Measuring hydrogen ortho-para composition with a microwave cavity resonator

The nuclear spin-isomer () composition of molecular hydrogen strongly influences the safety and efficiency of cryogenic hydrogen handling, as the energy of conversion exceeds the enthalpy of vaporization of liquid hydrogen, generating boil-off losses. Accurate measurement of spin-isomer composition at cryogenic conditions is therefore essential for liquefaction process design, spin-isomer catalyst characterization, and custody transfer. Here we demonstrate a microwave cavity resonator as a novel sensor for real-time quantification of hydrogen composition. The method exploits contrast in the dielectric permittivity of the nuclear-spin isomers, enabling precise tracking of natural conversion dynamics in solid (10.0 K), liquid (16.6–19.9 K), and supercritical (40.4 K) hydrogen. Measurement uncertainties averaged 0.007 mol/mol across all phases. During conversion, the resonant frequencies followed a second-order kinetic model consistent with theoretical predictions and prior studies, with final equilibrium compositions independently validated by Raman spectroscopy. The determined conversion rate constants agree with the empirical correlation of a previous study, while also revealing the potential errors introduced from measuring natural conversion kinetics over a limited concentration range. Furthermore, our results identify significant discrepancies between experimental dielectric data and current predictive models, underscoring the need to refine hydrogen property models at cryogenic conditions. This work establishes microwave cavity resonators as a powerful tool for hydrogen sensing at cryogenic conditions, with applications spanning industrial liquid hydrogen process monitoring, hydrogen isomer conversion catalyst development, and high-energy physics.