Fibre optic sensors offer promising solutions for accurate and precise temperature monitoring across diverse application areas. This study experimentally evaluates the performance of several widely-used fibre optic temperature sensing technologies, including distributed fibre sensing systems based on Raman, Brillouin, and Rayleigh scattering effects, as well as point sensing based on fibre Bragg gratings (FBGs). Under identical thermal conditions, each sensor type is assessed in terms of sensitivity, accuracy, and uncertainty. Distributed measurements are obtained through using Raman optical time domain reflectometry, Brillouin optical time domain analysis, and optical frequency domain reflectometry (OFDR). The results obtained show clear differences in the sensitivity and precision observed, with OFDR and FBGs demonstrating superior stability and repeatability. This comparative analysis, undertaken in this comprehensive way for the first time, offers industry a wealth of foundational data for selecting suitable fibre optic temperature sensors, which can be tailored to their application-specific requirements.

Distributed fiber sensing (DFS) is a powerful tool for structural health monitoring (SHM), allowing continuous and seamless measurements of temperature and strain along the fiber. The spatial accuracy of a DFS interrogator, as a key parameter of the system, is vital for precisely locating structural perturbations or defects. Its evaluation and calibration methods however attract little attention. A fiber optic artefact based on a fiber loop is developed to evaluate distance accuracy and signal quality for both self-developed and commercial sensing systems based on Rayleigh, Raman, and Brillouin scattering effects, respectively. The measured distance is corrected to remove the influence of the pulse width. Additionally, the obtained SNRs are compared for different loop trips and pulse widths, assisting to assess signal quality for SHM applications.

The measurement and control of temperature plays a key role in achieving the European Green Deal targets for a low carbon energy system. Fibre-optic thermometry is an emerging technology that can improve temperature measurement in extreme environments for energy providers and industry due to its distributed sensing and immunity to electromagnetic fields. Various applications for optimisation and monitoring in the energy sector are described, covering the whole range from energy generation to transmission and consumption. However, fibre-optic thermometers have cross sensitivities to other quantities (e.g., strain and humidity) and ageing effects that need to be investigated, quantified and minimised to obtain traceable and reliable measurements. This is particularly important so that applications in critical infrastructure can benefit from future measurements that are not possible with conventional sensors. The European INFOTherm project aims to overcome the limitations that currently prevent the widespread use of fibre-optic thermometry by creating a dedicated European metrology infrastructure for research, development and calibration. First results on measurement uncertainty, improvement of measurement techniques and practical field tests are presented. Bei der Erreichung der Ziele des europ & auml;ischen Green Deal f & uuml;r ein kohlenstoffarmes Energiesystem spielen Temperaturmessung und -& uuml;berwachung eine Schl & uuml;sselrolle. Die faseroptische Thermometrie ist eine aufstrebende Technologie, die aufgrund ihrer verteilten Messung und ihrer Unempfindlichkeit gegen & uuml;ber elektromagnetischen Feldern die Temperaturmessung in extremen Umgebungen f & uuml;r Energieversorger und die Industrie verbessern kann. Es werden verschiedene Anwendungen zur Optimierung und & Uuml;berwachung im Energiesektor beschrieben, die den gesamten Bereich von der Energieerzeugung & uuml;ber die & Uuml;bertragung bis hin zum Verbrauch abdecken. Faseroptische Thermometer weisen jedoch Querempfindlichkeiten gegen & uuml;ber anderen Gr & ouml;ss en (z. B. Dehnung und Feuchtigkeit) und Alterungseffekte auf, die untersucht, quantifiziert und minimiert werden m & uuml;ssen, um r & uuml;ckf & uuml;hrbare und zuverl & auml;ssige Messungen zu erhalten. Dies ist besonders wichtig, damit Anwendungen in der kritischen Infrastruktur zuk & uuml;nftig von Messungen profitieren k & ouml;nnen, die mit konventionellen Sensoren nicht m & ouml;glich sind. Das europ & auml;ische Projekt INFOTherm zielt darauf ab, die Hemmnisse zu & uuml;berwinden, die einen breiten Einsatz der faseroptischen Thermometrie derzeit verhindern, indem eine dedizierte europ & auml;ische metrologische Infrastruktur f & uuml;r Forschung, Enwicklung und Kalibrierung geschaffen wird. Erste Ergebnisse zur Messunsicherheit, zur Verbesserung der Messverfahren und zu praktischen Feldtests werden vorgestellt.

