This paper presents a proof-of-concept investigation into a novel hermetically sealed tunable-medium Extrinsic Fabry–Pérot Interferometer (EFPI) temperature sensor architecture. A series of tuneable-sensitivity EFPI temperature sensors is demonstrated, comprising a large-diameter fused silica diaphragm with a 800 $\mathsf{\mu }\mathrm{m}$ diameter, significantly exceeding conventional designs (typically ∼125 $\mathsf{\mu }\mathrm{m}$), with polished diaphragm thicknesses ranging from 28 to 49 $\mathsf{\mu }\mathrm{m}$, housed in hermetically sealed rigid melting point capillaries with a 1.8 mm internal diameter. By exploiting thermally induced pressure differentials generated by a tunable Krytox GPL 105 oil/air fill fraction within the sealed rigid cavity, the sensors demonstrate a continuously tuneable sensitivity design space spanning 0.45 to 190 $\mathrm{n}\mathrm{m}/\mathrm{K}$. An exact nonlinear thermal pressure model is derived and validated, replacing the linearised approximation which is shown to be inapplicable at fill fractions approaching unity. The low-sensitivity configuration (0.45 $\mathrm{n}\mathrm{m}/\mathrm{K}$) was characterised at the National Standards Authority of Ireland (NSAI) National Metrology Laboratory against ITS-90 fixed points: the Triple Point of Water (273.16 K) and the Gallium Fixed Point (302.9146 K), with traceability to the International Temperature Scale of 1990 (ITS-90), yielding an instrument-limited resolution of $<1.1$ mK, consistent with the metrological validation environment. The high-sensitivity configurations (21 and 190 $\mathrm{n}\mathrm{m}/\mathrm{K}$) were characterised on a laboratory bench, achieving instrument-limited theoretical resolutions of <24 $\mathsf{\mu }\mathrm{K}$ and $<2.6$ $\mathsf{\mu }\mathrm{K}$ respectively, pending future metrological validation. The 190 $\mathrm{n}\mathrm{m}/\mathrm{K}$ sensitivity represents an improvement of approximately 21.7× over the closest directly comparable prior Citationutilised fusion splicing and manual polishing. Future development priorities include metrological validation of the high-sensitivity configurations, long-term stability characterisation, thermal cycling, and progression towards an all-glass hermetically sealed construction.

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.