International Temperature Scale Research Articles

Potential for improved radiation thermometry measurement uncertainty through implementing a primary scale in an industrial laboratory

A primary temperature scale requires realising a unit in terms of its definition. For high temperature radiation thermometry in terms of the International Temperature Scale of 1990 this means extrapolating from the signal measured at the freezing temperature of gold, silver or copper using Planck’s radiation law. The difficulty in doing this means that primary scales above 1000 °C require specialist equipment and careful characterisation in order to achieve the extrapolation with sufficient accuracy. As such, maintenance of the scale at high temperatures is usually only practicable for National Metrology Institutes, and calibration laboratories have to rely on a scale calibrated against transfer standards. At lower temperatures it is practicable for an industrial calibration laboratory to have its own primary temperature scale, which reduces the number of steps between the primary scale and end user. Proposed changes to the SI that will introduce internationally accepted high temperature reference standards might make it practicable to have a primary high temperature scale in a calibration laboratory. In this study such a scale was established by calibrating radiation thermometers directly to high temperature reference standards. The possible reduction in uncertainty to an end user as a result of the reduced calibration chain was evaluated.

A study on ITS-90 type 3 non-uniqueness between freezing points of Al and Ag

This work presents an investigation about the type 3 non-uniqueness of the International Temperature Scale of 1990 (ITS-90) between the aluminium (660°C) and silver (962°C) freezing points, carried out at the Italian National Institute of Metrology (INRiM) within the framework of the EMRP—JRP SIB10 NOTED (Novel Techniques for Traceable Temperature Dissemination). For this purpose six High Temperature Standard Platinum Resistance Thermometers (HTSPRTs) from different manufacturers were calibrated at the ITS-90 fixed points, then intercompared in a sodium Gas Controlled Heat Pipe (GCHP) at 25K intervals between the aforementioned fixed points; a temperature uniformity and stability within ±0.5mK was achieved in the whole range between 660°C and 962°C. This paper presents the results of type 3 non-uniqueness analysis of these test HTSPRTs, together with problems deriving from their stability, which is the largest source of uncertainty in this work.

The triple point of sulfur hexafluoride

A cryogenic fixed point cell has been filled with high purity (99.999%) sulfur hexafluoride (SF6) and measured in an adiabatic closed-cycle cryostat system. Temperature measurements of the SF6 melting curve were performed using a capsule-type standard platinum resistance thermometer (CSPRT) calibrated over the International Temperature Scale of 1990 (ITS-90) subrange from the triple point of equilibrium hydrogen to the triple point of water. The measured temperatures were corrected by 0.37 mK for the effects of thermometer self-heating, and the liquidus-point temperature estimated by extrapolation to melted fraction F = 1 of a simple linear regression versus melted fraction F in the range F = 0.53 to 0.84. Based on this measurement, the temperature of the triple point of sulfur hexafluoride is shown to be 223.555 23(49) K (k = 1) on the ITS-90. This value is in excellent agreement with the best prior measurements reported in the literature, but with considerably smaller uncertainty. An analysis of the detailed uncertainty budget of this measurement suggests that if the triple point of sulfur hexafluoride were to be included as a defining fixed point of the next revision of the International Temperature Scale, it could do so with a total realization uncertainty of approximately 0.43 mK, slightly larger than the realization uncertainties of the defining fixed points of the ITS-90. Since the combined standard uncertainty of this SF6 triple point temperature determination is dominated by chemical impurity effects, further research exploring gas purification techniques and the influence of specific impurity species on the SF6 triple point temperature may bring the realization uncertainty of SF6 as a fixed point material into the range of the defining fixed points of the ITS-90.

Progress towards the determination of thermodynamic temperature with ultra-low uncertainty.

Previous research effort towards the determination of the Boltzmann constant has significantly improved the supporting theory and the experimental practice of several primary thermometry methods based on the measurement of a thermodynamic property of a macroscopic system at the temperature of the triple point of water. Presently, experiments are under way to demonstrate their accuracy in the determination of the thermodynamic temperature T over an extended range spanning the interval between a few kelvin and the copper freezing point (1358 K). We discuss how these activities will improve the link between thermodynamic temperature and the temperature as measured using the International Temperature Scale of 1990 (ITS-90) and report some preliminary results obtained by dielectric constant gas thermometry and acoustic gas thermometry. We also provide information on the status of other primary methods, such as Doppler broadening thermometry, Johnson noise thermometry and refractive index gas thermometry. Finally, we briefly consider the implications of these advancements for the dissemination of calibrated temperature standards.

