A vertical cobalt-carbon (Co-C) eutectic fixed point cell was constructed at PTB to demonstrate its use for improvement of the calibration of noble-metal thermocouples at temperatures above 1100 ˚C. The melting and freezing temperatures of the Co-C eutectic were measured in different high-temperature furnaces at PTB and INMETRO (Brazil) to show its stability by using a Pt/Pd thermocouple. The reproducibility of all measured electromotive forces at the inflection points of the melting curves amounts to a value of about 0.06 ˚C. No drift in the melting temperature was observed. Therefore, the Co-C eutectic fixed point cell can be used as an adequate transfer standard for dissemination of the International Temperature Scale of 1990 (ITS-90) in the temperature range above the freezing point of copper.
Three nickel-carbon (Ni-C) and three iron-carbon (Fe-C) eutectic fixed points cells of a new design, meeting the requirements for reliable applications and being suitable for the calibration of thermocouples, were constructed at PTB and Inmetro. Their melting temperatures were compared by using the high-temperature furnace of PTB (HTF-R) and two platinum/palladium (Pt/Pd) thermocouples. The measured emfs of the Ni-C eutectic fixed point cells at the inflection points of the melting curves agree within a temperature equivalent of about 0.29 • C, compared with an agreement of about 0.09 • C found for the Fe-C cells. Additional comparison measurements of two Fe-C eutectic fixed point cells in a second high-temperature furnace (HTF-M, Inmetro) demonstrate the applicability of the Fe-C eutectic cells as transfer standards for the dissemination of temperatures.
The objective of the present investigation was the determination of the melting temperatures of the eutectic compounds Fe-C, Co-C, and Ni-C. Six eutectic fixed-point cells of the Physikalisch-Technische Bundesanstalt (PTB) (Fe-C1, Fe-C2, Co-C1, Co-C2, Ni-C1, and Ni-C2) and two cells of the Brazilian National Metrological Institute (Inmetro) (Fe-C1V and Ni-C1V), useable for the calibration of contact thermometers, were investigated. Their melting temperatures were calculated by extrapolation of the emf-temperature characteristics of four stable Pt/Pd thermocouples, which were calibrated at the eutectic fixed points and at conventional fixed points of the International Temperature Scale of 1990 (ITS-90). On the basis of the eight eutectic fixed-point cells and seven independent calibration runs, the melting temperatures of the Fe-C, Co-C, and Ni-C eutectics resulted in 1153.67 ± 0.15 • C, 1323.81 ± 0.27 • C, and 1328.48 ± 0.20 • C, respectively, with expanded uncertainties corresponding to a coverage factor of k = 2.
This paper proposes to discuss some concepts whose importance is fundamental to any theory of measurement. From the epistemological point of view there is a very significant inconsistency in the concept of measurand contained in the GUM. According to the model adopted by the GUM, a particular measurand remains the same measurand after a change in its definition, notwithstanding the fact that new conditions had been included or other pre-existing conditions had been altered in the definition. And also notwithstanding the very probable fact that, in consequence, the uncertainty component associated with its definition had been changed. The problem that arises from this theoretical model is that a particular measurand is not completely characterized by a precise and unique definition. Instead, it is considered an 'ideal' utopian particular concept to which would converge a specific series of 'measurands' (more properly their definitions), continually redefined in an infinite succession. Such a theoretical basis seems very inappropriate: the model intends to determine the uncertainty of uncertainly defined particular measurands. Of great relevance is the proposal of a new definition for the concept of measurand, a concept that interfaces the realms of Metrology and experimentation. Other concepts have been created or redefined with sufficient precision and selectivity to consider some aspects that remain somewhat confused in the model of measurement currently applied.
The highest-temperature, defining fixed point of the International Temperature Scale of 1990 (ITS-90) is the copper freezing point (1,084.62 • C). Many international metrology institutes are investigating the use of transition temperatures of metal-carbon alloys as references for the calibration of temperature measuring instruments above the copper point, making it possible to reduce the calibration uncertainty of pyrometers in radiation thermometry and thermocouples in contact thermometry. This research is being performed mainly by radiation thermometry laboratories that have developed specific cells with blackbody cavities containing relatively small quantities of metal-carbon alloys. Parallel to this, some laboratories have also developed cells with these same alloys, but of a different design, suitable for the calibration of thermocouples. This report concerns the development of a nickel-carbon eutectic cell ( ∼ =1,329 • C) at Inmetro, with which either a radiation thermometer or thermocouple can be calibrated. The measurements of the temperature of this cell were performed using the reference radiation thermometer of the Pyrometry Laboratory and Pt/Pd thermocouples that were constructed, stabilized, and calibrated at the Thermometry Laboratory. Details of the cell fabrication, as well as the instrumentation used for the measurements are given. The results of a comparison between the two different types of measurement are reported, including the uncertainty budgets of both methods.
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