We propose, and demonstrate, the use of a fiber-optical measurement artefact as a metrological tool for traceable distance calibration of distributed optical fiber sensors. The constructed fiber artefact consists of a lead-in fiber coupled to a fiber loop using a 3-dB coupler and is used to calibrate both a home-build Brillouin-OTDR setup and a custom version of a commercial Brillouin-OTDR interrogator build for distributed temperature sensing. For both interrogators, we demonstrate distance calibrations with 1-meter uncertainty (k=1) in the offset length and 0.1 % (1 m/km) uncertainty (k = 1) in the distance scale factor. In addition, it is shown that the fiber artefact can be used to assess undesired distance-dependent measurement biases. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

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.

The TIR100-2 emissometer (manufactured by Inglas GmbH & Co.KG) is an emissivity measurement device used by several producers of thermal insulation products for buildings and by some organizations certifying performance of insulation products. A comparison of emissivity measurements on low-emissivity foils involving different measurement techniques, including the TIR100-2 emissometer, gave widely dispersed results; the discrepancies were not explained. The metrological performance of the TIR100-2 emissometer and the uncertainties for measurement on reflective foils was not known, which could be detrimental to users. In order to quantify the performance of TIR100-2 devices for measurement of total near-normal emissivity of low-emissivity foils, the Laboratoire National de Métrologie et d'Essais (LNE) analyzed in detail the measuring principle and listed the associated assumptions and uncertainty sources. A TIR100-2 emissometer actually measures the reflectance and, for opaque materials, the emissivity is calculated from the measured reflectance. The parameters analyzed experimentally are the temperature stability and uniformity of the thermal radiation source, the emissivity of the radiation source, the response function linearity and the spectral sensitivity of the radiometric detection system measuring the reflected radiation, the size of the measurement area, and the measurement repeatability and reproducibility. A detailed uncertainty budget was established. The uncertainty sources taken into account are the uncertainties of the emissivities of the two calibrated standards used for calibration, the stability and uniformity of the radiation source temperature, the non-linearity and the spectral sensitivity of the radiometric detection system, the specific measurement condition related to the radiation source temperature, the uncertainties related to the temperatures of the standards and the sample, the noises on results, and the non-homogeneity in emissivity of the tested material. The combined measurement uncertainty was calculated for different types of reflective foils; the expanded uncertainty is around 0.03 for total near-normal emissivity measurements on smooth low-emissivity foils. A measurement campaign on five types of low-emissivity foils, involving four TIR100-2 emissometers, and a comparison to a primary reference setup at the Physikalisch-Technische Bundesanstalt (PTB) confirmed the uncertainties assessed.

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.

An inter-laboratory comparison has been organized between LNE and Institute VINČA, respectively French National Metrology Institute and Serbian Designated Institute for thermal properties metrology, on thermal conductivity measurements by the guarded hot plate method. The main objective was to validate the measurement capabilities of VINČA in terms of thermal conductivity in the temperature range from 10 °C to 50 °C by using the facility improved in the frame of the European project Eura-Thermal. The measurements were carried out on expanded polystyrene boards using guarded hot plate apparatuses (two-specimen GHP apparatuses) in accordance with the international standard ISO 8302. The measurement programme was defined taking into account the major characteristics of the guarded hot plate apparatuses used, such as specimen dimensions and temperature and thermal conductivity ranges. Specimens were machined by LNE from a same batch for both participants. Prior to the measurements, the homogeneity of the set of specimens, as well as the influence of a variation of density of the expanded polystyrene on the thermal conductivity measurements were studied by VINČA.

