Vol.3 No.1 2010

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Research paper : Improving the reliability of temperature measurements up to 1550 ℃ (M. Arai et al.)−27−Synthesiology - English edition Vol.3 No.1 (2010) by them. To solve this issue, the national measurement standards are developed and the temperature standard system is constructed. The aim is that the developed national measurement standards are transferred to the user’s thermometers, and further, the calibration values declared by the calibration laboratories or thermometer manufacturers become verifiable, so that the reliability of thermometers widely used in society can be ensured. 3 Selection of scenario for the thermocouple traceabilityIn this chapter, the framework of the traceability system of thermocouples will first be described. It is followed by the description of the selection of scenarios that leads to the most-fit system design and the consideration on elements essential for implementing the selected scenario.3.1 The framework of the thermocouple traceability systemFigure 1 shows the basic framework of the national measurement standards for temperatures in the range between 1000 °C and 1550 °C, and the traceability system for the thermocouples that uses these standards.AIST maintains a series of temperature fixed points as national measurement standards. The freezing point of pure copper (1084.62 °C) and the freezing point of pure silver (961.78 °C) are temperature fixed points used conventionally by many metrology institutes around the world, including Japan. The temperature values at the freezing points are defined by an international agreement. The melting point of pure palladium (1553.5 °C) is also occasionally used as the high temperature fixed point in several countries.The metal-carbon eutectic points are the newly proposed fixed points by AIST. There is a wide temperature range of about 500 °C between the copper point and palladium point, and from the perspective of disseminating temperature standards, finding applicable fixed points along that temperature range has a great significance. To this concern, therefore, AIST initiated research at the earliest time on finding possibilities of metal-carbon eutectic points to be used as the national measurement standards.For transferring the national measurement standards from AIST to the thermocouple calibration laboratories, a transfer standard, which carries the temperature standard, is necessary. The transfer standard must possess high reliability to reproduce the standard accurately, and must be sufficiently robust to maintain the standard accurate during transportation. In practice, it must also be light-weight and low-cost. To these conditions, we considered two types of thermocouples as candidates. The first was the thermocouple that uses platinum-rhodium alloy with 13 % rhodium content and pure platinum as wires (type R thermocouple), and the other was the one that uses pure platinum and pure palladium as wires (platinum/palladium or Pt/Pd thermocouple). Both types are applicable for high temperature measurement. Although the type R thermocouple has already been used widely, there are some problems in stability at high temperatures. The Pt/Pd thermocouple was newly developed, and while a better stability was expected than the type R thermocouple, the related case studies were not significantly available, and the properties such as stability were not sufficiently understood.After receiving the transfer standard already calibrated at AIST, the calibration laboratories transfer the temperature standards to the working standard they are maintaining. In Japan, most of the calibration laboratories use temperature fixed-point device as their own working standard. In this case, the calibration laboratories can calibrate the field users’ thermocouples using their own temperature fixed-point device.3.2 Scenario selectionAIST considered the scenarios from two perspectives: (a) what artifact is to be selected as the transfer standard, (b) what temperature fixed points are to be used for disseminating the calibration service. The national metrology institutes worldwide are always in competition in research and development for setting the measurement standards, where some new methods are proposed or conventional methods are improved upon further evaluation. For a national metrology institute, it is ideal to start by integrating those achievements, setting measurement standards of top level quality, and creating a traceability system based on them. On the other hand, for a calibration laboratory, it is more convenient to employ conventional methods already adopted, because the equipments already owned can be operated as they used to be, along with the accustomed calibration procedures Four scenarios below were considered from these two opposing standing points, as shown in Fig. 2.Fig. 1 Basic framework of the high-temperature traceability for thermocouples.Cu 1084.62 ℃Ag 961.78 ℃Pd-C 1491.9 ℃Co-C 1324.0 ℃Pd 1553.5 ℃National measurement standards of AISTTravelling transfer standardsStandards of the calibration laboratoriesThermocouples used on siteTemperature fixed-point deviceFreezing point of pure metalMetal-carbon eutectic pointMelting point of pure metalPt/Pd thermocouple(Pure platinum/pure palladium)Type R thermocouple(Platinum-rhodium alloy/pure platinum)Various temperature fixed-point devices, type R thermocoupleVarious working thermocouples (1000 ℃ ~ 1550 ℃)

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