AVS 72 Session TS-WeA: The Future of Temperature Sensing III

Accurate temperature measurement and control are fundamental to advancements in industry and the energy sector. Transitioning to a clean industry is essential for ensuring future competitiveness. This transition necessitates not only the reduction of carbon emissions but also the assurance of reliable and affordable clean energy. Accurate thermometry is an enabling technology that facilitates the optimization of energy safety, storage, and transmission. It also contributes to the enhancement of industrial efficiency. Fiber optic thermometry has emerged as a promising solution for applications where conventional electrical temperature sensors are impractical. This technology offers unique advantages, including immunity to electromagnetic interference and the ability to make spatially distributed measurements over long distances. However, the lack of metrological characterization, and thus SI traceability, currently hinders its widespread adoption. As with all sensors, fiber optic sensors exhibit cross-sensitivities and aging effects that must be investigated, quantified, and minimized to ensure low uncertainty temperature measurements.

Over the past three years, the European project INFOTherm has begun to unlock the full potential of fiber optic thermometry by addressing current limitations and implementing calibration methods and infrastructure. A key aspect of this project was demonstrating fiber optic thermometry in real-world scenarios. Industrial case studies examined the ability of fiber optic thermometry to optimize energy infrastructure and improve the efficiency of processes in high-temperature industrial environments.

Case studies have been performed in thermal energy storage in molten salt tanks, geothermal systems, submarine cable monitoring, high-voltage component diagnostics, and silicone and glass production. These successful case studies were made possible by improving fiber optic thermometry throughout the project, particularly by enabling the traceable calibration and reducing measurement uncertainties, while extending the operational temperature range.

This presentation will provide a comprehensive overview of the INFOTherm project, with a focus on the practical implementation of fiber optics in the energy and industrial sectors. Recent progress will be highlighted. This includes improved calibration techniques for distributed sensing methods and advancements in fiber optic sensors for high temperatures.

Recent advances in distributed optical fiber sensing are enabling transformative approaches for high-resolution temperature metrology in harsh manufacturing environments. In this work, we developed a scalable, high-resolution distributed thermometry platform based on Rayleigh backscattering (RBS)-enabled optical frequency domain reflectometry (OFDR) for real-time spatial thermal mapping during the aluminum casting process. The system utilzed silica optical fibers (UHNA1, Thorlabs) interrogated with a high-speed OFDR platform, enabling high-density thermal metrology with a resolution of approximately 0.65 mm (~1538 points m^-1) and acquisition rates up to 62 Hz. The sensing approach was successfully integrated into elevated-temperature casting environments, including the casting of AA356 aluminum in 3D-printed investment shells. Distributed thermal data were acquired throughout the full casting cycle, including shell preheating, molten metal pouring, and subsequent solidification at temperatures approaching 700 °C, showing trends consistent with thermocouple measurements. The platform effectively captured localized thermal gradients, transient heat-transfer phenomena, and the spatio-temporal evolution of temperature during solidification, while the optical fiber remained intact after molten metal pouring and was reusable for subsequent measurements, demonstrating the robustness of the sensing platform in harsh casting environments. Unlike conventional thermocouple-based systems, the distributed fiber-optic approach provided near-continuous thermal measurements along the length of a single optical fiber, significantly improving spatial sensing capability while remaining minimally invasive. The results demonstrate the strong potential of RBS-enabled distributed fiber-optic sensors as robust thermometry platforms for metal casting and other advanced manufacturing applications operating in extreme environments. This work further establishes a foundation for intelligent process monitoring, real-time thermal diagnostics, and digital twin-enabled manufacturing systems, while highlighting the broader applicability of distributed fiber-optic thermometry for next-generation harsh-environment sensing technologies.

References:

1. N. Klimov, T. Purdy, and Z. Ahmed, "Towards replacing resistance thermometry with photonic thermometry," Sens. Actuators Phys. 269, 308–312 (2018).
2. T. P. Purdy, K. E. Grutter, K. Srinivasan, and J. M. Taylor, "Quantum correlations from a room-temperature optomechanical cavity," Science 356, 1265–1268 (2017).
3. A. J. Fleisher, Z. Ahmed, T. Herman, and M. R. Hartings, "Dual electro-optic frequency comb photonic thermometry," Opt. Lett. 48, 2210 (2023).
4. Y. Pang, L. Xu, X. Zhang, Z. Cao, Y. Ding, and G. Wang, "Accurate gas temperature measurement via iterative fitting of dual-comb spectral absorbance versus lower-state energy dependence," Opt. Express 33, 37913 (2025).

Johnson Noise Thermometry (JNT) offers a primary, electrically traceable approach to temperature measurement.

Achieved by directly linking the Johnson noise voltage across a resistive probe to temperature as defined by the Johnson-Nyquist equation. Presented is aconcise overview of JNT from its theory, through prior arts to modern techniques, and outlining real-world applications. Topics include noise signal acquisition and processing, andpractical challenges of implementation across laboratory and industrial environments. Recent advances in JNT development to realise a practical Johnson noise thermometer will also be examined, with a discussion of its evolving role within the broader landscape of precision thermometry.

Since the 2019 kelvin redefinition, thermodynamic temperature can be realized directly through methods anchored to the Boltzmann constant. Realization, however, is not dissemination - and dissemination determines whether a primary realization becomes a deployable, qualified measurement in the field. A calibrated kelvin still reaches accredited laboratories and industrial processes through standard platinum resistance thermometers and the ratio bridges that read them.

This contribution is from the manufacturer of one such instrument: the WIKA CTR9000, an AC resistance ratio bridge built on the ASL technology deployed at most national metrology institutes for ITS-90 realization and dissemination. One reason it occupies that role at parts-per-billion scale is architectural. Top-tier bridges derive accuracy from transformer-based ratio determination set by integer winding counts that cannot drift. The CTR9000 implements this in AC - continuously alternating excitation, inductive voltage divider ratio determination, quadrature balancing - where thermal EMFs sit at DC and never enter the measurement bandwidth, so each reading is a continuous measurement rather than a composite of two opposite-polarity readings combined by symmetry.

What gives the 20 parts-per-billion specification its standing is the framework of independent validations behind it: zero and unity checks, complement measurements by swapping resistors and evaluating the reciprocal, ratio test units against a calculable network, resistance bridge calibrators against a combinatorial standard, and long-term stabilization during manufacture. Each catches an error the others would miss, each works by physical manipulation of real ratios, independent of internal references. This carries the instrument from factory specification to qualified field deployment.

Drawing on a current cooperation with PTB on the CTR9000, the talk frames the AC bridge as the interface between primary realization and accredited dissemination. Of the methods enabled by the redefined kelvin, Johnson noise thermometry sits closest: a quantum voltage noise source provides the reference against which a sense resistor's thermal noise is compared, and the resistance contributes directly to realized temperature. As photonic, optomechanical, and noise-based primary thermometers move from laboratory demonstration toward deployable, qualified systems, each will face the question the bridge has answered over decades: not how accurate it can be in principle, but what architecture and framework will make the trust transferable.

Panelists: Isotech, WIKA, Fluke, Chi3 Optics, U.S. Air Force

This industry panel brings together leaders from Isotech, WIKA, Fluke, Chi3 Optics, and the U.S. Air Force to examine how next–generation temperature–sensing technologies can move from laboratory innovation to broad industrial deployment. Panelists will discuss commercialization challenges, key roadblocks, and practical strategies to accelerate worldwide adoption of advanced temperature–metrology solutions.