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Frequencies of acoustic and electromagnetic resonances were measured in a quasispherical acoustic resonator at the temperature of the triple point of water. The resonator was filled with helium 4He. Based on the analysis of experimental data, a value for the Boltzmann constant of kB = 1.380651·10–23 J·K–1 with uncertainty of 2.4 ppm was obtained.

We consider a method of transferring the temperature of the triple point of water with an acoustic resonator configuration for measuring the Boltzmann constant. It is shown that the transfer of the temperature of the triple point of water in a resonator using a highly sensitive differentiator does not require the use of absolute means of measurement and implemented with uncertainty limited only by thermal noise. The method enables reducing uncertainty in the temperature transfer when measuring the Boltzmann constant, whose accurate measurement is necessary for transition to a new definition of the unit of temperature.

There are multiple ways to increase the accuracy of temperature measurement by reducing uncertainties of temperature scale realization by means of standard platinum resistance thermometers (SPRT) calibrated at the defining fixed points (DFPs). This paper demonstrates the possibility of reducing uncertainty of temperature scale realization by taking into account correlations between the SPRT resistances at the DFPs in calibration and temperature measurements. The BIPM document Uncertainties in the Realisation of the SPRT Subranges of the ITS-90 issued in 2009 provides an overview of the different approaches. In most cases, SPRT resistances at DFPs were considered as uncorrelated. Only correlations caused by using the same SPRT resistance at the triple point of water (TPW) in calibration and temperature measurement were taken into account. Furthermore SPRT resistances at the DFPs can be correlated with respect to some of the same conditions in the laboratory for SPRT resistances measured at the DFPs. This paper demonstrates that the uncertainty of temperature scale ITS-90 realization is unaffected by errors originating form SPRT calibration and temperature measurements when all the measured resistances are measured with the same error. Furthermore, it is shown that the use of correlation can reduce the uncertainty of temperature scale realization, and dealing with this problem will bring improvement in the field of uncertainty evaluation. Formulas for the calculation of uncertainties of temperature scale realization are presented, together with the correlation between SPRT resistances and simulations that show the impact of correlation on the uncertainty of the temperature scale realization.

A thermodynamic description of a model for constructing a practical temperature scale based on Langevin paramagnetics is presented. A Carnot cycle with the paramagnetic as the working substance is described. A formula connecting the change in temperature with the magnetization and magnetic field strength is derived.

A new approach to the construction of a radiation temperature scale and the determination of the Boltzmann constant is considered, based on the derivation of formulas for determining the heat flux density from the equations of thermodynamics. The use of the total energy density of equilibrium radiation of a cell of water at the triple point and the recording of this radiation by an absolute cryogenic radiometer is proposed.

The triple point of water is the most important fixed point, both used as the defining fixed point for the definition of the Kelvin and also the pivotal fixed point for the realization of the International Temperature Scale of 1990 (ITS-90). As such, obtaining the best performance from the water triple point is vital. This comparative study focuses on the benefits of using long bushes (that cover most of the length of the thermowell) for improving the repeatability of the immersion profile for water triple point cells. It is shown that the use of long bushes helps by improving a) the thermal contact between the thermometer and the ice sheath and b) the repeatability of locating the thermometer inside the thermowell. To investigate the reproducability of the immersion profile for triple point of water cells, two different types of Standard Platinum Resistance Thermometers (SPRTs) have been used. The SPRTs have different designs, different length of sensing element and different self heating effects. These experiments were carried out using the same water triple point cell to identify thermometer specific effects. To measure the immersion profile, the following factors have to be taken into account: a) the effect of hydrostatic pressure, since the measurements are made at different depths below the water surface; b) the effect of heat flux along the thermometer stem; c) the need of long thermal stabilization at every immersion depth. The experimental results reported here show that repeatability in the determination of immersion profiles was significantly improved (by a factor of at least two) through the use of long bushes and when measured with a SPRT with a low self-heating effect.

An equation is obtained for calculating the compressibility factor and saturated vapor density of n-alkanes (C1–C10), parahydrogen, and water at temperatures of the triple point up to critical point. The calculation error is close to that for experimental data.

The effect of long-term natural variations in the isotopic composition on the temperature of the reproduction of the triple point of water – the main reference point of the ITS-90 International Temperature Scale – is investigated.

