Vol.3 No.1 2010

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Research paper : Improving the reliability of temperature measurements up to 1550 ℃ (M. Arai et al.)−29−Synthesiology - English edition Vol.3 No.1 (2010) After numerous sessions of such information and opinion exchanges, the following agreement was reached between AIST and industry. While using the type R thermocouple as the transfer standard might be an easy solution, the uncertainty due to the inhomogeneity of the thermocouple will increase (explained in section 4.2.1). The demand from the thermometer manufacturers was to achieve a sufficiently small uncertainty that matches the quality of Class 1 thermocouple as designated by the IEC standards. It was recognized that the development of the transfer standards with small uncertainty and their dissemination in synch with the establishment of the national measurement standards would be a great advantage. The details of this R&D will be explained in section 4.2. Since there was a technical prospect for achieving small uncertainty of approximately 0.1 °C, we decided to select Scenario (3) that employed the Pt/Pd thermocouples as the transfer standards for the Ag and Cu fixed points.3.3 Elements for the implementation of the scenarioThe policy taken was to first quickly employ Scenario (3) to provide the advanced standard to industry, and then move toward Scenario (4) for further advancement. Elements to were selected as the elements necessary for AIST to implement these scenarios.Fabrication of temperature fixed-point devices that would serve as the national measurement standards for high temperatures and the evaluation of their uncertaintyDevelopment of the stabilization technique of Pt/Pd thermocouples as transfer standardsTechnique to calibrate transfer standards at temperature fixed points and the evaluation of their uncertainlyConfirmation of equivalence of the national measurement standards by international comparison Building and operation of a quality management system to ensure regular and accurate provision of the calibration serviceDesigning a practical traceability system and drafting of the technical documentsThe traceability system for high temperatures was attempted to be built by integrating and synthesizing the above elements.Element involved the setting of the temperature fixed points as the national measurement standards. To realize Scenario (3), the freezing point of silver (961.78 °C), the freezing point of copper (1084.62 °C), and the melting point of palladium (1553.5 °C) were developed as the national measurement standards. Also, to progress to Scenario (4) in the future, Co-C eutectic point (1324.0 °C) and Pd-C eutectic point (1491.9 °C) were selected as new national measurement standards, and their researches were started. To provide the emf of thermocouples at each temperature fixed point as standards to the calibration laboratories, the type R thermocouple was selected as the transfer standard for the palladium melting point. For temperatures below 1500 °C, Pt/Pd thermocouple was selected as the transfer standard. Element was the development of this Pt/Pd thermocouple.Element was the technique to calibrate the transfer standard using the prepared fixed-point device, and the evaluation of the calibration uncertainly. Element involved the comparison of the standards with the other competitive national standard institutes. This was carried out to confirm the international equivalence of the national measurement standards and the calibration technique. The method for conducting the international comparison was determined internationally, and 12 metrology institutes under the international organization called the Asia Pacific Metrology Programme (APMP) that was established for the Asia Pacific region participated.Element concerned the development of the system to ensure that AIST could conduct regular and accurate provision of the calibration service using the established national measurement standards. Element was the traceability system of the temperature standards where the calibration laboratories would use the prepared measurement standards to calibrate the thermometers that would be used by industry. The drafting of the technical document that served as the basis for the third party to check the technical level of the calibration laboratory was also an important element. This document confirmed the calibration capability of the calibration laboratories.The details of Elements to will be explained in sections 4.1 to 4.5. Element is discussed in chapter 5.4 Preparation of the national measurement standards and development of the transfer standards 4.1 Element 1: Fabrication and evaluation of the fixed-point device To realize the temperature fixed points for thermocouple calibration, it is known that there are two methods. One is the method using a crucible (crucible method) where the temperature fixed point is realized by melting or freezing of the metal founded in a crucible, and the other is the wire-bridge method where the temperature fixed point is realized by melting of metal directly attached at the measuring junction of the thermocouple for calibration. Since the crucible method indicates excellent reproducibility of the fixed-point temperature, and maintains the melting or freezing state over a long period, this method is generally used to realize the many temperature fixed points accurately. On the other hand, the wire-bridge method does not require a crucible to realize the temperature fixed point, and accordingly, it is

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