Fixed-point Cells Research Articles

Final report for CCT comparison of ITS-90 realisations above the silver point using two transfer radiation thermometers and a set of high temperature fixed-point blackbody cells

Main textThis report outlines the equipment, measurement method, results and uncertainties associated with the participant measurements for the Consultative Committee of Thermometry Key Comparison K10 (CCT-K10), "ITS-90 realisations above the silver point using two transfer radiation thermometers and a set of high temperature fixed-point blackbody cells", over the period from around summer 2014 to the final measurements made during January 2020. The report presents differences of the participant data from the KCRV values for both the radiation thermometer and HTFP measurements. Note: this interim report excludes the measurement data of VNIIM, Russia - see explanation at the end of Section 6.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 https://www.bipm.org/kcdb/.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).

Large-area Fe–C eutectic fixed-points for radiation and contact thermometry

Abstract High-quality metal (carbide)–carbon eutectic materials based on high-temperature fixed points (HTFPs) are widely used in radiometry and thermometry as reference standards. HTFPs on the base of iron–carbon (Fe–C) binary eutectic alloys, with a nominal melting temperature of about 1154 °C (just above the copper freezing point of 1084.62 °C), are one of the promising candidates among the eutectic materials. To establish new HTFPs as reference metrological tools for high-temperature thermometry, their performance should be thoroughly investigated regarding reproducibility and stability. In this work, two large-area (8 mm aperture, 107 mm cavity/thermowell length) Fe–C fixed-point cells were constructed and studied in detail using a radiation thermometer and two different thermocouples (TCs). Three different furnaces were used to explore the thermal behaviors of the cells at various furnace gradients and furnace offsets. The melting temperature at the inflections point of the melting curves of the cells studied across extensive measurement campaigns demonstrated good performance with repeatability of less than 9 mK (assessed from four successive runs) and reproducibility—less than 100 mK (at different furnaces and furnace offsets). The melting temperature agreement between both cells in the same experimental conditions was better than 30 mK. In addition, the equivalence of the developed large-area cells and a small-area radiometric cell (3 mm cavity aperture, and 35 mm cavity length) were comparatively examined in the same experimental conditions. The coherence of the obtained results for the melting temperature of large-area Fe–C cells indicates the feasibility of using large-volume cells for precise calibration of both radiation thermometers and TCs.

Pyroreflectometry as a technique for the accurate measurement of very high temperatures in molten materials.

Experimental research into severe nuclear accidents often requires the accurate measurement of high temperatures of molten materials. Measurements of very high temperatures (1500-2500 °C) in liquid materials using standard pyrometry can entail uncertainties in the order of 5%-10%. Pyroreflectometry is a powerful technique with the potential to significantly reduce these uncertainties. A method is proposed to optimize pyroreflectometry temperature measurements in the 1500-2500 °C range and to allow more easily the detection of the solid-liquid phase transition. The originality of this research essentially relies on the use of pyroreflectometry based on two wavelengths (1.3 and 1.55μm) and its application to liquid materials at high temperature, which implies to adapt technological elements and metrological procedures. The proposed procedure first requires temperature calibration, which is undertaken using three eutectic fixed-point cells, reducing temperature uncertainty. Second, precise settings are adopted to enable reflectivity measurements on specular surfaces, such as the surfaces of molten metals. Pyroreflectometry measurements on liquid surfaces have been validated on an iron sample. Subsequently, the application of pyroreflectometry at very high temperatures was validated on various materials: metal (iron and 18MND5 steel), oxide (alumina), and carbide (rhenium-carbon eutectic). For each of these samples, the uncertainties of temperature measurements in the 1500-2500 °C range were estimated in the range of 1%-2%, performing well against standard pyrometry measurements. The principal difficulties encountered during the pyroreflectometry characterization were the fine-tuning of parameters (optical head orientation and lens focusing) to enable measurements on highly specular surfaces and ensuring inert interactions between the samples and the crucible.