Systems for realizing the fixed points of the ITS-90 for calibrating column and capsule standard platinum thermometers, namely, the triple points of argon and mercury and the melting point of gallium, are constructed and investigated. The errors of the values of the metrological characteristics of the systems obtained enable one, using platinum resistance thermometers, to reproduce and transfer the temperature scale in the 83.8–302.9 K range. The extended uncertainty in reproducing the temperatures of the fixed points does not exceed 0.4 mK.

Apparatus for reproducing the unit of absolute pressure by a thermodynamic method in the 133 – 1·105 Pa range is described. Argon, nitrogen and oxygen are used as the working materials. The pressure is reproduced at triple points and also on the p–T-equilibrium curves of two phases. The processes of attaining a specified point on the p–T-curve and of stabilizing the process at this point are automated using the AKSAMIT measurement-control device. The relative error in reproducing the pressure is 0.01% of the reproduced value.

A method of replacing the triple point of mercury by the melting point of gallium when reproducing the ITS-90 International Temperature Scale is considered. A method of estimating the systematic error and of determining the conditions under which the method can be used is proposed.

Platinum oxidation in PTS-10 and PTS-25 standard platinum resistance thermometers has been studied. The experimental results show that heat treatment of a thermometer at 100–300°C can increase its resistance at the triple point of water by the equivalent of 1.5 mK. A procedure for calibration in the range 0–450°C is recommended.

IMGC has established the fixed points required by the International Temperature Scale of 1990 (ITS-90) for long-stem platinum resistance thermometers (PRTs) at the highest accuracy level. The present paper describes the fixed point apparatus and the measuring procedures used at IMGC for the accurate realization of each fixed point, and gives some data on the typical phase transitions thereby obtained. Moreover, since IMGC has developed both traditional and sealed cells for the realization of the freesing or melting points of metals, the different types of cells and the procedure used for their construction and preparation are also supplied. Finally, the reproducibilities of the freezing points of Sn, Zn, Al, and Ag were evaluated by means of several determinatiaons with one standard thermometer.

The adoption of the international temperature scale of 1990 (ITS-90) is producing significant innovation in thermal metrology. The temperature range of the ITS is wider than that of the previous editions. Also the scale precision and the accuracy of the thermodynamic temperature values forming its base have been improved. A short presentation of the ITS-90 will introduce its most important features, describing briefly the types of interpolating instruments and fixed points. To realize the ITS-90, many national laboratories will have to develop within their premises relatively new techniques of measurement such as gas thermometry to interpolate between fixed points from 3 K and 24.6 K and high-temperature platinum resistance thermometry. The highest precision is acheived in the temperature range −40°C to 30° C, where the ITS-90 combines the most reproducible fixed points with the interpolating instrument exhibiting the highest accuracy and the lowest nonuniqueness. Since a reproducibility better than 0.2 mK can be achieved everywhere in that range, except in the proximity of the triple point of water where it is lower than 0.1 mK (all estimates being at the 1σ level), the size of the kelvin is reporoduced to within 7·10−6 in the worst case, and to within 2·10−7 at the triple point of water. Some practical problems may result from the dissemination of the iTS-90 to users of temperature measurements in the temperature range 420–1085°C. They are related to the construction and use of high-temperature platinum resistance thermometers (HTPRTs), to the practical exploitation of radiation thermometry, and to the availabiality of suitable transfer standards in this temperature range. Several practical realizations of the ITS-90 that are underway in various national laboratories will enable one to better estimate its precision. Such an operation will essentially be carried out through intercomparisons, which will be successful if highly-reproducible transfer standards like the sealed cells for fixed points will be made available. The basic level of the scale precision, i.e., its irreducible component, is given by the scale nonuniqueness. Some results are already available and some experiments have recently been proposed in order to provide further data in critical areas. Another important aspect is the scale smoothness and its effect on the measured thermal properties. With the introduction of the ITS-90 the projects for the determination of thermodynamic properties have not completely stopped. Radiometric determinations and noise thermometry are mainly carried out at temperatures between 660°C and 1085°C. Other experiments involving gas thermomery may lead to better determinations below 3 K. Thermal noise measurements are presently considered for the redetermination of the Boltzmann constant, kB. Their uncertainty should be within ± 10ppm, still too high to propose such a method for the definition of the kelvin in terms of kB and of the SI unit of energy. As regards the determination of thermophysical properties of matter, special importance is now attributed to the temperature dependence of the vapor pressure of sodium. Such an interest in connection with the use of pressure-controlled sodium heat pipes for temperature reference. In addition, some information is provided on methods for the determination of pulsed-energy excitation. There, advantage is taken from improvements in radiation thermometry.

