Vol.3 No.1 2010

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Research paper : Improving the reliability of temperature measurements up to 1550 ℃ (M. Arai et al.)−42−Synthesiology - English edition Vol.3 No.1 (2010) Question (Akira Ono)That the drift and inhomogeneity can be significantly reduced by final heat treatment of the Pt/Pd thermocouple at a specific temperature is a major finding of this research. The authors stated that the cause of drift and inhomogeneity is in the palladium wire, but palladium is pure metal, and I don’t think its composition changes by exposure to high temperature. I suppose the cause of decreased drift and inhomogeneity may be some suppression of microscopic structural change in the palladium wire, but what is the view of the authors? To what extent can the microscopic changes that occur in the palladium wire be explained in terms of material science and solid state physics?Is there any other way than maneuvering the heat treatment condition based on some different principle that can reduce the drift and inhomogeneity of the thermocouple?Answer (Masaru Arai, Hideki Ogura, and Masaya Izuchi)We are certain that the microscopic structure change of the palladium wire has something to do with this. Currently, other researchers report the cause of the drift and inhomogeneity of the Pt/Pd thermocouple, and these can be roughly divided into: the oxidation of the impurities in the palladium wire, and the growth of crystal grain in the palladium wire.If the cause is the impurities in the palladium wire, the impurities in palladium oxidize, and change from conductor to insulator, thereby changing the emf. Therefore, if the refining technique advances in the future, and we are able to fabricate highly pure palladium wire or remove the impurities that enhance the inhomogeneity, then we may be able to inhibit the inhomogeneity. On the other hand, if it is caused by the growth of crystal grain in the palladium wire, we can sufficiently grow the crystal by preliminary heat treatment, or add additives to inhibit the crystal growth to the level that it will not alter the emf.Currently, the cause of the drift and inhomogeneity of the Pt/Pd thermocouple is not fully clarified, and to further reduce the calibration uncertainty of the thermocouple is a future research topic.4 Contribution to the Japanese industry and its levelComment and question (Akira Ono)The authors established the traceability system for thermocouples with thorough consideration of the technological status of the Japanese industry. I think there is a Japanese characteristic in the traceability system compared to the that in other countries. Many of the Japanese private calibration laboratories possess fixed-point furnaces of high temperatures that can be used for calibration services, though the furnaces may not be advanced like the AIST’s national measurement standards. The authors took into account the equipment and experiences of the private calibration laboratories, and I think that it is the reason the authors were able to construct a traceability system at the highest level of reliability in the world. What do you think about that point? The authors repeatedly emphasize “the technique that has been nurtured by the private companies” in the paper and I wonder whether you are referring to this.It was demonstrated by the international comparison that the technological level of AIST is high. If there was an international comparison among the I suppose the Japanese calibration laboratories would perform extremely well. What is your thought? I would suggest some activities so that the high technological reliability of the Japanese private calibration laboratories are recognized better in the world.Answer (Masaru Arai, Hideki Ogura, and Masaya Izuchi)In Japan, the introduction of the fixed-point device to raise the reliability of temperature measurement started early. Also, for the thermocouple calibration device using the Co-C eutectic point, which is our latest research mentioned in the paper, the Japanese calibration laboratories have developed products jointly with AIST, and several laboratories are already preparing to use these devices. As you can see, the technological level of the Japanese calibration laboratories is extremely high. If there is an international comparison among the calibration laboratories, the Japanese laboratories will certainly demonstrate high reliability. Also, in the working group of the Temperature Measurement Subcommittee, under the 36th Committee on Industrial Instrumentation of the Japan Society for the Promotion of Science, the thermocouple calibration laboratories and AIST are collaborating to study the thermocouple calibration technique, and we would like to present the research results at international conferences.5 Reliability of the thermocouples at temperatures above 1550 °CQuestion (Akira Ono)This paper describes the traceability system of thermocouples in the temperature range up to 1550 °C. Thermocouples are also used as the major thermometer for above 1550 °C. What do you think is the reliability of the thermocouples in this temperature range? If you were constructing the traceability system in the temperature range above 1550 °C in the future, what kind of research approach would you take?Answer (Masaru Arai, Hideki Ogura, and Masaya Izuchi)The thermocouple is a major thermometer for high temperatures above 1550 °C. In industry, tungsten-rhenium thermocouples are used for temperatures up to over 2000 °C. However, in practice, the reliability of tungsten-rhenium thermocouples are not really known. For example, with approximately 100 hours of use, approximately 5 °C thermoelectric drift is expected at around 1700 °C, and the drift will increase in higher temperatures.In this temperature range, it is difficult to create a stable and homogeneous temperature field to begin with, and in addition, since the reactivity of the substance increases, the effect of the insulation and protection tubes on the thermocouple wire must be studied. To construct highly reliable thermocouple traceability, we think it is important to engage in R&D to solve the elements for evaluating the stability of the thermocouple. We would like to continue the research by actively incorporating the high reproducibility of metal-carbon eutectic point that is the result of our latest research.

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