Thermodynamic temperature by primary radiometry.

Above the freezing temperature of silver (1234.93 K), the International Temperature Scale of 1990 (ITS-90) gives a temperature, T90, in terms of a defining fixed-point blackbody and Planck's law of thermal radiation in ratio form. Alternatively, by using Planck's law directly, thermodynamic temperature can be determined by applying radiation detectors calibrated in absolute terms for their spectral responsivity. With the advent of high-quality semiconductor photodiodes and the development of high-accuracy cryogenic radiometers during the last two decades radiometric detector standards with very small uncertainties in the range of 0.01-0.02% have been developed for direct, absolute radiation thermometry with uncertainties comparable to those for the realization of the ITS-90. This article gives an overview of a number of design variants of different types of radiometer used for primary radiometry and describes their calibration. Furthermore, details and requirements regarding the experimental procedure for obtaining low uncertainty thermodynamic temperatures with these radiometers are presented, noting that such radiometers can also be used at temperatures well below the silver point. Finally, typical results obtained by these methods are reviewed.

The kelvin redefinition and its mise en pratique.

In 2018, it is expected that there will be a major revision of the International System of Units (SI) which will result in all of the seven base units being defined by fixing the values of certain atomic or fundamental constants. As part of this revision, the kelvin, unit of thermodynamic temperature, will be redefined by assigning a value to the Boltzmann constant k. This explicit-constant definition will define the kelvin in terms of the SI derived unit of energy, the joule. It is sufficiently wide to encompass any form of thermometry. The planned redefinition has motivated the creation of an extended mise en pratique ('practical realization') of the definition of the kelvin (MeP-K), which describes how the new definition can be put into practice. The MeP-K incorporates both of the defined International Temperature Scales (ITS-90 and PLTS-2000) in current use and approved primary-thermometry methods for determining thermodynamic temperature values. The MeP-K is a guide that provides or makes reference to the information needed to perform measurements of temperature in accord with the SI at the highest level. In this article, the background and the content of the extended second version of the MeP-K are presented.

Estimates of the difference between thermodynamic temperature and the International Temperature Scale of 1990 in the range 118 K to 303 K.

Using exceptionally accurate measurements of the speed of sound in argon, we have made estimates of the difference between thermodynamic temperature, T, and the temperature estimated using the International Temperature Scale of 1990, T90, in the range 118 K to 303 K. Thermodynamic temperature was estimated using the technique of relative primary acoustic thermometry in the NPL-Cranfield combined microwave and acoustic resonator. Our values of (T-T90) agree well with most recent estimates, but because we have taken data at closely spaced temperature intervals, the data reveal previously unseen detail. Most strikingly, we see undulations in (T-T90) below 273.16 K, and the discontinuity in the slope of (T-T90) at 273.16 K appears to have the opposite sign to that previously reported.

A SQUID-based primary noise thermometer for low-temperature metrology.

Practical temperature measurements in accordance with the international system of units require traceability to the international temperature scales currently in force. Along with the awaited redefinition of the unit of temperature, the kelvin, on the basis of the Boltzmann constant, in future its mise en pratique will allow the use of approved methods of primary thermometry for the realization and dissemination of the kelvin. To support this process, we have developed a DC superconducting quantum interference device-based noise thermometer especially designed for measurements of thermodynamic temperature in a broad temperature range from 5 K down to below 1 mK. In this paper, we describe in detail the primary magnetic field fluctuation thermometer and the underlying model applied for the temperature determination. Experimental measurement results are presented for a comparison with the Provisional Low Temperature Scale 2000 between 0.7 K and 16 mK including an uncertainty budget for the measured thermodynamic temperatures. In this set-up, the relative combined standard uncertainty is equal to 0.6%.

