junctions are conspicuous. However, the many incidental junctions are inconspicuous and may not be recognized yet, uncontrolled; their temperatures can significantly

Thermocouple Hardware Material Sources Thermoelectric alloys and thermoelements are manufactured by a very few specialized sources. These manufacture bare thermoelement bar stock, wire, and foil, and selectively pair thermoelements for thermocouples. Fewer sources fabricate the MIMS thermocouple materials that are incorporated by others into finished products. These producers of basic forms and very many other

Characteristics of Thermocouples Standardized Thermocouple Types In the United States, nine material combinations presently are letter-designated by the American Society for Testing and Materials (ASTM) as thermocouple types: B, C, E, J, K, N, R, S, and T. Representative properties of letter-designated thermocouples are summarized in Table 1. To view that Table, Please Follow the

Applications of Functional Model A representative T/X sketch, Figure 1, for a dual-reference junction thermocouple with two extensions, Figure 2, illustrates T/X application. It shows one deliberately inappropriate yet plausible temperature distribution. It reveals how particular segments of thermoelements, variably, are thermally paired in electrical effect, with different segments that may be remote in the

T/X Sketch of Dual-Reference Junction Circuit An informal T/X sketch, Figure 1, plots one present distribution of temperatures, T, of all junctions against the sequence, X, in which those junctions occur in the circuit. (X is not a space variable; slopes do not represent temperature gradients.) A T/X sketch is not rendered to scale. It

Functional Model of Thermoelectric Circuits The commonplace obscure problems of thermoelectric circuits are difficult to visualize without a graphic tool. Conventional electrical circuit diagrams conceal the actual thermoelectric emf sources, unrecognized thermoelements, incorrect pairings, incidental junctions, and changing functions that often cause significant hidden thermometry error. The spurious “junction-source” model conceals the fact that all

Extensions of Thermocouple In the Figure given below, pairs P′N′, P″N″ and any additional thermoelement pairs are optional circuit extensions. A connector is an extension. Every thermoelement must be homogeneous. Only two junctions may function as the essential reference junctions. Which two depends on the type of extensions? If there are no extensions, b and

Basic Thermocouple Circuits Conventional electric circuit schematics represent passive conductors that often all are of copper. Distinctively, in thermoelectric circuits, paired legs are of very different materials and the “conductors” are the emf sources. Much more is involved than circuit connectivity. True thermoelectric schematics must explicitly acknowledge each leg material, all junctions, essential relative junction