Inseok Yang

Features of Co/C and Ni/C eutectic transitions for use in thermocouple thermometry

Eutectic cells of Co/C and Ni/C for use in thermocouple calibration were manufactured and tested to investigate their melting and freezing characteristics using type B thermocouples. It was observed that the melting and freezing behaviour of Co/C and Ni/C systems are very similar. The freezing plateaus were found to be flatter than those of melting, but the melting points were closer to the ideal transition temperatures, which is an extrapolated value to zero temperature difference from the set temperature to induce melting/freezing, than the freezing points. Based on the observed results, the melting process is recommended for use when calibrating thermocouples.

Improving acoustic determinations of the Boltzmann constant with mass spectrometer measurements of the molar mass of argon

We determined accurate values of ratios among the average molar masses MAr of 9 argon samples using two completely-independent techniques: (1) mass spectrometry and (2) measured ratios of acoustic resonance frequencies. The two techniques yielded mutually consistent ratios (RMS deviation of 0.16 × 10−6 MAr from the expected correlation) for the 9 samples of highly-purified, commercially-purchased argon with values of MAr spanning a range of 2 × 10−6 MAr. Among the 9 argon samples, two were traceable to recent, accurate, argon-based measurements of the Boltzmann constant kB using primary acoustic gas thermometers (AGT). Additionally we determined our absolute values of MAr traceable to two, completely-independent, isotopic-reference standards; one standard was prepared gravimetrically at KRISS in 2006; the other standard was isotopically-enriched 40Ar that was used during NISTs 1988 measurement of kB and was sent to NIM for this research. The absolute values of MAr determined using the KRISS standard have the relative standard uncertainty ur(MAr) = 0.70 × 10−6 (Uncertainties here are one standard uncertainty.); they agree with values of MAr determined at NIM using an AGT within the uncertainty of the comparison ur(MAr) = 0.93 × 10−6. If our measurements of MAr are accepted, the difference between two, recent, argon-based, AGT measurements of kB decreases from (2.77 ± 1.43) × 10−6 kB to (0.16 ± 1.28) × 10−6 kB. This decrease enables the calculation of a meaningful, weighted average value of kB with a uncertainty ur(kB) ≈ 0.6 × 10−6.

Correction of NPL-2013 estimate of the Boltzmann constant for argon isotopic composition and thermal conductivity

In 2013, a team from NPL, Cranfield University and SUERC published an estimate of the Boltzmann constant based on precision measurements of the speed of sound in argon. A key component of our results was an estimate of the molar mass of the argon gas used in our measurements. To achieve this we made precision comparison measurements of the isotope ratios found in our experimental argon against the ratios of argon isotopes found in atmospheric air. We then used a previous measurement of the atmospheric argon isotope ratios to calibrate the relative sensitivity of the mass spectrometer to different argon isotopes. The previous measurement of the atmospheric argon isotope ratios was carried out at KRISS using a mass spectrometer calibrated using argon samples of known isotopic composition, which had been prepared gravimetrically.We report here a new measurement made at KRISS in October 2014, which directly compared a sample of our experimental gas against the same gravimetrically-prepared argon samples. We consider that this direct comparison has to take precedence over our previous more indirect comparison. This measurement implies a molar mass which is 2.73(60) parts in 106 lighter than our 2013 estimate, a shift which is seven times our 2013 estimate of the uncertainty in the molar mass.In this paper we review the procedures used in our 2013 estimate of molar mass; describe the 2014 measurement; highlight some questions raised by the large change in our estimate of molar mass; and describe how we intend to address the inconsistencies between them. We also consider the effect of a new estimate of the low pressure thermal conductivity of argon at 273.16 K. Finally we report our new best estimate of the Boltzmann constant with revised uncertainty, taking account of the new estimates for the molar mass and the thermal conductivity of the argon.

The dependence of the melting temperatures of the eutectics Fe?C and Co?C, designed for thermocouple thermometry, on the furnace offset temperature during the preceding freeze

We have studied the pre-freezing furnace offset temperature dependence of the melting temperature of the Fe–C eutectic system, designed for thermocouple thermometry, and compared the results to those measured accordingly for the Co–C eutectic system. When the pre-freezing offset temperature was changed from −3 K to −70 K, the shift of the melting temperature of the Fe–C eutectic system increased by 0.1 K. Our results suggest that care must be taken on deciding and controlling the furnace offset temperature during the preceding freeze as well as the furnace offset temperature during the melt when the melting temperature of the Fe–C system is used as a reference temperature for thermocouple calibrations. Under careful control of the offset temperature of the furnace, its effect via pre-freeze or melt on the melting temperature of the Fe–C cell can be confined to as low as 0.04 K for thermocouple calibrations.

Validation of the fixed-point realization of the thermocouple scale using a radiation thermometer in the temperature range between the freezing points of Ag and Pd

We have developed a blackbody comparison system to validate the fixed-point realization of the thermocouple temperature scale using the radiation thermometry scale. Platinum/palladium, type S and type B thermocouples were calibrated at the fixed points of Ag, Cu, Fe?C, Co?C and Pd (only for type B). The radiation thermometer used was an LP4 linear pyrometer operating at a central wavelength of 650?nm. To assign the Fe?C eutectic melting temperature in our laboratory, a radiometric Fe?C cell was fabricated, and its melting temperature was determined as (1154.0 ? 0.1)??C with k = 2. Two scales were compared from 962??C to 1555??C in the blackbody comparison system. The two scales were consistent within 0.5??C up to 1400??C, but the discrepancy for the type B thermocouple increased to 2.3??C at 1554.8??C. The thermocouple and radiometric scales realized at our laboratory were in agreement up to the freezing temperature of Pd within the measurement uncertainty.