Metrological Characterization of a High-Temperature Hybrid Sensor Using Thermal Radiation and Calibrated Sapphire Fiber Bragg Grating for Process Monitoring in Harsh Environments.

Fiber Bragg gratings inscribed in single crystalline multimode sapphire fibers (S-FBG) are suitable for monitoring applications in harsh environments up to 1900 °C. Despite many approaches to optimize the S-FBG sensor, a metrological investigation of the achievable temperature uncertainties is still missing. In this paper, we developed a hybrid optical temperature sensor using S-FBG and thermal radiation signals. In addition, the sensor also includes a thermocouple for reference and process control during a field test. We analyzed the influence of the thermal gradient and hotspot position along the sensor for all three detection methods using an industrial draw tower and fixed point cells. Moreover, the signal processing of the reflected S-FBG spectrum was investigated and enhanced to determine the reachable measurement repeatability and uncertainty. For that purpose, we developed an analytical expression for the long-wavelength edge of the peak. Our findings show a higher stability against mechanical-caused mode variations for this method to measure the wavelength shift compared to established methods. Additionally, our approach offers a high robustness against aging effects caused by high-temperature processes (above 1700 °C) or harsh environments. Using temperature-fixed points, directly traceable to the International System of Units, we calibrated the S-FBG and thermocouple of the hybrid sensor, including the corresponding uncertainty budgets. Within the scope of an over 3-weeks-long field trial, 25 production cycles of an industrial silicon manufacturing process with temperatures up to 1600 °C were monitored with over 100,000 single measurements. The absolute calibrated thermocouple () and S-FBG () measurements agreed within their combined uncertainty. We also discuss possible strategies to significantly reduce the uncertainty of the S-FBG calibration. A follow-up measurement of the sensor after the long-term operation at high temperatures and the transport of the measuring system together with the sensor resulted in a change of less than 0.5 K. Thus, both the presented hybrid sensor and the measuring principle are very robust for applications in harsh environments.

On the uncertainties in the realization of the kelvin based on thermodynamic temperatures of high-temperature fixed-point cells

In July 2016 the Consultative Committee for Thermometry’s Non-Contact Thermometry Working Group formed a task group to compile a complete list of uncertainties for high-temperature fixed points, categorize them as well specified or not, and recommend areas of future research. We describe herein two paths to realizing T by indirect primary radiometry using high-temperature fixed-point blackbody cells: one in which published values for the metal–carbon eutectic material transition temperatures are used and the other where a set of cells has their transition temperatures determined directly. The uncertainty components that need to be considered for each path are given together with typically achievable values and how well those values are known. This work concentrates on Co–C, Pt–C and Re–C.

Metrological Evaluation of Deep-Ocean Thermometers

An accurate metrological investigation was performed on several units of the two de facto standards for deep-ocean temperature measurements—the SBE35 (the reference thermometer) and the SBE3 (the working thermometer) from SeaBird Scientific. Four SBE35 units were repeatedly calibrated against state-of-the-art fixed-point cells (triple point of water at 0.01 °C and melting point of gallium at 29.7646 °C), with calibration uncertainties of approximately 0.2 mK and 0.5 mK, respectively. Three SBE35 units and one SBE3 unit were calibrated in the temperature range 0 °C to 30 °C, again with sub-millikelvin calibration uncertainties, in a recently-developed water–bath calibration facility. All these calibrations evidenced (1) the deviation of each unit from its original manufacturer’s calibration (up to 1.7 mK), which were found to be inconsistent with the manufacturer’s uncertainty claims and (2) unexplained irreproducibilities, which could not be attributed to the calibration system of up to 1.5 mK. The effect of high pressures (up to 60 MPa) on the response of two SBE35 units was investigated by pressurizing the SBE35 units inside a purpose-built compact pressure enclosure. The results of the pressure investigation confirmed the existence of a small device-dependent pressure effect (approximately 0.3 mK at 60 MPa) and the need for individual temperature–pressure calibration of each SBE35 unit.

