Vol.3 No.1 2010

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Research paper : Improving the reliability of temperature measurements up to 1550 ℃ (M. Arai et al.)−31−Synthesiology - English edition Vol.3 No.1 (2010) the pure metal under high temperature, contaminated the pure metal, and decreased the melting and freezing points. This is the major reason that the temperature fixed point above 1100 °C could not be realized by using the pure metal with the graphite crucible. To solve this issue, it was proposed to mix the metal and carbon according to the ratio of the composition of eutectic alloy and then founding this mixture in the graphite crucible[4]. This enabled the realization of a melting temperature with good reproducibility in the crucible and could be used as the temperature fixed point. The metal-carbon eutectic points are currently studied in the advanced national metrology institutes around the world as new fixed points for the high temperature range. Using this technique, the Co-C eutectic point device for the thermocouple calibration was developed. We succeeded in fabricating a large-sized Co-C eutectic point cell for the first time in the world, and demonstrated that it can be used for accurate thermocouple calibrations[5]. In designing the device, the technique for the Cu fixed-point device was applied. Although a quartz tube was used to seal the crucible in the Cu fixed-point device, an alumina tube was used instead of quartz in the new device since devitrification or softening of quartz occur at the Co-C eutectic point temperature.Figure 6 shows the melting and freezing curves of the Co-C eutectic point. As a result of the uncertainty evaluation, it was found that thermocouples could be calibrated with the expanded uncertainly of 0.53 °C (level of confidence of approximately 95 %) at the Co-C eutectic point[6][7].4.1.4 Pd fixed-point deviceThe wire-bridge method for realizing the melting point of palladium was mentioned earlier, and there were several techniques for attaching the fixed-point material to the thermocouple. As a result of experimental evaluation, it was found that attaching the coil-shaped palladium wire, as shown in Fig. 7, was effective in realizing a stable melting temperature[8]. Figure 8 shows the emf of a type R thermocouple with coil-shaped palladium wire when the furnace temperature was increased gradually after inserting the thermocouple in the Pd fixed-point device. In 150 seconds, the sustained melting temperature in the range of ±0.05 °C was observed while the attached palladium wire was melting, and the average emf value in this range was obtained as the calibration emf value of the thermocouple at the Pd fixed point. As a result of the investigations, it was confirmed that the melting temperature could be realized with reproducibility Fig. 5 Freezing curve at the Cu fixed point.Fig. 6 Melting and freezing curves at the Co-C eutectic point.Supercooling/ mintΔ / mKTSolid and liquid phases coexist and show constant temperature(Calibration of thermocouple is done in this range)0010020030040050020-20-40-60010020030040018630186201861018600185900.5 ℃1st2nd3rd4th5th6thTime / minEmf / µVFig. 7 Wire-bridge method to realize the melting point of palladium.Alumina tubeType R thermocouple wirePd wire3 mmFig. 8 Melting curve at the Pd fixed point.Time / sEmf / µVAftermeltingBeforemeltingDuring meltingDuring meltingMelting plateau1 ℃Emf value at the Pd fixed point4008006001200140010001818018200182201824018260

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