This paper presents the effects of short-term and long-term temperature exposure on noble metal thermocouples in the range from 1000 °C to 1720 °C in oxidizing atmosphere (air). As thermocouples voltage output depends on the wire material of which the sensors are constructed, high-temperature and long continuous exposure to limit temperatures can introduce changes to the materials composition and structure. This can result in drift of generated voltage independent of the thermal environment and into reduced lifetime of the sensors. The intensions of conducted measurements were to determine the drift and lifetime of commonly available alumina-sheathed noble metal thermocouples and furthermore to establish traceable techniques to enable lifetime testing and thermoelectric stability evaluation of noble metal thermocouples at high temperatures. Result presented within this paper is not intended to test the capabilities of thermocouple Types B, R and S in general, but to show a possible behavior in the above-specified temperature conditions. This work puts more emphasis on the procedures that can be used for regular checks of intensely used thermocouple. Results obtained by this study show that the long-term temperature drift of Type B thermocouples at 1600 °C, and R-, S-type thermocouples at 1000 °C are much smaller than thermoelectric stability declared by IEC 60584-1:2013 standard (International standard IEC 60584-1:2013, Thermocouples—Part 1: EMF specifications and tolerances, 2013 ) tolerance classes after exposure to thermal stress up to 4 months. More specifically tolerance for class 1 thermocouple Types R, S from 0 °C up to 1100 °C is ± 1 °C and for class 2 tolerance for Type B at 1600 °C is ± 1.5 °C. The short-term thermoelectric stability of R- and S-type thermocouples exposed to 1600 °C for repeated 8 h periods has been within 2 °C when measured by comparison with a reference Pt–Pd thermocouple at 960 °C. The short-term thermoelectric stability of B-type thermocouple when exposed to 1720 °C for repetition of 8 h has been within 1 °C.

Decommissioning of nuclear facilities incurs high costs regarding the accurate characterisation and correct disposal of the decommissioned materials. Therefore, there is a need for the implementation of new and traceable measurement technologies to select the appropriate release or disposal route of radioactive wastes. This paper addresses some of the innovative outcomes of the project “Metrology for Decommissioning Nuclear Facilities” related to mapping of contamination inside nuclear facilities, waste clearance measurement, Raman distributed temperature sensing for long term repository integrity monitoring and validation of radiochemical procedures.

The estimation and control of the thermal power released by the radioactive waste packages are a key parameter in the management of radioactive waste geological repository sites. In the framework of the European project “Metrology for decommissioning nuclear facilities,” the French National Agency of Radioactive Waste Management (ANDRA) collaborates with Laboratoire National de Métrologie et D’essais in order to measure the thermal power up to 500 W of typical real size radioactive waste packages (of at least 0.175 m3) with an uncertainty better than 5% by using a measurement method traceable to the international system of units. One of the selected metrological approaches is based on the principles of air flow calorimetry. This paper describes in detail the development of the air flow calorimeter prototype as well as the design of a radioactive waste package simulator used for its calibration. Results obtained from the calibration of the calorimeter and from the determination of thermal powers are presented here with an investigation of the measurement uncertainties.

Raman distributed temperature sensing techniques (Raman-DTS) are currently meeting a growing interest from the industry as they are a promising and cheap alternative to classical temperature measurements which require the deployment of many sensors. The reliability of the DTS measurements, as well as the traceability to the temperature standards, must be ensured throughout the entire period of use (typically over a few tens of years for nuclear waste repositories and hydraulic structures). LNE, in association with Andra and EDF has developed facilities dedicated to the metrological characterization of Raman DTS devices. A first benchmark performed on five devices from different manufacturers has been performed following experimental procedures which enable a relevant comparison of these instruments. This paper defines the proposed metrological features to be evaluated for each Raman DTS system, and presents a ranking method enabling to provide useful pieces of information to the final users for the selection of the most appropriate device to the requirements of their applications.

Within the frame of a European project called Eura-Thermal, the general objective was to upgrade the regional metrological infrastructure (Bosnia & Herzegovina, Croatia, Ireland, Serbia...) with new capabilities, especially in the field of thermal measurements. This paper highlights the strategy used for improving in the short term, scientific knowledge transfer and the capabilities of different emerging institutes. Furthermore, as a main output, the impacts and benefit for Industry and for the end-users are also presented as examples.

This paper complements the existing measurement standards and literature for high-temperature guarded hot plates (HTGHPs) by addressing specific issues relating to thermal conductivity measurement of technical insulation at high temperatures. The examples given are focused on the designs of HTGHPs for measuring thin thermal insulation. The sensitivity studies have been carried out on major influencing factors that affect the thermal conductivity measurements using HTGHPs, e.g., the uncertainty of temperature measurements, plate flatness and center-guard gap design and imbalance. A new configuration of center-guard gap with triangular shape cross section has been optimized to obtain the same thermal resistance as a 2 mm wide gap with rectangular shape cross section that has been used in the HTGHPs at NPL and LNE. Recommendations have been made on the selections of heater plate materials, high-temperature high-emissivity coatings and miniature temperature sensors. For the first time, thermal stress analysis method has been applied to the field of HTGHPs, in order to estimate the effect of differential thermal expansion on the flatness of thin rigid specimens during thermal conductivity tests in a GHP.