Metal-carbon eutectic high temperature fixed points for in-situ calibration of radiation thermometers

The diffusivimeter of LNE has been modified by improving the inductive furnace used to heat the tested specimens in order to extend the operating temperature range up to 3000 �C. The temperature of specimen is one of the tricky parameters to be measured to ensure the relevance of the thermal diffusivity measurement and the associated uncertainty. At high temperature, radiation thermometers are used to determine the temperature of the specimens at which the thermal diffusivity measurements are performed. In addition to the periodic calibration of the radiation thermometers performed outside the experimental facility with black body sources, LNE proposes an in-situ verification method based on miniature high temperature fixed-point cells filled with metal-carbon eutectic alloys in order to detect and correct potential drift of the radiation thermometers between two out-of-process calibration operations. The proposed method enables high repeatable and reproducible temperature measurements on eutectic fixed-points (Pd-C, Pt-C and Ir-C) distributed in the range from 1500 �C to 2300 �C.

Understanding immersion in zinc fixed-point cells using factorial design of experiments

The objective of this study was to investigate the ‘lift-off effect’ in zinc open cells, where an anomalously large change in temperature is observed when the standard platinum resistance thermometer (SPRT) is lifted off the bottom of the re-entrant tube during an immersion profile measurement. The goal was to determine the most important influences and hence what measures could be taken to mitigate it. The influences investigated were the filling gas, re-entrant tube material, re-entrant tube exterior surface condition, and the SPRT design. For each of these influence variables, three options were chosen. Since testing all combinations of different configurations arising would be extremely time consuming, the authors employed the optimal ‘design of experiments’ method using orthogonal arrays described by the Taguchi method, which is commonly used in manufacturing to optimise processes. Nine test configurations were identified, giving the necessary information on the interrelation between all influence variables. According to these results, the ideal configuration that yields the optimum immersion profile is a borosilicate tube, with full sandblasting and using nitrogen as filling gas for the cell. In addition, one particular SPRT design was found to be the least sensitive to the lift-off effect.

Development and Realization of Fe–C and Co–C Eutectic Fixed-Point Blackbodies for Radiation Thermometry at CSIR-NPL

A series of metal–carbon (M–C) eutectics with robust reproducible melting phase transitions are evolving as the reference high-temperature fixed-points up to 3000 °C for radiation thermometry. The mise-en-pratique prepared for the realization and dissemination of new kelvin allows the use of set of M-C blackbody fixed-points or a combination of M-C fixed-points along with silver (Ag), gold (Au) or copper (Cu) blackbodies for the direct measurement of thermodynamic temperatures at high-temperature range. The iron–carbon (Fe–C) [1153 °C] and cobalt–carbon (Co–C) [1324 °C] fixed-point cells are the doorway to high temperature above a copper fixed-point. This paper describes the in-house development of Fe–C and Co–C eutectic graphite crucible blackbodies and their realizations by linear spectral pyrometer (LP4, 650 nm) at CSIR-NPL, National Metrology Institute (NMI) of India. The novel design and assembly approaches were employed to fabricate the blackbody crucibles with 120o inner-well cone, 100 mm ingot length and 8 mm aperture diameter with 0.9997 emissivity and robust thermo-mechanical stability. The eutectic ingots were prepared by multiple fillings of the homogeneously ground and mixed powders with 4.2 and 2.6 weight percentage of carbon in Fe and Co, respectively. Repeatability of measured melting plateau and uncertainty of the temperature measurement for both Fe–C and Co–C cells are presented. The ITS-90 temperatures (T90) to these M-C fixed-points were assigned by a spectral radiance ratio measurement (Planck’s Law) relative to the freezing point of Ag [961.78 °C]. The T90 temperatures thus determined for Fe–C and Co–C blackbody fixed-point cells are 1154.05 °C and 1323.93 °C with the expanded uncertainty of 0.37 °C and 0.50 °C, respectively.

