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Research paper : Improving the reliability of temperature measurements up to 1550 ℃ (M. Arai et al.)−38−Synthesiology - English edition Vol.3 No.1 (2010) four labs AIST” to investigate the degree of change of calibration values in the Cu fixed-point calibration by each calibration laboratories. Prior to the tests, investigations were performed based on the thermocouple fabrication facilities and calibration equipment of the calibration laboratories and AIST, of the specification of the materials such as the thermocouple wires and insulation tube to fabricate the Pt/Pd thermocouple, the assembling conditions such as the pretreatment of wires and insulation tubes as well as the heat treatment after assembly, and the condition of usage.In the same working group, two joint experiments were conducted in 2004 by seven laboratories including AIST as the “Joint Experiment including the Calibration of Type R Thermocouples at Pd Fixed Point ”[23]-[25]. For the calibration laboratory to register to JCSS, it must comply with the guideline for the technical requirement items. This guideline is to clarify the technical requirements established by the international standard ISO/IEC 17025 and to provide explanation to the calibration laboratories. This technical document was published and issued by NITE. This also serves as technical criteria by which the third party recognizes the technical competence for the calibration conducted by a calibration laboratory. The conditions for the secondary standards or the regular reference standards were determined based on the results of the above joint experiments. Cautions pertaining to the handling of inhomogeneity were also described [26].6 Impact of standard development and future issues6.1 Effect of the development of the traceability system for thermocouples As the actual effect of constructing the traceability system for thermocouples, the temperature range of the JCSS has increased, and also the number of JCSS certificates issued by the registered calibration laboratories has increased. The number of JCSS certificates issued was approximately 2000 in FY 2002[27] while there were approximately 10000 in FY 2008[28]. It indeed increased five times in these six years. The certifications of standard conformity and calibration values issued by the thermometer manufacturers and the calibration laboratories ensure the reliability of the thermometers used widely in our society.6.2 Dissemination of the Pt/Pd thermocouples to industry Research work has shown that the Pt/Pd thermocouple developed as the transfer standard can gain extremely high performance by conducting appropriate heat treatment. In the beginning, this thermocouple was distributed as an R&D product with charge by AIST, and the target was limited to the calibration laboratories that had plans to ask AIST for calibration. We transferred this technique for fabricating the thermocouple to a private company, thereby enabling a wider range of users to use the Pt/Pd thermocouple. The developed Pt/Pd thermocouple was launched for sale in April 2006 from Chino Corporation to which the fabrication method was transferred[29].The Pt/Pd thermocouple is expected to become a thermocouple for general temperature measurement, as well as for a transfer standard. To promote its industrial use, AIST worked on the standardization by IEC, and as an outcome, it was standardized as IEC 62460 in 2008[30]. 6.3 Development of the Pd-C eutectic pointAs described in chapter 3, we aim to provide the temperature standards at four fixed points (Ag fixed point, Cu fixed point, Co-C eutectic point, and Pd-C eutectic point) using the Pt/Pd thermocouples as the transfer standards in the future. Therefore, we are currently working actively to develop the Pd-C eutectic point and its evaluation[5]. We have participated in the joint project (Euromet Project 857) as in the Co-C eutectic point, to conduct the international comparison of the temperature values at the Pd-C eutectic points with the representative European national metrology institutes including PTB (Germany), NPL (UK), and LNE-Cnam (France). Table 2 shows the projection of how the calibration uncertainties at each temperature fixed point including the Pd-C eutectic point can possibly be reduced in the future. Currently, the metal-carbon eutectic point is not assigned as the defining fixed point in the 1990 International Temperature Scale (ITS-90)Term 5. Therefore, the temperature value using the Co-C eutectic point cell for thermocouple calibration was measured and determined using the radiation thermometer calibrated based on ITS-90. One of the sources of calibration uncertainty of the thermocouple at the Co-C eutectic point shown in Table 1 includes “the uncertainty of the fixed-point device,” and this is the largest uncertainty component of 0.26 °C. This uncertainly component includes the uncertainty of measurement using a radiation thermometer[6]. In the future, the uncertainty due to the radiation thermometer measurement is expected to decrease, and as a result, the expanded uncertainties (level of confidence of approximately 95 %) of the thermocouple at the Co-C and Pd-C eutectic points are expected to fall to approximately 0.3 °C. When this is achieved, it will be Table 2 Calibration uncertainty of the Pt/Pd thermocouple at each temperature fixed point expected in the future.Temperature fixed pointAgfixed pointCufixed pointCo-Ceutectic pointPd-Ceutectic point961.780.091084.620.111324.00.31491.90.3Temperature of fixed point / ℃Expanded uncertainty of calibration / ℃ (Level of confidence of approx. 95 %)

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