A new in situ high temperature electrical calibration system was developed at Laboratoire National de Metrologie et d'Essais, Laboratoire Commun de Metrologie and integrated into a heat flux Calvet calorimeter in order to perform accurate and reliable measurements of enthalpy of fusion that are directly traceable to the International System of Units (SI). This system has been designed to enable the calibration of the calorimeter by electrical substitution (Joule effect) as well as the measurement of enthalpy of fusion in perfectly identical experimental conditions. The metrological features (repeatability, linearity, etc.) of the calorimeter have been evaluated with this system by investigating the influence of some parameters (level of energy, dissipation time, and temperature) on the determination of the sensitivity factor of its thermopiles. Two different procedures, for the calibration and the enthalpy measurements with this new electrical calibration system, have been implemented and tested by measuring the enthalpy of fusion of high purity 6N tin. The results obtained are in very good agreement with those measured by other National Metrology Institutes on the same material.
Raman distributed temperature sensing (DTS) technologies are currently under evaluation by the nuclear and hydraulic industries as it may bring promising alternatives to classical measurement techniques. The reliability of the DTS measurements, as well as the traceability to the temperature standards, must be ensured throughout the entire period of monitoring (typically over a few tens of years). In order to achieve this goal, one key task consists in the verification of the performances claimed by the DTS devices manufacturers. Thus, the metrological performances and characteristics of the DTS devices, such as their limitations and accuracies, as well as the practical aspects of systems implemented on site should be evaluated step by step. This paper describes the dedicated facilities which have been developed at LNE in order to evaluate and to qualify DTS devices for very demanding applications. A first case study performed on one specific DTS device is detailed. A systematic bias has been observed among others on the spatial resolution. The DTS response to a temperature variation step over 1 m (spatial resolution typically claimed by the manufacturers) of sensing optical fibre corresponds indeed to only 90% of the temperature step magnitude, whereas the full DTS response is obtained in fact for 10 m (the practical spatial resolution) of sensing optical fibre solicited by this temperature step variation.
The French National Metrology Institute LNE has improved its homemade laser flash apparatus in order to perform accurate and reliable measurements of thermal diffusivity of homogeneous solid materials at very high temperature. The inductive furnace and the associated infrared (IR) detection systems have been modified and a specific procedure for the in situ calibration of the used radiation thermometers has been developed. This new configuration of the LNE’s diffusivimeter has been then applied for measuring the thermal diffusivity of molybdenum up to 2200 °C, tungsten up to 2400 °C and isotropic graphite up to 3000 °C. Uncertainties associated with these high temperature thermal diffusivity measurements have been assessed for the first time according to the principles of the “Guide to the Expression of Uncertainty in Measurement” (GUM). Detailed uncertainty budgets are here presented in the case of the isotropic graphite for measurements performed at 1000 °C, 2000 °C and 3000 °C. The relative expanded uncertainty (coverage factor k = 2) of the thermal diffusivity measurement is estimated to be between 3 % and 5 % in the whole temperature range for the three investigated refractory materials.
Abstract. The French National Metrology Laboratory LNE-LCM has developed a high temperature reference facility for accurate measurements of the specific heat capacity and of the enthalpy of fusion of materials over the temperature range [23 °C, 1000 °C]. The metrological approach was to modify a commercial Calvet calorimeter in order to lower the uncertainty of measurement and to insure the metrological traceability of the measurements to the SI units, in particular by designing a new calibration system. The enthalpies of fusion of pure metals (indium, tin and silver) and of a binary alloy Ag-28Cu have been measured. The results obtained on the three pure metallic materials are in very good agreement with data obtained by other National Metrology Institutes (NMIs) using adiabatic calorimetry.
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