Calibration of standard radiation thermometers using two fixed points

Radiation thermometry based on the measurement of two fixed points, the so-called n = 2 scheme, is studied in this work. Verification of the scheme was undertaken for various measurement conditions using seven radiation thermometers with nominal centre wavelengths of 650 nm, 890 nm, 900 nm, and 1600 nm. Comparison between the signals calculated using the n = 2 scheme and the signals measured directly with fixed-point cells was studied. For temperatures between the two reference fixed points, the equivalent temperature difference was less than 0.14 K in the temperature range from the In to Cu fixed points, and less than 0.3 K in the temperature range from the Cu to Re–C fixed points. The maximum normalised error was 0.35, confirming the validity of the n = 2 scheme; the consistency was observed not only in the interpolation temperature range, but also in the extrapolation range. The results also indicate that temperature realisation in the range from 150 °C to 2500 °C can be achieved by maintaining only the Zn, Cu, and Re–C fixed-point facilities. The n = 2 scheme would be of great benefit to developing national metrology institutes in terms of time consumed for scale realisation and cost for maintenance of the measurement system.

Miniature fixed-point cells for self-calibration of type C high-temperature thermocouples

A high-temperature tungsten-rhenium (W-Re) thermocouple is commonly used at temperatures up to 2000 °C for the high melting temperature of the thermoelement materials. When exposed to high temperatures, a thermocouple can show significant thermoelectric drift, which increases measurement uncertainty. The uncontrolled drift may come from the change in the crystallographic structure, oxidization of the thermoelement materials, interaction between the thermoelement materials and insulator materials and some other unknown reasons. Therefore, periodic recalibration of the thermocouple should be performed; however, sometimes it is not possible to remove the sensor out of the process, especially in some special fields, such as a nuclear power plant. Self-validation methods for thermocouples provide a solution to avoid this drawback. In this paper, miniature eutectic fixed-point cells are presented for self-validation of W-Re thermocouples. To prevent the breakage of the graphite crucible, these cylindrical miniature fixed-point cells contain several small independent crucibles and one thermocouple well, which increase the robustness of the miniature cell. The melting temperature was assigned by a radiation thermometer traced to the primary radiation standard at NIM. The effect of the temperature offset and temperature ramping rate on the melting temperature were checked. The performance of the miniature fixed-point cells and type C thermocouple, including characterization of the stability and repeatability, is presented.

Immersion effects in zinc ITS-90 fixed-point cells

The freezing point of zinc (419.527 °C) is an important defining fixed point of the International Temperature Scale of 1990 (ITS-90). To realize it, a crucible of high purity graphite containing an ingot of high purity zinc is placed in a glass container containing an inert gas, with a central re-entrant tube, also glass, to allow insertion of a standard platinum resistance thermometer (SPRT). A key problem with these cells is difficulty in obtaining good immersion of SPRTs; it is commonly thought that the glass construction causes ‘light piping’ whereby the glass could facilitate heat transfer away from the region close to the SPRT sensing element by radiation, both directly and via reflections. In this paper it is shown that at the zinc freezing temperature light piping via reflections between the surfaces of the glass components is not an important heat transfer mechanism. It is also shown that light piping via transmission within the glass components is insignificant because of attenuation at wavelengths where the magnitude of the Planck function is important. In the case of the zinc fixed point, direct line-of-sight radiative heat transfer and thermal effects within the ingot appear to be the most important contributors to spurious immersion effects. It is shown that where the immersion profile is poor, a short platinum coil wrapped around the SPRT in the vicinity of the sensing element to improve radial conduction and to block vertical radiative heat transfer substantially improves the performance.

Modelling of thermal effects in a slim tin fixed-point cell

Slim fixed-point cells are an economic and suitable alternative for calibration of industrial platinum resistance thermometers, which are widely used in research and industrial applications. The most relevant disadvantage of slim cells is related with thermal effects produced by interactions between the cell and the furnace used for the realization of the fixed-points. This work presents the implementation of a two-dimensional numerical model that considers the transient heat transfer due to conduction, radiation and latent heat. The main goal was to evaluate different geometrical parameters in the design of slim fixed-point cells. Results show that thermal effects are mitigated when the annular distance between interior and exterior walls of the crucible is maximum, while the length of the cell and the thermo-well must be setup according to the temperature gradients in the furnace to avoid undesirable heat fluxes.

