Weiß, NicoNicoWeißKrafft, AndreasAndreasKrafftBullermann, JasminJasminBullermannMentzer, RasmusRasmusMentzerWirz, FriedrichFriedrichWirzKölpin, AlexanderAlexanderKölpin2026-04-152026-04-152026-04-07IEEE Sensors Letters (in Press): (2026)https://hdl.handle.net/11420/62723The maritime industry faces significant pressure to reduce CO2 emissions by up to 80% by 2050, as mandated by the “FuelEU Maritime” initiative of the European Union. Blending conventional fossil fuels with renewable drop-in fuels, such as hydrotreated vegetable oils (HVO), offers a viable medium-term solution. This letter presents a sensor system based on four cylindrical cavity resonators to monitor these mixing processes in real-time. By measuring the relative permittivity and applying an extended Clausius-Mossotti model, the system determines the mixing ratio within the fuel circulation loop of a marine engine. Experimental validation using a reduced-scale setup demonstrates a high accuracy with a maximum deviation of 4.5% compared to flow-rate sensors. The system effectively detects supply blockages, thereby providing a robust solution for ensuring compliance for possible regulations in maritime fuel systems.en2475-14722026IEEETechnology::600: TechnologyJournal Article10.1109/lsens.2026.3681676