Miniature Fixed-point Cells Research Articles

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.

Design and Investigation of Pd–C Eutectic Fixed-Point Cells for Thermocouple Calibration at NMIA

The calibration of Pt/Rh thermocouples up to 1560 \(^{\circ }\hbox {C}\) at NMIA currently uses the conventional ‘melt-wire technique’ to realize Gold (Au) and Palladium (Pd) melting points, resulting in the loss of 20 mm of wire from the junction end for each calibration. To avoid this loss, NMIA intends to replace the melt-wire technique with the use of miniature fixed-point cells. NMIA has established Copper (Cu) and Cobalt–Carbon (Co–C) eutectic cells for calibration of thermocouples to 1324 \(^{\circ }\hbox {C}\). To extend the calibration up to 1500 \(^{\circ }\hbox {C}\), miniature Palladium–Carbon (Pd–C) eutectic cells (1492 \(^{\circ }\hbox {C}\)) have been constructed and tested in collaboration with NMIJ, AIST. Although these cells are made of high-purity reference materials, careful consideration must be given to contamination introduced during the manufacture and filling of the crucibles and by their long-term use. These issues can only be assessed by measurement of cell-to-cell temperature differences within the ensemble of cells traceable to ITS-90. In the work presented here, 3 NMIA-design mini Pd–C cells were constructed: 1 at NMIA and 2 at NMIJ. These cells were compared, together with a “large” NMIJ Pd–C cell, using type-R, type-B and Pt/Pd thermocouples and radiation thermometry. Although the cells are found to be stable and repeatable, significant problems arising from migration of Pd to the thermocouples were identified.

The Melting Point of Palladium Using Miniature Fixed Points of Different Ceramic Materials: Part I—Principles and Performances

Fifteen miniature fixed-point cells made of three different ceramic crucible materials ( $$\hbox {Al}_{2}\hbox {O}_{3},\, \hbox {ZrO}_{2}$$ , and $$\hbox {Al}_{2}\hbox {O}_{3} (86\,\%)+\hbox {ZrO}_{2}$$ (14 %)) were filled with pure palladium and used for the calibration of type B thermocouples (Pt30%Rh/Pt6%Rh). The melting behavior of the palladium was investigated by using different high-temperature furnaces usable in horizontal and vertical positions. It was found that the electromotive forces measured at the melting temperature of palladium are consistent with a temperature equivalent of ±0.25 K when using a furnace with an adequate temperature homogeneity (±1 K over a length of 12 cm), independent of the ceramic crucible materials. The emfs measured in the one-zone furnaces with larger temperature gradients along the crucibles are sensitive related to the position of the crucibles in the temperature gradient of these furnaces. This is caused by higher parasitic heat flux effects which can cause measurement errors up to about $$\text {-}$$ (1 $$\text {-}$$ 2) K, depending on the thermal conductivity of the ceramic material. It was found that the emfs measured by using crucibles with lower thermal conductivity $$(\hbox {ZrO}_{2})$$ were less dependent on parasitic heat flux effects than crucibles made of material of higher thermal conductivity $$(\hbox {Al}_{2}\hbox {O}_{3})$$ . The investigated miniature fixed points are suitable for the repeatable realization of the melting point of palladium to calibrate noble metal thermocouples without the disadvantages of the wire-bridge method or the wire-coil method.

The Melting Point of Palladium Using Miniature Fixed Points of Different Ceramic Materials: Part II—Analysis of Melting Curves and Long-Term Investigation