Integrated self-validating thermocouples with a reference temperature up to 1329 °C

Thermoelectric instability, or calibration drift, is a major problem for users of thermocouples at high temperature. NPL, in collaboration with CCPI Europe, AFRC and ICPE-CA/BRML-INM has designed, made and industrially tested Type S thermocouples with integrated temperature fixed-point cells. These in situ, self-validating thermocouples (denoted ‘inseva’) have the same external dimensions as conventional industrial thermocouples (the recrystallised alumina sheath has an outer diameter 7 of mm). The device can be used to detect the melting and/or freezing temperature of the integrated temperature fixed-point ingot, which enables a self-validation to be performed whilst in situ. During the testing, three different reference ingots were used in the cells, namely: copper (1084 °C), cobalt-carbon (1324 °C) and nickel-carbon (1329 °C). The metrological performance for two iterations of the design are presented, with an emphasis on the ability of the inseva thermocouples to indicate their own thermoelectric stability. A measurement uncertainty budget is also given for the case of a Ni–C inseva thermocouple. This paper demonstrates that inseva thermocouples can be successfully validated during industrial processes through the observation of the melting plateau, as well as their robustness over time in industrial conditions. A key finding is that the Ni–C eutectic alloy is much more robust than the Co–C eutectic alloy for the used type of graphite crucible, making the Ni–C inseva thermocouple more suitable for industrial applications, and a good alternative.

Aluminium fixed-point cells for thermodynamic temperature assignment

In the frame of the EMPIR-funded joint research project 15SIB02 “Implementing the new kelvin – 2” (InK2) the determination of T-T90 over the temperature range between 430 K and 1358 K with a target uncertainty around the 10 mK level is under preparation in several NMIs, among which LNE-Cnam and CEM. The two laboratories have joined their efforts for the thermal characterisation of a set of three aluminium fixed-point pyrometric cells. The cells were constructed using the hybrid design based crucibles and the piston filling method both developed by LNE-Cnam over the past decade. To study the effect of the temperature gradients on the freezing temperatures of the cells, two three-zone furnaces were used for the implementations of the cells with different temperature distributions. Finally, a first determination of the thermodynamic temperature of the freezing plateau was determined by extrapolation from the thermodynamic temperature of the copper point of LNE-Cnam. The results of the characterisations and the temperature determination are presented.

Validating the impurity component of high-temperature fixed-point uncertainty budgets with the Scheil solidification model

A high temperature fixed-point cell based on a melting or freezing transition can only be as good as the purity of the metal used to make it, and that purity can be hard to quantify. Least squares fitting of the Scheil solidification model has been applied to four cobalt-carbon and four platinum-carbon fixed point cells, as measured variously at eight different laboratories, to determine the pure material liquidus temperature for each cell. If this value does not agree with the liquidus temperatures of the same melting transitions, within the assigned uncertainties, as determined directly from the melting curve then the implication is that the allowed uncertainty component for impurities may not be large enough. We find the method can be accurate enough to be useful in validating a high-temperature fixed-point cell.

