Vol.3 No.1 2010

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Research paper : Improving the reliability of temperature measurements up to 1550 ℃ (M. Arai et al.)−30−Synthesiology - English edition Vol.3 No.1 (2010) simple and the calibration can be conducted with only a small amount of the fixed-point material of 0.1 g or less. The wire-bridge method is generally used in cases where the crucible material may contaminate the fixed-point material, or metals indicating high melting point, such as noble metals, is used as the fixed-point material. In order to start the calibration service of the thermocouples, AIST employed the crucible method for realizing the Ag fixed point, the Cu fixed point, and the Co-C eutectic point because of good reproducibility of the freezing and melting points. The wire-bridge method was used for realizing the Pd fixed point. 4.1.1 Ag fixed-point deviceFigure 3 shows the schematic cross section of the Ag fixed-point device. The fixed-point device was mainly composed of the “fixed-point furnace” consisting of a heater and a control system, and the “fixed-point cell” that held the silver ingot as fixed-point material. A vertical electric furnace containing a sealed sodium heat pipe was used as the fixed-point furnace which indicated the high temperature stability within ±25 mK for 9 hours around 960 °C, and excellent temperature uniformity within ±6 mK over 14 cm height. The silver ingot with nominal purity 99.9999 % was 1390 g in weight and founded in a graphite crucible which was sealed in the fixed-point cell. This cell was an open type that allowed monitoring of gas pressure on the ingot. The freezing point temperature could be maintained for 5 hours within ±10 mK, and the standard deviation of reproducibility in 14 measurements was 3.8 mK[2]. By using this device, it was evaluated that thermocouples could be calibrated with the expanded uncertainty of 0.09 °C at the Ag fixed point (level of confidence of approximately 95 %). 4.1.2 Cu fixed-point deviceFigure 4 shows the schematic cross section of the Cu fixed-point device. As in the aforementioned Ag fixed-point device, this device consisted of the “fixed-point furnace” composed of the heating element, control system and the “fixed-point cell” that held the fixed-point material. The fixed-point furnace was a vertical electric furnace with three-zone heater in a vertical direction for temperature control. Kanthal wires were wound noninductively as heaters, and type R thermocouples were installed in the center of each zone as the controlling thermocouple. The design characteristics of this fixed-point furnace compared with the conventional 3-zone electric furnace were the improvement of the heat contact between controlling thermocouples and heaters, and the improvement of heat-retention of the furnace by thickening insulation material. The quick response of the controlling thermocouple was realized by attaching them to the alumina tube on the heater, to improve the temperature control of the furnace. The high-temperature refractory fiber (ceramic fiber) was used as insulation around the heating elements, the thickness of which was over 150 mm to increase the heat-retention of the furnace. Due to these improvements, the electric power consumption was reduced below 1 kW. The fixed-point cell was the open type as shown in the Ag fixed-point, and 1450 g copper with nominal purity 99.9999 % was founded in a graphite crucible. Figure 5 shows the temperature inside the fixed-point cell during the freezing of the copper. The temperature at the freezing point was maintained for 8 hours within ±2 mK. The standard deviation of reproducibility of the freezing point measured by repeating the melting and freezing 26 times was 11.7 mK[3], and it was evaluated that by using this device, thermocouples could be calibrated with expanded uncertainly of 0.11 °C at the Cu fixed point (level of confidence of approximately 95 %). 4.1.3 Co-C eutectic-point deviceWhen a pure metal was founded in the graphite crucible for realizing its melting and freezing points above 1100 °C , the carbon that is the component of the crucible dissolved into Gas portQuartz tubeGraphite diskThermometer wellGraphite woolSilverGraphite crucibleHeat pipeHeaterFixed-point cellFixed-pointfurnace33050140160575730Desirable uniform temperature zoneFig. 3 Schematic diagram of the Ag fixed-point device.Fig. 4 Schematic diagram of the Cu fixed-point device.Gas portQuartz tubeGraphite diskThermometer wellGraphite crucibleHeaterLiquid phase of metalSolid phase of metalFlangeThermocouple98065550400Fixed-point cellFixed-pointfurnace

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