First-order compensation for time lag in the dynamic calibration of industrial thermometers

A new calibration method that provides first-order compensation for the time-lag error of dynamic temperature comparisons is suggested. The calibration uses the ramp response of the first-order system, assuming that the temperature change of the system can be approximately linearized in a narrow region near the calibration point. The calibration is done by measuring the rate of temperature change and comparing the temperature readings of a reference thermometer and test thermometers while the temperature of the system is decreased and then increased. This calibration is useful for industrial thermometers at low temperatures without using a sophisticated temperature-control system. We demonstrate in this work that the deviation of the result of time-lag compensation from the calibration in a stable thermal enclosure was less than 20 mK. The result indicated that the time-lag compensation can be used for calibrations where the required uncertainties are around 40 mK, and the calibration procedure is much simpler than that for comparison in a stable enclosure. Furthermore, the compensation suggested here has wider applicability at higher temperature ranges where thermometers are calibrated dynamically in liquid baths or furnaces.

Study on the melting and freezing behaviour of high temperature binary eutectic fixed points using differential scanning calorimetry

We have measured the heat flux accompanying the melting or freezing of metal (or metalloid)?carbon eutectics, using differential scanning calorimetry (DSC) to identify appropriate binary systems for secondary thermometry fixed points. Well-known alloy systems such as Fe?C and Ni?C showed reproducible endothermic and exothermic peaks that represent melting and freezing reactions in the DSC measurement. Furthermore, a new Si?C system with a eutectic composition showed reproducible melting and freezing peaks in the DSC measurements. Based on the results by DSC, we identified the Si?SiC eutectic point as a possible eutectic fixed point. To confirm this possibility, we made a Si?SiC cell for thermocouple thermometry and measured its melting and freezing characteristics using a Pt/Pd thermocouple. The melting temperature of the Si?SiC eutectic was reproducible to within 0.02??C (one standard deviation). From the results, we found that Si?SiC has possibility as a new eutectic fixed point at temperatures around 1400??C. We also concluded that DSC analysis could be used to measure the reproducibility of freezing and melting reactions that are to be used as fixed points for thermometry, because it is a rapid and easy-to-use tool for characterizing the thermal behaviour of materials with only a small sample.

First measurement using three-in-one cell: a new design of the fixed-point cell for the calibration of thermometers

A new design of the fixed-point cell that contains multiple metal elements in a single crucible has been introduced for fixed-point calibration of secondary thermometers such as thermocouples. A multicell with aluminium, silver and copper was fabricated and used for the calibration of a Pt/Pd thermocouple. The expanded uncertainties (k = 2) of the realization of the multicell were 0.07 °C, 0.12 °C and 0.24 °C, plus an additional term from the inhomogeneity of the thermocouple, for aluminium, silver and copper, respectively. These uncertainties are considerably smaller than those obtained from the method of calibration by comparison in similar temperature ranges. This proves the suitability of the metal multicells for the calibration of secondary thermometers.

Measurement of the Inhomogeneity of Pt/Pd Thermocouples using an Immersion-into-Liquid Method

Abstract We report on the behavior of Pt/Pd thermocouples after heat‐treatment at 1,000°C. The thermal EMF is measured at the silver freezing point for different heat‐treatment periods. The thermal EMF at the silver freezing point initially increases with heat‐treatment time, before it saturates at a cumulative heat‐treatment time of 200 hours or longer. The thermal EMF is higher for the thermocouple with a higher purity Pd wire. The inhomogeneity profile along the length of the thermocouples at 180°C is also investigated using immersion into an isothermal liquid bath. When the heat‐treatment time is 200 hours or longer, an inhomogeneity of about 2.8 µV is observed. To determine the origin of this inhomogeneity, the thermal emf of each thermocouple wire is measured relative to a reference platinum wire. The results indicate that the change causing the inhomogeneity appears to be irreversible and comes mostly from the palladium wires. Our results indicate that, for good thermoelectric stability, the Pt/Pd thermocouple is best heat‐treated for ∼200 h at 1,000°C if it is used up to that temperature.

The potential for using the hypereutectoid transition in Fe–C in thermocouple thermometry

The eutectoid transition in the iron–carbon system around 730 °C has been tested as a potential fixed point for use in thermocouple thermometry. Using a heating rate ≤ 1.5 °C min−1, the α–γ transition showed a short, but repeatable, plateau with a repeatability of 0.07 °C. However, the γ–α transition only exhibited a small discontinuity in a plot of emf versus time. Although only preliminary results were obtained here, once the thermodynamic (or the ITS-90) temperature, as well as its uncertainty, is assigned to this eutectoid transition, then the hypereutectoid transition in the iron–carbon system is potentially very useful, in that this fixed point can be obtained additionally while realizing the iron–carbon eutectic point.