Freezing and melting in the peritectic W–C–X

The tungsten–carbon (WC) peritectic, with a liquidus temperature near to 3020 K, shows promise as a high-temperature reference fixed point. It appears that the WC phase is a discrete phase with full stoichiometry of the components W and C, in which impurities cannot be dissolved. In this paper we describe the use of Thermo-Calc software to derive the liquidus and solidus temperatures versus the impurity concentration for the impurities V and Si. It is found that the liquidus is univariant, whereas the solidus is invariant. Additionally, the transition curves between the solidus and the liquidus are modeled. The liquid fraction at the liquidus point is found to be less than 1; in contrast, it is unity for metal–carbon eutectics. Measurements are presented for a series of melting and freezing curves for two WC fixed-point cells. The Gibbs phase rule requires segregation of a fourth compound phase, involving the impurity in question, at the solidus point. The observations confirm that during supercooling, impurities are trapped in compound phases consisting of the impurity together with W and/or C, encapsulating the impurity, and resulting in a flat freezing plateau characteristic of the pure system. Finally, an expression for the melting temperature, as a function of the liquid fraction, is derived, spanning the temperature range from an initial melting temperature to the liquidus temperature. Within this range just free impurities are involved.

Methods of total spectral radiant flux realization at VNIIOFI

VNIIOFI carries out works on realization of independent methods for realization of the total spectral radiant flux (TSRF) of incoherent optical radiation sources - reference high-temperature blackbodies (BB), halogen lamps, and LED with quasi-Lambert spatial distribution of radiance. The paper describes three schemes for measuring facilities using photometers, spectroradiometers and computer-controlled high class goniometer. The paper describes different approaches for TSRF realization at the VNIIOFI National radiometric standard on the basis of high-temperature BB and LED sources, and gonio-spectroradiometer. Further, they are planned to be compared, and the use of fixed-point cells (in particular, based on the high-temperature δ(MoC)-C metal-carbon eutectic with a phase transition temperature of 2583 °C corresponding to the metrological optical “source-A”) as an option instead of the BB is considered in order to enhance calibration accuracy.

Co–C and Pd–C Eutectic Fixed Points for Radiation Thermometry and Thermocouple Thermometry

Two Co–C and Pd–C eutectic fixed point cells for both radiation thermometry and thermocouple thermometry were constructed at NMC. This paper describes details of the cell design, materials used, and fabrication of the cells. The melting curves of the Co–C and Pd–C cells were measured with a reference radiation thermometer realized in both a single-zone furnace and a three-zone furnace in order to investigate furnace effect. The transition temperatures in terms of ITS-90 were determined to be \(1324.18\,{^{\circ }}\hbox {C}\) and \(1491.61\,{^{\circ }}\hbox {C}\) with the corresponding combined standard uncertainty of \(0.44\,{^{\circ }}\hbox {C}\) and \(0.31\,{^{\circ }}\hbox {C}\) for Co–C and Pd–C, respectively, taking into account of the differences of two different types of furnaces used. The determined ITS-90 temperatures are also compared with that of INRIM cells obtained using the same reference radiation thermometer and the same furnaces with the same settings during a previous bilateral comparison exercise (Battuello et al. in Int J Thermophys 35:535–546, 2014). The agreements are within \(k=1\) uncertainty for Co–C cell and \(k = 2\) uncertainty for Pd–C cell. Shapes of the plateaus of NMC cells and INRIM cells are compared too and furnace effects are analyzed as well. The melting curves of the Co–C and Pd–C cells realized in the single-zone furnace are also measured by a Pt/Pd thermocouple, and the preliminary results are presented as well.

Reliability of High-Temperature Fixed-Point Installations over 8 Years

At NPL, high-temperature metal-carbon eutectic fixed points have been set up for thermocouple calibration purposes since 2006, for realising reference temperatures above the highest point specified in the International Temperature Scale of 1990 for contact thermometer calibrations. Additionally, cells of the same design have been provided by NPL to other national measurement institutes (NMIs) and calibration laboratories over this period, creating traceable and ISO 17025 accredited facilities around the world for calibrating noble metal thermocouples at $$1324~{^{\circ }}$$ C (Co–C) and $$1492~{^{\circ }}$$ C (Pd–C). This paper shows collections of thermocouple calibration results obtained during use of the high-temperature fixed-point cells at NPL and, as further examples, the use of cells installed at CCPI Europe (UK) and NIMT (Thailand). The lifetime of the cells can now be shown to be in excess of 7 years, whether used on a weekly or monthly basis, and whether used in an NMI or industrial calibration laboratory.