Use of a phenomenological chemical scale for the identification of high distribution coefficient impurities within the ITS-90

The International Temperature Scale of 1990 standardises practical temperature measurement based on the reproducibility of the solidification phase transition temperature of highly pure metals. The effect of impurities can be the biggest block to establishing international equivalence of temperature metrology and so it is important to have knowledge of the distribution coefficients for all potential impurities. In particular impurities with a high distribution coefficient cause problems with applying corrections, and so it is helpful to identify which impurities are likely to be an issue. By plotting the published measured distribution coefficients of impurities in the metals used as references in the International Temperature Scale of 1990, as a function of a quantum mechanical based scale intended to provide separation of binary alloy crystal structure, an apparent resonance peak is found. This could allow a simple fit equation to be applied to determine whether a given impurity is likely to cause an error to any correction strategy.

The use of computational fluid dynamics to study furnace effects in ITS-90 fixed points realizations

Together with the impurities, the thermal fluxes are one of the major sources of uncertainty during the realization of the International Temperature Scale of 1990 (ITS-90) defining fixed points. The use of computational fluid dynamics (CFD) is a valuable tool to develop models that describe the time evolution of the phase transformation (essentially the evolution of the solid–liquid interface) as a function of given theoretical assumptions and given parameters (furnace thermal gradients, freezing initiation, ambient temperature and insulation). The models can be validated by observing the impact of the selected parameters on the observed corresponding melting curves and used to achieve a full understanding of these thermal effects and their impact on uncertainty. This paper proposes an ITS-90 metallic fixed points CFD model together with some results about the influence of the furnace thermal gradients and the freezing initiation techniques.

Report on NIM-NMC bilateral comparison: SPRT calibration comparison from −190°C to 420°C

A bilateral comparison of local realization of the International temperature scale of 1990 (ITS-90) between National Institute of Metrology (NIM) and National Metrology Centre (NMC) was carried out over the temperature range from -190°C to 420°C. It involved six fixed points including the argon triple point, the mercury triple point, the triple point of water, the melting point of gallium, the freezing point of tin and the freezing point of zinc. In 2009, NMC asked NIM to participate in a bilateral comparison to link the NMC results to the Consultative Committee for Thermometry Key comparison 3 (CCT-K3) and facilitate the NMC's Calibration and measurement capabilities (CMCs) submission. This comparison was agreed by NIM and Asia Pacific Metrology Programme (APMP) in 2009, and registered in the Key Comparison Database (KCDB) in 2010 as CCT-K3.2. NMC supplied two 25 Ω fused silica sheath standard platinum resistance thermometers (SPRTs) as traveling standards. One of them was used at the Ga, Sn and Zn fixed points, while the other one was used at the Ar and Hg fixed point. NMC measured them before and after NIM measurement. During the comparison, a criterion for the SPRT was set as the stability at the triple point of water to be less than 0.3 mK. The results for both laboratories are summarized. A proposal for linking the NMC's comparison results to CCT-K3 is presented. The difference between NMC and NIM and the difference between NMC and the CCT-K3 Average Reference Value (ARV) using NIM as a link are reported with expanded uncertainties at each measured fixed point.Main textTo 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 (CIPM MRA).

Advances in thermometry.

The last 25 years have seen tremendous progress in thermometry in the moderate temperature range (1 K to 1235 K). Various primary thermometers - based on different physics -have uncovered errors in the International Temperature Scale of 1990 and set the stage for the planned redefinition of the kelvin.

The influence of some relevant metallic impurities in the triple point of mercury temperature

The influence of impurities on the defined fixed points of the International Temperature Scale of 1990 is one of the most relevant sources of uncertainty in their use. Namely, the triple point of mercury is one of the fixed points most used because its assigned triple point temperature (−39,8344 °C) is within a temperature range of high interest for science and industry. Mercury can be obtained with high purity, usually better that 8N and it is realized with a high accuracy, what makes impurities a significant contributor on the final uncertainty. The best procedure to quantify this influence is by means of doping experiments which consist of the addition of a controlled amount of an impurity, determined gravimetrically, in a fixed point cell. This paper presents a summary of the results obtained from a series of doping experiments performed with some relevant metallic impurities on the triple point of mercury temperature.