The SI units are employed [i] together with the international practical temperature scale IPTS-68. The triple point of water is Ttr = 273.16K, while the temperature is in degrees Celsius t = T--To with To = 273.15 K; when using IPTS-68 one employs the relation tee = T68 -To. The deviations of the international practical temperature T68 from the thermodynamic temperature T are given in [2]. In the range from --40 to + 100~ the maximum deviation is T68 -T = 0.025 K. The atmospheric pressure under normal conditions is Po = 101325 N/M 2 = 1013.25 hPa. Pressures in pascal are too large, so the World Meteorological Organization (WMO) has suggested [3] that the hectopascal should be generally used (I hPa = i00 Pa = 1 mbar).

1) For the study of the reproducibility of the change of phase temperatures of solid oxygen a calorimetric method was suggested and a simplified vacuum calorimeter was made with a compensation of heat losses, thus providing a temperature variation in the calorimeter < 0.001–0.0006°/min. 2) By means of this technique the phase-change temperatures suitable for thermometry can be produced with an error up to 0.004°. 3) Numerical values for change of phase temperatures of solid oxygen and at the triple point were obtained in terms of the practical VNIIFTRI scale. The numerical values in degrees of the thermodynamic scale are determined with the accuracy of the formula, which accounts for the possible difference between the practical and thermodynamic scales, and the possible variation in the boiling temperature of oxygen.

The TSPN-3 differs from the TSPN-I [3] in having improved stability over a wide temperature range (up to IO0~ while having the same dimensions. This has been attained by using comparatively thick platinum wire and mounts of a new design. The spiral is made of PL-O thermometer-grade platinum, while the holder consists of glass-coated platinum rods (the high quality of the PL-O platinum has meant that chemical-grade glass has been used in the holder). The rods lie parallel around a circle of diameter 3 mm, and the ends are brought together and welded. The number of rods is varied to suit the diameter of the platinum wire and the required sensitivity. The spiral is mounted on these without welding. The gap between the spiral and the rods is about 0.02 mm. Each thermometer is hermetically sealed and has a four-wire connection system. The design completely eliminates any displacement or deformation of the platinum spiral on acceleration perpendicular to the axis, as well as strain in the spiral in response to changing temperatures. The mechanical strength of this thermometer under acceleration along the axis is higher than that of the thermometer described in [4, 5], which has been attained by reducing the length and diameter of the spiral. For example, a TSPN-3 with Ro= i00 ~ has the diameter of the platinum spiral twice that used in the thermometers of [4-6], while the length is reduced by a factor of 3, although the latter thermometers were made from platinum wire of the same diameter. This means that the strength of a TESPN-3 thermometer has been improved by about afactor of 5, while the length has been halved. In addition, the spiral can be attached to the glass-coated rods with lacquer, which makes the thermometer insensitive to vibration. Preliminary tests have shown that the drift in the TSPN-3 thermometers is not more than 3"I0-3~ and that they can be used as first-class standards [I]. The scope for using the TSPN-3 in thermometric standardization was examined by making 13 thermometers from PL-O platinum: eight of them were TSPN-3 and five were TSPN-I. The latter employed platinum wire of diameter 0.07 mm, whereas the former used wires of diameters 0.05, 0.07, and 0.092 mm. The thermometers were annealed at about 500~ before filling with the heat-transfer gas (helium or neon). The nominal values of the resistance Ro at 0~ varied with the design, the spiral diameter, and the platinum wire used; the ratio of R,oo to Ro (where the former is for t = 100~ was in the range 1.392562-1.392574. These different thermometers were compared for stability during tests under identical conditions. First the stability was tested at the triple point of water. The resistance was measured by the method of [7] with an R-348 potentiometer of class 2"10-s. * The sequence was as follows: two cooling--heating cycles over the range from --200 to +I00~ measurement at the triple point, 20 cooling--heating cycles, measurement at the triple point, �9 An error of 2"I0-S% in measuring the above nominal resistances corresponds to 176 and is largely dependent on the stability of the temperature of the potentiometer. Careful adjustment of the potentiometer has allowed the figure to be reduced to 5"i0-~% (176