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.

In order to enhance the availability of facilities in the field of high temperature contact thermometry in European emerging National Metrology Institutes (NMIs) and Designated Institutes (DIs), where access to types of facilities is currently limited an EMPIR Research Potential Project named Eura- Thermal has been launched. This project develop skills and tools for less experienced NMIs/DIs in order to acquire the required knowledge and expertise in temperature metrology.

This paper deals with a new European project in which the overall objective is to enhance the availability of facilities in the field of thermal metrology (e.g. high temperature contact thermometry, non-contact thermometry and thermo-physical properties of materials characterisation) in European emerging National Metrology Institutes (NMIs), where access to these types of facilities is currently limited.

Temperature measurements in the nuclear field require a high degree of reliability and accuracy. Despite their sheathed form, thermocouples subjected to nuclear radiations undergo changes due to radiation damage and transmutation that lead to significant EMF drift during long-term fuel irradiation experiment. For the purpose of a High Temperature Reactor fuel irradiation to take place in the High Flux Reactor Petten, a dedicated fixed-point cell was jointly developed by LNE-Cnam and JRC-IET. The developed cell to be housed in the irradiation rig was tailor made to quantify the thermocouple drift during the irradiation (about two year duration) and withstand high temperature (in the range 950 °C-1100 °C) in the presence of contaminated helium in a graphite environment. Considering the different levels of temperature achieved in the irradiation facility and the large palette of thermocouple types aimed at surveying the HTR fuel pebble during the qualification test both copper (1084.62 °C) and gold (1064.18 °C) fixed-point materials were considered. The aim of this paper is to first describe the fixed-point mini-cell designed to be embedded in the reactor rig and to discuss the preliminary results achieved during some out of pile tests as much as some robustness tests representative of the reactor scram scenarios.

Thermocouples are often exposed to harsh conditions when used for high-temperature measurements in industry. They suffer commonly from unavoidable drift effects which influence the required process efficiency and control. A self-validation concept for thermocouples to monitor their performance in the temperature range between 1000 C and about 1800 C in oxidizing atmospheres by using integrated miniature fixed-point units of different designs was tested. Two different models of fixed-point crucibles filled with high purity palladium (1553.4 °C) and platinum (1769 °C) have been constructed and assembled with type B thermocouples to be used as traceable references. Furthermore, two innovative self-validation methods by using thick wires of high purity gold (1064.18 °C), nickel (1455 °C), and palladium in multi-bore insulators as fixed-point materials were developed and investigated by assembling them with type B thermocouples. The measurement results obtained have demonstrated the suitability of the integrated fixed-point units to provide long-term confidence in industrial high-temperature measurements within about (2−3) K.

Thermocouples are prone to significant drift in use particularly when they are exposed to high temperatures. Indeed, high-temperature exposure can affect the response of a thermocouple progressively by changing the structure of the thermoelements and inducing inhomogeneities. Moreover, an oxidizing atmosphere contributes to thermocouple drift by changing the chemical nature of the metallic wires by the effect of oxidation. In general, severe uncontrolled drift of thermocouples results from these combined influences. A periodic recalibration of the thermocouple can be performed, but sometimes it is not possible to remove the sensor out of the process. Self-validation methods for thermocouples provide a solution to avoid this drawback, but there are currently no high-temperature contact thermometers with self-validation capability at temperatures up to \(1600\,^{\circ }\hbox {C}\). LNE-Cnam has developed fixed-point devices integrated to the thermocouples consisting of machined alumina-based devices for operation under oxidizing atmospheres. These devices require small amounts of pure metals (typically less than 2 g). They are suitable for self-validation of high-temperature thermocouples up to \(1600\,^{\circ }\hbox {C}\). In this paper the construction and the characterization of these integrated fixed-point devices are described. The phase-transition plateaus of gold, nickel, and palladium, which enable coverage of the temperature range between \(1000\,^{\circ }\hbox {C}\) and \(1600\,^{\circ }\hbox {C}\), are assessed with this self-validation technique. Results of measurements performed at LNE-Cnam with the integrated self-validation module at several levels of temperature will be presented. The performance of the devices are assessed and discussed, in terms of robustness and metrological characteristics. Uncertainty budgets are also proposed and detailed.