J. V. Pearce

Evaluation of the Pd–C eutectic fixed point and the Pt/Pd thermocouple

A Pd–C eutectic fixed point cell (1492 °C) was constructed to investigate its utility for thermocouple calibration. The primary aim of the study was to evaluate the long-term stability, immersion characteristics (influence of heat conduction along the thermocouple stem) and robustness of a Pd–C fixed point using a Pt/Pd thermocouple, especially constructed for this purpose. The performance of both devices at this relatively high temperature could therefore be tested. The melting and freezing plateaux at the Pd–C eutectic point were measured using the Pt/Pd thermocouple. The total exposure to the Pd–C melting temperature was about 850 h for the fixed point cell and 550 h for the thermocouple. The standard deviations of the melting and freezing points were 1.03 µV (0.041 °C) and 0.77 µV (0.031 °C) respectively. The emfs of the thermocouple at the melting point were observed to drift by about 0.1 °C. The immersion measurements show that for the current cell design, the measuring junction should be at most 30 mm from the bottom of the thermowell to be properly immersed. The long-term performance and robustness of the fixed point indicate a promising future for its use as a fixed point for calibration of noble metal thermocouples.

Investigation of Pd–C cells to improve thermocouple calibration

An intercomparison of the melting temperatures of four Pd–C eutectic fixed-point cells was performed using four Pt/Pd thermocouples. The cells are designed for the calibration of thermocouples and were constructed in the participating laboratories of NPL, LNE, NMIJ and PTB. The measurements were performed in four different high-temperature furnaces but by applying the same measurement procedure. In spite of slightly different cell designs and different material sources, the melting temperatures of three of the four Pd–C cells (NPL, LNE and NMIJ) agreed very well within their expanded uncertainties of k = 2.

MTDATA and the Prediction of Phase Equilibria in Oxide Systems: 30 Years of Industrial Collaboration

This paper gives an introduction to MTDATA, Phase Equilibrium Software from the National Physical Laboratory (NPL), and describes the latest advances in the development of a comprehensive database of thermodynamic parameters to underpin calculations of phase equilibria in large oxide, sulfide, and fluoride systems of industrial interest. The database, MTOX, has been developed over a period of thirty years based upon modeling work at NPL and funded by industrial partners in a project co-ordinated by Mineral Industry Research Organisation. Applications drawn from the fields of modern copper scrap smelting, high-temperature behavior of basic oxygen steelmaking slags, flash smelting of nickel, electric furnace smelting of ilmenite, and production of pure TiO2via a low-temperature molten salt route are discussed along with calculations to assess the impact of impurities on the uncertainty of fixed points used to realize the SI unit of temperature, the kelvin.

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.

A miniature high-temperature fixed point for self-validation of type C thermocouples

Reliable high-temperature (>1500 °C) measurement is crucial for a wide range of industrial processes as well as specialized applications, e.g. aerospace. The most common type of sensor used for high-temperature measurement is the thermocouple. At and above 1500 °C, tungsten–rhenium (W–Re) thermocouples are the most commonly used temperature sensors due to their utility up to 2300 °C. However, the achievable accuracy of W–Re thermocouples is seriously limited by the effects of their inhomogeneity, drift and hysteresis. Furthermore, due to their embrittlement at high temperature, the removal of these thermocouples from environments such as nuclear power plants or materials processing furnaces for recalibration is generally not possible. Even if removal for recalibration were possible, this would be of, at best, very limited use due to large inhomogeneity effects. Ideally, these thermocouples require some mechanism to monitor their drift in situ. In this study, we describe a miniature Co–C eutectic fixed-point cell to evaluate the stability of type C (W5%Re/W26%Re) thermocouples by means of in situ calibration.

Self-validating thermocouples based on high temperature fixed points

We report the development of a self-validating high temperature thermocouple, whereby a high temperature fixed point of metal?carbon eutectic alloy forms an integral part of the thermocouple measuring junction, permitting in situ calibration.

HiTeMS: A project to solve high temperature measurement problems in industry

HiTeMS is a three-year research project involving fifteen partners, part funded by the European Metrology Research Programme. The objective of the research is to develop a suite of methods and techniques that will significantly improve the practice of industrial high temperature non-contact and contact thermometry, up to at least 2500 °C. Special emphasis is given to facilitating in-situ traceability, i.e. ensuring traceability to the International Temperature Scale of 1990 (ITS-90) directly within the industrial process. HiTeMS will do this by seeking to address common problems encountered in high temperature thermometry such as unknown emissivity and varying path transmission for non-contact thermometry and sensor characterization (including thermocouple reference function determination) and, characterizing and mitigating sensor drift (to at least 2000 °C). The project started in September 2011 and this paper focuses on the anticipated research outcomes by the time of the project conclusion in Aug 2014.

Fe?C eutectic fixed-point cells for contact thermometry: an investigation and comparison

Five iron?carbon (Fe?C) eutectic fixed-point cells have been constructed between NPL and LNE-Cnam to investigate the robustness and to measure the agreement of their melting temperatures. Each cell was constructed with a different selection of materials sourced by NPL and LNE-Cnam. The measured emfs at the Fe?C fixed-point temperature (~1153??C), compared between cells, agree within around 1.98??V (~90?mK), where the most important contribution to the uncertainty of each measurement is the inhomogeneity associated with the measuring Pt/Pd thermocouple. This demonstrates that these cells are suitable for use as secondary fixed-point cells in contact thermometry but the robustness of the presented cells is not found to be sufficient for maintaining their integrity during repeated cycling procedures.

Bose-Einstein condensation in solid 4He.

We present neutron scattering measurements of the atomic momentum distribution n(k) in solid helium under a pressure p=41 bar (molar volume Vm=20.01+/-0.02 cm3/mol) and at temperatures between 80 and 500 mK. The aim is to determine whether there is Bose-Einstein condensation (BEC) below the critical temperature, Tc=200 mK, where a superfluid density has been observed. Assuming BEC appears as a macroscopic occupation of the k=0 state below Tc, we find a condensate fraction of n0=(-0.10+/-1.20)% at T=80 mK and n0=(0.08+/-0.78)% at T=120 mK, consistent with zero. The shape of n(k) also does not change on crossing Tc within measurement precision.

A simple device for substantially improving metal?carbon eutectic fixed point performance by reducing temperature gradients

Metal–carbon eutectic fixed points are revolutionizing high temperature metrology. However, the quality of the melting curve relies heavily on achieving good axial temperature uniformity along the fixed point crucible. This is not easy to achieve with conventional tube furnaces, and much work has been published in which temperature uniformity is clearly a problem. Here is presented a simple, inexpensive and effective way of reducing temperature gradients along the fixed point crucible by at least a factor of two. This leads to a corresponding increase in melt duration and quality. A key feature of the device, essential to its operation, is the fact that it mechanically isolates the fixed point crucible from its surroundings.