Kee Sool Gam

Final Report on CCT-K7: Key comparison of water triple point cells

The triple point of water serves to define the kelvin, the unit of thermodynamic temperature, in the International System of Units (SI). Furthermore, it is the most important fixed point of the International Temperature Scale of 1990 (ITS-90). Any uncertainty in the realization of the triple point of water contributes directly to the measurement uncertainty over the wide temperature range from 13.8033 K to 1234.93 K. The Consultative Committee for Thermometry (CCT) decided at its 21st meeting in 2001 to carry out a comparison of water triple point cells and charged the BIPM with its organization. Water triple point cells from 20 national metrology institutes were carried to the BIPM and were compared with highest accuracy with two reference cells. The small day-to-day changes of the reference cells were determined by a least-squares technique. Prior to the measurements at the BIPM, the transfer cells were compared with the corresponding national references and therefore also allow comparison of the national references of the water triple point. This report presents the results of this comparison and gives detailed information about the measurements made at the BIPM and in the participating laboratories. It was found that the transfer cells show a standard deviation of 50 ?K; the difference between the extremes is 160 ?K. The same spread is observed between the national references. The most important result of this work is that a correlation between the isotopic composition of the cell water and the triple point temperature was observed. To reduce the spread between different realizations, it is therefore proposed that the definition of the kelvin should refer to water of a specified isotopic composition. The CCT recommended to the International Committee of Weights and Measures (CIPM) to clarify the definition of the kelvin in the SI brochure by explicitly referring to water with the isotopic composition of Vienna Standard Mean Ocean Water (VSMOW). The CIPM accepted this recommendation and the next edition of the SI brochure will include this specification. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

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.

Thermoelectric properties of the Au/Pt thermocouple

An Au/Pt thermocouple having a gold-bridged junction instead of a Pt coil is fabricated and its thermoelectric properties are investigated at the Sn, Zn, Al, and Ag freezing points. Reproducibility is found to be excellent and standard deviation is as small as ±4 mK at the Ag fixed point. Immersion properties, which indicate homogeneity of the thermocouple wire, are also shown to be very good, with standard deviation estimated as about 10 mK. In order to investigate interchangeability, 20 sets of Au/Pt thermocouples are made and their emfs are measured at the same fixed points. Thermocouples from the same spool are reproducible within ±0.02 °C for the whole temperature range. For practical use of the Au/Pt thermocouple, an inverse function and a deviation function for calibration are proposed. The inverse function has two temperature ranges and is accurate within ±1 mK, while the deviation function is a third-order polynomial without a constant term. The Au/Pt thermocouple is stable at high temperatures for long periods. After 1500 h at 1000 °C, the thermal emf at the Ag fixed point is changed by about 0.9 μV (about 40 mK). After the other experiments, calibration uncertainties at the fixed points are evaluated. At the Ag fixed point, the maximum expanded uncertainty is calculated and is about ±30 mK at a confidence interval of 95%. It is shown that the gold-bridged Au/Pt thermocouple is reproducible, accurate, and stable. At present, the Korea Research Institute of Standards and Science (KRISS) uses Au/Pt thermocouples as secondary standard thermometers; they are supplied to industry as certified references.

A nickel freezing-point cell for thermocouple calibration

The melting and freezing behaviour of nickel was investigated using a vertical alumina crucible specially constructed for thermocouple calibration. The melting and freezing emfs coincided to within 0.02 °C, and the freezing plateau depression was only 0.04 °C for 45 min. The effect of the thermocouple insulator was examined, and the heat loss through the insulator stem was estimated to lower the recorded temperature about 0.6 °C below the true freezing temperature. The expanded uncertainty for the calibration of a type B thermocouple was calculated to be about 2.1 °C (k = 2). The uncertainty factor related to the fixed-point temperature of nickel played a major role in the combined uncertainty. Exact determination of the true freezing temperature of nickel on the International Temperature Scale of 1990 (ITS-90) will reduce this uncertainty. From the observed results on the melting and freezing behaviour of the nickel cell, it is suggested that the Ni freezing point can be used as a new primary-grade fixed point at high temperature.

Realization of the palladium freezing point for thermocouple calibrations

A vertical alumina palladium (Pd) freezing-point cell was made and used for the calibration of noble metal thermocouples. Various characteristics of the freezing process were investigated using type B thermocouples, and the best method of freezing is discussed. It was found that the freezing behaviour was influenced by the thermal history: it is preferable to freeze the Pd melt completely before carrying out the next test. The thermocouple emfs during freezing were determined by calculating the average emf during the first 10 minutes of the plateau just after the depression in the supercooling curve. This emf was reproducible within a standard deviation of 0.1 °C. The expanded uncertainty of calibrating a type B thermocouple in the Pd freezing-point cell was estimated to be 1.0 °C at the 95% confidence level (k = 2).

Immersion Characteristics and Reproducibility of the Gold/Palladium Thermocouple

Thermoelectric properties of a new gold/palladium (Au/Pd) thermocouple are investigated at the aluminium and silver freezing point cells. The Au/Pd thermocouple generates a high thermal emf and the reproducibility is ±0,2 μV at 961,78 °C. The immersion characteristics of the Au/Pd thermocouple are found to be superior to the conventional type S (Pt-10% Rh/Pt) thermocouple, and it has a larger sensitivity than the extensively studied gold-platinum (Au/Pt) thermocouple. Its easy maintenance and good thermoelectric properties merit serious consideration as an alternative but preferred temperature sensor in the range of 0 °C to 1 000 °C.

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.

The thermoelectric inhomogeneity of palladium wire

Thermoelectric inhomogeneity tests were performed on pure palladium wire using a moving temperature gradient method and the results were analysed quantitatively. The Seebeck coefficient of palladium was found to differ with the heat-treatment conditions. The variation of extra EMF was explained well with the analytical approach. The calculation and experimental results closely coincided with each other. To keep the palladium homogeneous, high-temperature annealing for removal of plastic deformation and low-temperature heat-treatment for annealing out of quenched-in vacancies should be carried out.

1000 h stability test of the Au/Pd thermocouple at 1000 °C

A newly developed Au/Pd thermocouple has shown very high reproducibility and good immersion characteristics. A stability test of the Au/Pd thermocouple has been conducted for 1000 h at 1000 °C in air to assess its suitability for practical application. Two thermocouples from two manufacturers were fabricated and their emf values were checked in a silver freezing-point cell before and after the soaking at 1000 °C. It was found that the high-temperature stability of the Au/Pd thermocouple is within ±30 mK (2σ) throughout the test time. This result implies that the Au/Pd thermocouple possesses characteristics which may make it suitable for use as a secondary high-temperature thermometer in industry.

Measurement of the Seebeck coefficients of binary Cu-Ni alloys

In order to identify materials suitable as a compensating wire pair for the Pt/Pd thermocouple, binary Cu100−xNix (x = 3, 5, 8 and 10 wt%) alloys were prepared and their Seebeck coefficients were measured using the temperature gradient method from room temperature to approximately 120 °C using a pure platinum reference material. The Cu–Ni alloys showed negative Seebeck coefficients at all test temperatures. As the temperature increased, the Seebeck coefficients of the Cu–Ni alloys became more negative. At a fixed temperature, the Seebeck coefficients became more negative as the Ni content increased. Among the test alloys, Cu95Ni5 and Cu90Ni10 (designated as CuNi5/10) both showed a calculated thermal emf value that was very similar to that of the Pt/Pd thermocouple. It was concluded that the CuNi5/10 system could be a good compensating material for the Pt/Pd thermocouple, and by a small adjustment of the Ni content the temperature error could be reduced even further.