Fifteen miniature fixed-point cells made of three different ceramic crucible materials (\(\hbox {Al}_{2}\hbox {O}_{3}\), \(\hbox {ZrO}_{2}\), and \(\hbox {Al}_{2}\hbox {O}_{3}(86\,\%)+\hbox {ZrO}_{2}\)(14 %)) were filled with pure palladium and used to calibrate type B thermocouples (Pt30 %Rh/Pt6 %Rh). A critical point by using miniature fixed points with small amounts of fixed-point material is the analysis of the melting curves, which are characterized by significant slopes during the melting process compared to flat melting plateaus obtainable using conventional fixed-point cells. The method of the extrapolated starting point temperature using straight line approximation of the melting plateau was applied to analyze the melting curves. This method allowed an unambiguous determination of an electromotive force (emf) assignable as melting temperature. The strict consideration of two constraints resulted in a unique, repeatable and objective method to determine the emf at the melting temperature within an uncertainty of about \(0.1\,\upmu \hbox {V}\). The lifetime and long-term stability of the miniature fixed points was investigated by performing more than 100 melt/freeze cycles for each crucible of the different ceramic materials. No failure of the crucibles occurred indicating an excellent mechanical stability of the investigated miniature cells. The consequent limitation of heating rates to values below \({\pm }3.5\,\hbox {K}\,\hbox {min}^{-1}\) above \(1100\,{^{\circ }}\hbox {C}\) and the carefully and completely filled crucibles (the liquid palladium occupies the whole volume of the crucible) are the reasons for successfully preventing the crucibles from breaking. The thermal stability of the melting temperature of palladium was excellent when using the crucibles made of \(\hbox {Al}_{2}\hbox {O}_{3}(86\,\%)+\hbox {ZrO}_{2}(14\,\%)\) and \(\hbox {ZrO}_{2}\). Emf drifts over the total duration of the long-term investigation were below a temperature equivalent of about 0.1 K–0.2 K.

Zinc–Bismuth and Aluminum–Indium Monotectic Alloy-Based Fixed-Point Cells with Double Phase Transition for In Situ Calibration of Thermocouples

Re-calibration of a thermocouple after it has been installed in a process is often not practical. In situ monitoring of performance is desirable and can be done with built-in reference standards based on melting or freezing phase transitions. Binary alloys with a monotectic reaction frequently have two invariant melt/freeze phase transitions taking place in the same material over a range of compositions. This makes them potentially well suited to be in situ temperature calibration artifacts, enabling correction for thermocouple drift without the need to disturb the thermocouple. A zinc–bismuth fixed-point cell was constructed and has been shown to be stable with two well-defined melting plateaus at nominally 255 $$^\circ \hbox {C}$$ and 415 $$^\circ \hbox {C}$$ . Two miniature fixed-point cells (each designed to be permanently installed with a thermocouple) based on zinc–bismuth and aluminum–indium alloys were made. Measurements have shown that the phase transitions can be identified despite the small quantity of metals used and that the alloys were sufficiently stable to have the potential to provide improved long-term confidence in process control and monitoring.

Miniature Fixed-Point Cell Approaches for $${{\varvec{In\,Situ}}}$$ I n S i t u Monitoring of Thermocouple Stability

In the framework of the European Metrology Research Project ENG08 “MetroFission,” LNE-Cnam and NPL have undertaken cooperative research into the development of temperature measurement solutions for the next generation of nuclear fission power plants. Currently, in-pile temperature monitoring is usually performed with nickel-based (Type K or N) thermocouples. When these thermocouples are exposed to a neutron flux, the thermoelements transmute, leading to large and unknown drifts in output. In addition, it is impossible to routinely recalibrate the thermocouples after irradiation for obvious reasons of safety. To alleviate this problem, both LNE-Cnam and NPL have developed, via differing approaches, in situ calibration methods for the thermocouples. The self-validating thermocouple methodologies are based on the principle of a miniature fixed-point cell to be co-located with the thermocouple measurement junction in use. The drift of the thermocouple can be monitored and corrected for by regular determination of the output at the phase transition of the fixed-point material: in effect performing regular in situ calibration checks. The two institutes have constructed miniature fixed-point cells for use at three different temperatures; the freezing point of silver $$(961.78\,^{\circ }\mathrm{C}$$ ; LNE-Cnam), the freezing point of copper $$(1084.62\,^{\circ }\mathrm{C}$$ ; LNE-Cnam and NPL), and the melting point of Fe–C ( $${\sim }1154\,^{\circ }\mathrm{C}$$ ; NPL). This paper introduces the construction and validation of the miniature fixed-point cells prior to use, to ensure traceability to the ITS-90. A comparison of the performance of the two cell designs is discussed, where typical industrial Type N thermocouples have been used for measurement of the fixed-point cells. Such initial measurements demonstrate the feasibility of each of these two approaches.