Fabrication and Determination of an Aluminium Fixed Point Cell as a Secondary Standard in Thermocouple Calibration

A new open-type aluminium (Al) fixed-point cell has been fabricated in Puslit Metrologi-LIPI. The cell was successfully characterized by direct comparison with a commercial Al cell. The cell plateau lasted for four hours with a flatness of 0.014 °C, measured as the difference between the beginning and the end of the plateau using a type S thermocouple. The reproducibility of realizations is 0.04 °C; the immersion profile of the cell was measured for 30 mm depth, from the bottom of the cell, and the variation was found to be 0.02 °C. A good agreement within 0.16 °C was measured at the freezing point. Furthermore, the cell improves the entry in Appendix C, calibration and measurement capability (CMC) by 0.24 °C, allowing the cell to be included as a secondary standard of the international temperature scale of 1990 realization in Puslit Metrologi-LIPI.

Effect of bushings in thermometric fixed-point cells

Bushings are tubular inserts that can be used with temperature fixed-point cells for the calibration of standard platinum resistance thermometers in accordance with the International Temperature Scale of 1990. They are made out of different materials such as glass and various metals with high thermal conductivities. Their function is to increase the thermal contact between the thermometer’s sensor and the phase boundary in the thermometer well of the cell. In the paper, the effect of bushings on the self-heating and on the immersion profiles of thermometers in fixed-point cells was researched. Three different thermometers were used: one fused silica sheathed and two Inconel sheathed. The self-heating was measured in the temperature range from the mercury triple-point to the zinc freezing-point with and without metal bushings: brass, aluminum and copper. The immersion profiles were measured in the zinc cell with and without an aluminum and a glass bushing. Besides experimental measurements, some qualitative results of numerical modeling are also presented. The bushings considerably lower the self-heating, in some cases even by more than 50%. But, on the other hand, bushings seem to increase the measured temperature, worsen the immersion profile and act as a radiation shield in the gap between the thermometer and the fixed-point cell at higher temperatures, where radiation is the dominant mode of heat transfer.

Infrared cameras are potential traceable “fixed points” for future thermometry studies

The National physical laboratory (NPL) requires “fixed points” whose temperatures have been established by the International Temperature Scale of 1990 (ITS 90) be used for device calibration. In practice, “near” blackbody radiators together with the standard platinum resistance thermometer is accepted as a standard. The aim of this study was to report the correlation and limits of agreement (LOA) of the thermal infrared camera and non-contact infrared temporal thermometer against each other and the “near” blackbody radiator. Temperature readings from an infrared thermography camera (FLIR T650sc) and a non-contact infrared temporal thermometer (Hubdic FS-700) were compared to a near blackbody (Hyperion R blackbody model 982) at 0.5 °C increments between 20–40 °C. At each increment, blackbody cavity temperature was confirmed with the platinum resistance thermometer. Measurements were taken initially with the thermal infrared camera followed by the infrared thermometer, with each device mounted in turn on a stand at a fixed distance of 20 cm and 5 cm from the blackbody aperture, respectively. The platinum thermometer under-estimated the blackbody temperature by 0.015 °C (95% LOA: −0.08 °C to 0.05 °C), in contrast to the thermal infrared camera and infrared thermometer which over-estimated the blackbody temperature by 0.16 °C (95% LOA: 0.03 °C to 0.28 °C) and 0.75 °C (95% LOA: −0.30 °C to 1.79 °C), respectively. Infrared thermometer over-estimates thermal infrared camera measurements by 0.6 °C (95% LOA: −0.46 °C to 1.65 °C). In conclusion, the thermal infrared camera is a potential temperature reference “fixed point” that could substitute mercury thermometers. However, further repeatability and reproducibility studies will be required with different models of thermal infrared cameras.

