Vol.3 No.1 2010

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Research paper : Improving the reliability of temperature measurements up to 1550 ℃ (M. Arai et al.)−28−Synthesiology - English edition Vol.3 No.1 (2010) • In the case where only type R thermocouple is the transfer standardScenario (1): AIST calibrates one type R thermocouple at the three temperature fixed points (Ag, Cu, and Pd fixed points), and expresses the relationship of temperature and thermal electromotive force (emf) between 960 °C and 1550 °C as a function based on the emf values (calibration values) at each point. This function is provided to the calibration laboratory as the standard.Scenario (2): AIST either calibrates one type R thermocouple at the three temperature fixed points (Ag, Cu, and Pd fixed points) or, otherwise, calibrates three separate type R thermocouples, one at each of the three temperature fixed points (Ag, Cu, and Pd fixed points), and provides the emf values (calibration values) as the standards to the calibration laboratory.• In the case where both Pt/Pd thermocouple and type R thermocouple are the transfer standardsScenario (3): AIST calibrates two Pt/Pd thermocouples, one at each of the two fixed points (Ag and Cu fixed points), calibrates the type R thermocouple at one temperature fixed point (Pd fixed point), and provides the emf values (calibration values) as the standards to the calibration laboratory.• In the case where Pt/Pd thermocouples are the transfer standardsScenario (4): AIST calibrates four separate Pt/Pd thermocouples, one at each of the four temperature fixed points (Ag fixed point, Cu fixed point, Co-C eutectic pointTerm 3, and Pd-C eutectic point), and provides the emf values (calibration values) as the standards to the calibration laboratory.The important considerations for the above four scenarios are (a) what is to be selected as the transfer standard, and (b) what temperature fixed points are to be selected as the calibration temperature. The technical difficulties increase in the order from the Scenario (1) to the Scenario (4).In the Scenario (1), it is more efficient to provide the calibration service from AIST directly to the thermometer users without intermediation by any calibration laboratory. This scenario is occasionally selected by the metrology institutes in the developing nations. The calibration uncertainty is the largest among the four scenarios.Scenario (2) is convenient for the calibration laboratories that would be receiving the transfer standard because they can use the devices they already operate. However, for example, if the uncertainty of calibration service of the type R thermocouple at 1000 °C is set to be 0.3 °C, the uncertainly of the working standard of the calibration laboratory should be within 0.1 °C, which is approximately one-third the value. This figure is not easy to achieve in this scenario. As will be explained in chapter 4, there is a limit in the stability of the type R thermocouple when being used as the transfer standard.Scenario (4) places the metal-carbon eutectic point, which is a recent research product, as the national measurement standard. It is an excellent method that can incorporate the findings of upcoming future research. However, the dissemination of the standard under this scenario involves the vast investment to introduce new facilities and the learning of new skills by the calibration laboratories, and their workload will increase. Although AIST positions Scenario (4) as an optimum solution for the future, Scenarios (2) and (3) that use the temperature fixed points are the practical solutions that could be readily adopted by the thermometer manufacturers and the calibration laboratories. Thus we have initially selected Scenarios (2) and (3) for the time being, and the preparations toward Scenario (4) has also been started.AIST has been disclosing and updating the Measurement Standards Development Program for various quantities including temperature since 2001. Since this program clearly states when and which national measurement standards will be prepared, it is possible for the industry to prepare the necessary facilities and personnel for providing the calibration service, and to start the calibration service according to the release for the dissemination of the national measurement standards. AIST announced the commencement of the calibration service for thermocouples using the Ag and Cu fixed points as 2002 in this the Measurement Standards Development Program, and conducted research on the transfer standards using the available interval. The study sessions of the temperature-related academic societies and industrial associations were held almost every month. There, the status of standard development at AIST was reported, the thermometer calibration by the thermometer manufacturers and calibration laboratories were discussed, and the technical data obtained by AIST for the calibration of the transfer standard using the Ag and Cu fixed points were presented.Fig. 2 Comparison of the scenarios for thermocouple traceability.“R” and “Pt/Pd” represent the type R thermocouple and the Pt/Pd thermocouple that are used as transfer standards, and the circled numbers are for individual identification.Scenario(1)Scenario(2)Scenario(3)Scenario(4)SmallSmallLargeLargeR ②R ①R ① 960 ℃~1550 ℃R ①R ①R ①R ① R ③961.78 ℃Ag1324.0 ℃Co-C1084.62 ℃Cu1491.9 ℃Pd-C1553.5 ℃PdPt/Pd ①Pt/Pd ②Pt/Pd ①Pt/Pd ②Pt/Pd ③Pt/Pd ④CalibrationuncertaintyTechnical issues ･Burden for calibration labs

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