Long-Term Monitoring of Thermocouple Stability with Miniature Fixed-Point Cells

In the framework of the European Metrology Research Programme ENG08 “MetroFission” project, two National Measurement Institutes, LNE-Cnam (France) and NPL (UK), have cooperatively developed methods of in situ validation of thermocouple output for application in next-generation nuclear fission power plants. Miniature fixed-point cells for use at three temperatures were constructed in the first step of this project: at the freezing point of silver (\(961.78\,^{\circ }\mathrm{C}\)), the freezing point of copper (\(1084.62\,^{\circ }\mathrm{C}\)), and the melting point of the iron–carbon eutectic (\(1154\,^{\circ }\mathrm{C}\)). This paper reports the results of a second step in the study, where the robustness of the self-validation method has been investigated. Typical industrial Type N thermocouples have been employed with each of the miniature fixed-point devices installed, and repeatedly thermally cycled through the melting and freezing transitions of the fixed-point ingots. The devices have been exposed to a total of up to 90 h in the molten state. Furthermore, the LNE-Cnam devices were also subjected to fast cool-down rates, on five occasions, where the rate is estimated to have been between \(150\,^{\circ }\mathrm{C}\,{\cdot }\min ^{-1}\) and \(200\,^{\circ }\mathrm{C}\,{\cdot } \min ^{-1}\). The devices are shown to be repeatable, reliable, and robust over the course of these tests. The drift of the Type N thermocouple has been identified separately to the behavior of the device. A reliable method for improving thermocouple performance and process control is therefore demonstrated. Requirements for implementation and the advantages of each approach for monitoring and correcting thermocouple drift are discussed, and an uncertainty budget for self-validation is presented.

Investigation of differently designed Pd–C eutectic fixed-point cells for the calibration of thermocouples

An interlaboratory comparison of the melting temperatures of three palladium–carbon (Pd–C) eutectic fixed-point cells of different designs and usable for the calibration of thermocouples was performed at NIM. Two Pt/Pd thermocouples were constructed for this particular purpose. One of the Pd–C eutectic cells is of conventional design and size; the two other cells are miniature fixed-point cells of different designs. The melting temperature of the conventional cell, PdC1-NIM, was slightly higher than the melting temperatures of the miniature cells by about 160 mK (PdCM2 of PTB) and 290 mK (PdC2-NIM). The melting temperatures of the two miniature Pd–C cells agree within their measurement uncertainties for k = 2 of about 135 mK. Furthermore, a new aspect of leakage effects often observed in the use of Pd–C cells is presented.

Miniature Co–C eutectic fixed-point cells for self-validating thermocouples

Miniature fixed-point cells have been constructed using ingots of Co–C eutectic alloy, with the aim of constructing self-validating thermocouples for in situ calibration. The design consists of a cylindrical graphite crucible, diameter 6 mm, containing the ingot, integrated into the measuring junction of a type R thermocouple. Two sizes of cell were studied, with internal volumes 124 and 64 mm3 respectively. A detailed study of the characteristics of the two designs shows a correlation between the emf at the melting temperature, and the furnace temperature. This correlation is dramatically reduced for the smaller cell. The correlation is characterized extensively. The smaller cell provides the best overall performance. The repeatability of both cells is of the order of 0.1 °C.

Estimation of fixed-point temperatures – A practical approach

Phase transition temperatures can be used for thermometer calibrations in accordance with the ITS-90. For this the thermometer is inserted into a fixed-point cell filled with a highly pure substance. While the substance is melting or freezing the temperature inside the cell remains almost constant and the thermometer measures a plateau in the temperature curve. From this plateau the phase transition temperature needs to be estimated for the calibration of the thermometer. This can be done using different mathematical methods, taking into account various systematic deviations as well as reproducibilities of the results depending on the chosen method, the fixed-point material’s purity and above all the amount of fixed-point material. Hence this article presents results from measurements in miniature fixed-point cells filled with zinc of various purities. The plateau curves were measured at different heating rates and comparatively analysed using five different estimation methods.

In-situ calibration of inhomogeneous thermocouples by integrated miniature fixed-point cells

In-situ calibration of inhomogeneous thermocouples by integrated miniature fixed-point cells