Comparison of extrapolated and interpolated temperature scales from 1000 °C to 2500 °C between a national measurement institute and an ISO17025 accredited calibration laboratory

The current high temperature part of the international temperature scale of 1990 (ITS-90) is based on extrapolation from a reference fixed point, using Plancks law in ratio form. However in the near future the mise en pratique for the definition of the kelvin will also allow an interpolated thermodynamic scale based on high temperature references whose temperatures are known a priori. Both approaches to dissemination will be considered equally valid. A transfer radiation thermometer specifically designed with the dissemination to industry of a temperature scale based on the mise en pratique-kelvin approach has been constructed. It was calibrated at a national measurement institute and used to compare the two possible realisation schemes (i.e. radiation thermometer calibrated to ITS-90, and the same thermometer calibrated using high temperature references) to the local ITS-90 scale at an accredited industrial calibration laboratory. All the scales agreed within their respective uncertainties. It was found that the interpolation scheme could significantly reduce the uncertainties in a calibration laboratory without the full cost of maintaining ITS-90.

Dielectric-constant gas thermometry

The principles, techniques and results from dielectric-constant gas thermometry (DCGT) are reviewed. Primary DCGT with helium has been used for measuring T–T90 below the triple point of water (TPW), where T is the thermodynamic temperature and T90 is the temperature on the international temperature scale of 1990 (ITS-90), and, in an inverse regime with T as input quantity, for determining the Boltzmann constant at the TPW. Furthermore, DCGT allows the determination of several important material properties including the polarizability of neon and argon as well as the virial coefficients of helium, neon, and argon. With interpolating DCGT (IDCGT), the ITS-90 has been approximated in the temperature range from 4 K to 25 K. An overview and uncertainty budget for each of these applications of DCGT is provided, accompanied by corroborating evidence from the literature or, for IDCGT, a CIPM key comparison.

Development of a Standard Platinum Resistance Thermometer for Use up to the Copper Point

The international temperature scale of 1990 defines temperatures in the range from 13.8 K to 1234.93 K (\(961.78\,^{\circ }\hbox {C}\)) using a standard platinum resistance thermometer (SPRT) as an interpolating instrument. For temperatures above \(660\,^{\circ }\hbox {C}\), the current designs of an SPRT require extreme care to avoid contamination, especially by metallic impurities, which can cause rapid and irreversible drift. This study investigates the performance of a new design of a high-temperature SPRT with the aim of improving the stability of the SPRTs and extending their temperature range. The prototype SPRTs have an alumina sheath, a sapphire support for the sensing element, which are aspirated with dry air and operated with a dc bias voltage to suppress the diffusion of metal-ion contaminants. Three prototype thermometers were exposed to temperatures near or above the copper freezing point, \(1085\,^{\circ }\hbox {C}\), for total exposure times in excess of 500 h and exhibited drifts in the triple-point resistance of less than 10 mK. The new design eliminates some of the problems associated with fused-silica sheaths and sensor-support structures and is a viable option for a high-accuracy thermometer for temperatures approaching \(1100\,^{\circ }\hbox {C}\).

Effect of changes in temperature scales on historical temperature data

ABSTRACTTemperature has always been a central quantity in meteorology and plays a key role in weather forecast and climate determination. Long temperature series are a useful tool both as a direct indicator of global climatic trends and as input to climate models. However, ensuring the quality of the data records is challenging, with issues arising from the wide range of sensors used, how the sensors were calibrated, and how the data were recorded and written down. In particular, the very definition of the temperature scales has evolved. While the temperature scales have always been tied to phase transitions of substances such as the freezing and boiling points of water, the temperatures assigned to those phase transitions have been revised occasionally when new sensors and measuring techniques suggested discrepancies between the temperature scales and thermodynamic temperature. Invariably this has led to alterations in the reference scales of temperature that give rise to biases when compiling historic temperature records, an issue that appears to be neglected in the meteorological literature. The present work deals with this issue for 20th century data by proposing a mathematical model to allow the conversion from historical scales to the International Temperature Scale of 1990 (ITS‐90). This work also presents the implementation of this mathematical model into a software tool, which can convert large files of historical records to the ITS‐90. The correction equation is applied to example observations, and it is found that the correction becomes an issue in special cases involving aggregated temperature data. However, the corrections are significantly smaller than the global warming trend seen in the 20th century. The corrections applied have shown a level of change, which has not significantly affected the global warming trend.